Type Iax Supernovae

Saurabh W. Jha

Author’s version of a review chapter in the Handbook of Supernovae, edited by Athem W. Alsabti and Paul Murdin, Springer International Publishing, DOI 10.1007/978-3-319-20794-0

Abstract Type Iax supernovae (SN Iax), also called SN 2002cx-like supernovae, are the largest class of “peculiar” white dwarf (thermonuclear) supernovae, with over fifty mem- bers known. SN Iax have lower ejecta velocity and lower luminosities, and these pa- rameters span a much wider range, than normal type Ia supernovae (SN Ia). SN Iax are spectroscopically similar to some SN Ia near maximum light, but are unique among all supernovae in their late-time spectra, which never become fully “nebular”. SN Iax over- whelmingly occur in late-type host galaxies, implying a relatively young population. The SN Iax 2012Z is the only white dwarf supernova for which a pre-explosion progenitor sys- tem has been detected. A variety of models have been proposed, but one leading scenario has emerged: a type Iax supernova may be a pure-deflagration explosion of a carbon- oxygen (or hybrid carbon-oxygen-neon) white dwarf, triggered by helium accretion to the Chandrasekhar mass, that does not necessarily fully disrupt the star.

1 Introduction

Type Iax supernovae (SN Iax) are a class of objects similar in some observational prop- erties to normal type-Ia supernovae (SN Ia), but with clear differences in their light-curve and spectroscopic evolution. SN Iax are also called “02cx-like” supernovae, based on the exemplar SN 2002cx, which was described by Li et al. (2003) as “the most peculiar known” SN Ia. Later, Jha et al. (2006) presented other similar objects, making SN 2002cx arXiv:1707.01110v2 [astro-ph.HE] 9 Aug 2017 “the prototype of a new subclass” of SN Ia. Foley et al. (2013) coined the SN Iax classi- fication, arguing these objects should be separated from SN Ia as “a new class of stellar explosion.”

Saurabh W. Jha Department of Physics and Astronomy, Rutgers, the State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854 USA, e-mail: [email protected]

1 2 Saurabh W. Jha

Here I summarize the properties of SN Iax, describing their observational properties in Section 2 and models in Section 3. I discuss analogues of SN 2002cx and well-studied examples like SN 2005hk (Chornock et al. 2006; Phillips et al. 2007; Stanishev et al. 2007; Sahu et al. 2008), SN 2008A (McCully et al. 2014b), SN 2012Z (Stritzinger et al. 2015; Yamanaka et al. 2015), and SN 2014ck (Tomasella et al. 2016) as well as more extreme members of the class like SN 2008ha (Foley et al. 2009; Valenti et al. 2009) and SN 2010ae (Stritzinger et al. 2014). There may be some connection between SN Iax and other classes of peculiar white-dwarf supernovae (like SN 2002es-like objects; Gane- shalingam et al. 2012; White et al. 2015; Cao et al. 2016) but I restrict my focus to SN Iax here; other peculiar objects are explored by Taubenberger (2016).

2 Observations

In this section I discuss the identification and classification of SN Iax, followed by their photometric and spectral properties from early to late time. I also discuss the host envi- ronments of SN Iax, their rates, and pre- and post-explosion high-resolution imaging.

2.1 Identification and Classification

Supernovae are traditionally classified by their maximum light optical spectra, and SN Iax are no exception. These objects have spectra very similar to some normal SN Ia, particu- larly the “hot” SN 1991T-like or SN 1999aa-like objects, with typically weak Si II absorp- tion and prominent Fe III lines (Nugent et al. 1995; Li et al. 2001). The key discriminant for SN Iax is the expansion velocity; unlike typical SN Ia where the line velocity (usually measured with Si II) is ∼10,000 km s−1, SN Iax have much lower line velocities any- where from 6000 − 7000 km s−1 (for objects like SN 2002cx, 2005hk, 2008A, 2012Z, and 2014dt) down to 2000 km s−1 (for objects like SN 2009J). Figure 1 shows maximum- light spectra of SN Iax with a range of expansion velocities, compared to normal SN Ia. Because line velocities are not always measured or reported when SN are classified and host redshifts are sometimes uncertain, on occasion SN Iax have been misclassified as normal SN Ia. Indeed, it is possible to identify unrecognized SN Iax in past observations, like SN 1991bj1 (Stanishev et al. 2007). Table 1 gives a list of 53 objects that have been classified as SN Iax.

2.2 Photometric properties

The optical light curves of SN Iax show a general similarity to SN Ia, though with more di- versity. SN Iax typically have faster rises (∼10 to 20 days) in all bands, with pre-maximum

1 One speculates how the history of supernova cosmology would have changed if the diversity in SN Ia were not only typified by the extremes SN 1991T and SN 1991bg, but SN 1991bj as well, a SN Iax that would fall off the luminosity/light-curve decline rate relationship! Type Iax Supernovae 3

Table 1 Partial list of type Iax supernovae. Host galaxies, redshifts, and estimated peak absolute magnitudes are from the Open Supernova Catalog (Guillochon et al. 2017). The references listed present the classification and/or maximum-light data and are incomplete.

Supernova Host Galaxy z Mpeak References SN 1991bj IC 344 0.018 −15.3 Stanishev et al. (2007) SN 1999ax A140357+1551 0.023 −18.3 Foley et al. (2013) SN 2002bp UGC 6332 0.020 −16.4 Silverman et al. (2012) SN 2002cx CGCG 044−035 0.023 −18.8 Li et al. (2003) SN 2003gq NGC 7407 0.021 −17.2 Jha et al. (2006) SN 2004cs UGC 11001 0.014 −16.1 Foley et al. (2013) SN 2004gw CGCG 283−003 0.017 −16.9 Foley et al. (2009) SN 2005P NGC 5468 0.009 −14.8 Jha et al. (2006) SN 2005cc NGC 5383 0.007 −17.6 Antilogus et al. (2005) SN 2005hk UGC 272 0.013 −18.5 Chornock et al. (2006); Phillips et al. (2007); Stanishev et al. (2007); Sahu et al. (2008) SN 2006hn UGC 6154 0.017 −18.9 Foley et al. (2009) SN 2007J UGC 1778 0.017 −17.2 Filippenko et al. (2007) SN 2007ie SDSS J21736.67+003647.6 0.093 −18.2 Östman et al. (2011) SN 2007qd SDSS J20932.72−005959.7 0.043 −16.2 McClelland et al. (2010) SN 2008A NGC 634 0.016 −19.0 McCully et al. (2014b) SN 2008ae IC 577 0.030 −18.8 Blondin and Calkins (2008) SN 2008ge NGC 1527 0.003 −17.4 Foley et al. (2010b) SN 2008ha UGC 12682 0.004 −14.0 Foley et al. (2009); Valenti et al. (2009) SN 2009J IC 2160 0.015 −15.9 Stritzinger (2009) SN 2009ho UGC 1941 0.048 −18.2 Steele et al. (2009) SN 2009ku A032953−2805 0.079 −17.9 Narayan et al. (2011) PTF 09ego SDSS J172625.23+625821.4 0.104 −18.6 White et al. (2015) PTF 09eiy 0.06 White et al. (2015) PTF 09eoi SDSS J232412.96··· +124646.6 0.042 −16···.7 White et al. (2015) SN 2010ae ESO 162-G17 0.003 −14.0 Stritzinger et al. (2014) SN 2010el NGC 1566 0.005 −13.0 Bessell and Schmidt (2010) PTF 10xk 0.066 −17.1 White et al. (2015) SN 2011ay NGC··· 2315 0.021 −18.1 Szalai et al. (2015) SN 2011ce NGC 6708 0.008 −17.1 Maza et al. (2011) PTF 11hyh SDSS J014550.57+143501.9 0.057 −18.7 White et al. (2015) SN 2012Z NGC 1309 0.007 −18.1 Stritzinger et al. (2015); Yamanaka et al. (2015) PS1-12bwh CGCG 205−021 0.023 −16.2 Magee et al. (2017) LSQ12fhs 0.033 −18.2 Copin et al. (2012) SN 2013dh NGC··· 5936 0.013 −17.3 Jha et al. (2013) SN 2013en UGC 11369 0.015 −17.9 Liu et al. (2015c) SN 2013gr ESO 114−G7 0.007 −15.4 Hsiao et al. (2013a,b) OGLE-2013-SN-130 0.09 −18.0 Bersier et al. (2013) OGLE-2013-SN-147 ··· 0.099 −19.3 Le Guillou et al. (2013) iPTF 13an 2MASX J12141590··· +1532096 0.080 White et al. (2015) SN 2014ck UGC 12182 0.005 −15···.5 Tomasella et al. (2016) SN 2014cr NGC 6806 0.019 −17.0 Childress et al. (2014) LSQ14dtt 0.05 −18.0 Elias-Rosa et al. (2014) SN 2014dt NGC··· 4303 0.005 −18.6 Foley et al. (2015); Fox et al. (2016) SN 2014ek UGC 12850 0.023 −18.0 Zhang and Wang (2014) SN 2014ey CGCG 048−099 0.032 −18.1 Lyman et al. (2017) SN 2015H NGC 3464 0.012 −17.7 Magee et al. (2016) PS15aic 2MASX J13304792+3806450 0.056 −17.9 Pan et al. (2015) SN 2015ce UGC 12156 0.017 −17.9 Balam (2017) PS15csd 0.044 −17.5 Harmanen et al. (2015) SN 2016atw ··· 0.065 −18.0 Pan et al. (2016) OGLE16erd ··· 0.035 −17.1 Dimitriadis et al. (2016) SN 2016ilf 2MASX J02351956··· +3511426 0.045 −17.6 Zhang et al. (2016) iPTF 16fnm UGC 00755 0.022 −15.0 Miller et al. (2017) 4The Astrophysical Journal,767:57(28pp),2013April10 Saurabh W. Jha Foley et al. SNe Ia and Ic. However, exclusively using the previously listed adapted from criteria for classification would include specific SNe that do Foley et al. (2013) not appear to be physically related to SN 2002cx. For instance, SN 2005E (Perets et al. 2010)lackshydrogen,hasalowejecta 4 SN 2009J velocity, and a low luminosity for its light-curve shape, but Smoothed by is clearly spectroscopically different from SN 2002cx at all −1 300 km s epochs. SNe 2005E and 2008ha have somewhat similar spectra Blueshifted by at 2 months after maximum brightness. Both have strong 3000 km s−1 [Ca∼ii]andCaii emission, but there are several significant 3 differences, including SN 2008ha lacking [O i](Foleyetal. SN 2005cc S II 2009). Significant differences in the host-galaxy morphologies Si II Smoothed by for SNe similar to SNe 2002cx and 2005E further suggest that −1 + Constant adapted from λ 900 km s these SNe have significantly different progenitor systems (Foley McCully et al. (2014) 2 Blueshifted by et al. 2009; Perets et al. 2010). SNe 2002es (Ganeshalingam 3000 km s−1 et al. 2012)andPTF09dav(Sullivanetal.2011,whichfails

Relative f our first criterion) have low luminosity and low velocities, but also have significantly cooler spectra (lacking Fe iii and having relative flux + constant relative SN 2008A strong Ti ii features) than SN 2002cx near maximum brightness. 1 Smoothed by 1800 km s−1 Although SN 2002es and PTF 09dav may be physically related to the SN 2002cx-like class in the same way that SN 1991bg Blueshifted by is similar to the hotter, more common “Branch-normal SNe Ia” 6000 km s−1 (Branch et al. 1993), we do not currently link these SNe to 0 SN 1999aa SNe Iax. The progenitor environments of the SN 2002es and PTF 09dav are also suggestive of older progenitor systems: 4000 4500 5000 5500 6000 6500 7000 Rest Wavelength (Å) SN 2002es was hosted in an S0 galaxy, a fairly unusual host rest wavelength (Å) for an SN Iax (Foley et al. 2009), and PTF 09dav was found Figure 1. Spectra of SNe Iax, from lowest to highest ejecta velocity (SNe 2009J, 40 kpc from its host, which may indicate a particularly old 2005cc, and 2008A, respectively), in comparison with the high-luminosity ∼ Fig. 1 Near-maximum light spectra of SNSN Iax Ia 1999aa compared (Li et al. to2001b normal;Garavinietal. and 91T/99aa-like2004). The black SN spectra Ia. The are progenitor system, unlike what is inferred for the majority of unaltered. The red spectra are smoothed using a Gaussian filter of 300, 900, SNe Iax. We also exclude SN 2002bj (Poznanski et al. 2010), left panel shows the similarity of SN Iax to normal SN1 Ia (including the weak presence of S II lines and 1800 km s− for SNe 2009J, 2005cc, and 2008A, respectively. The blue which is somewhat spectroscopically similar to SN 2002cx, but sometimes considered hallmarks of thermonuclearspectra are the SN), smoothed but spectra also after the being lower blueshifted expansion by 3000, velocities 3000, and 1 also different in several ways. Its light curve was extremely 6000 km s− , respectively. The smoothed, blueshifted SN 2009J spectrum is (e.g., compare the locations of the Si II lines).visually The similar right to the panel unaltered shows SN 2005cc the range spectrum; of the SN smoothed, Iax expansion blueshifted fast (∆m15 > 4mag),yetitwasstillfairlyluminousatpeak − SN 2005cc spectrum≈ is visually similar1 to the unaltered SN 2008A spectrum;≈ (M 18 mag). We consider there to be too many significant velocities, from the lowest velocity SN 2009J (vexp 2200 km s ) at the top, SN 2005cc (vexp ≈− 5000 km s−1) intermediate, and SN 2008Aand ( thev smoothed,≈ 6400 blueshifted km s−1 SN) at 2008A the bottom, spectrum is also visually compared similar to tothe differences to include SN 2002bj in the class. A list of our unalteredexp SN 1999aa spectrum. criteria and how various SN classes and particular objects pass the normal SN 1999aa below. Note the similarity(A color version in ofthe this spectra figure is available as the inlower-velocity the online journal.) objects are or fail the criteria is presented in Table 1. smoothed and shifted to resemble the higher-velocity objects. Figure adapted from McCully et al. When applying the criteria to our sample, we note that three (2014b) and Foley et al. (2013). To be a member of the SN Iax class, we require (1) no evi- of the four criteria can be determined from a single spectrum dence of hydrogen in any spectrum, (2) a maximum-brightness near maximum light. We also note that there are no SNe photospheric velocity lower than that of any normal SN Ia at that are spectroscopically similar to SN 2002cx and also have 1 luminosities equal to or larger than SNe Ia with the same light- maximum brightness ( v ! 8000 km s− ), and (3) if near- light curves showing significant varietymaximum (Magee light etcurves al. 2016, are| | available, 2017). The an absoluteB and magnitudeV-band de- that curve shape. Therefore, a single near-maximum-light spectrum cline rates are similar to normal SN Ia,is though low for a generally normal SNalso Ia given on its the light-curve faster side shape (Stritzinger (i.e., falling appears to be sufficient for classification. However, because of et al. 2015), and the optical color evolutionbelow the in WLR SN Iax for SNe (e.g., Ia). in ByB the−V first) has criterion, a shape we exclude roughly all the spectral similarities with other SNe Ia (except for the lower SNe II and any SN that obviously has a hydrogen envelope. The ejecta velocities; see Figure 1), SNe Iax are easily mistaken as similar to normal SN Ia (Foley et al. 2013).second However, criterion will SN exclude Iax have all “normal” significantly SNe Ia, slower Ib, and de- Ic, normal SNe Ia in initial classifications. In particular, spectral clines in redder bands, e.g., ∆m15(R) including' 0.2 to high-luminosity 0.8 mag, compared and low-luminosity to normal SN SNe Ia Ia which similar classification software such as SNID (Blondin & Tonry 2007) to SN 1991T (Filippenko et al. 1992b; Phillips et al. 1992)and can easily misclassify an SN Iax as an SN 1991T-like SN Ia if have ∆m15(R) ' 0.6 to 0.8 mag (Magee et al. 2016). Faster rising SN Iax are generally SN 1991bg (Filippenko et al. 1992a;Leibundgutetal.1993), one does not know the redshift of the SN or does not restrict faster fading as well, with some exceptions,respectively, like as well SN as 2007qd the unique (McClelland SN 2000cx (Li et et al. al. 2010).2001a), the redshift to be the host-galaxy redshift. Misclassification SN Iax do not show the prominent “second-peak”SNe similar to SN in the 2006bt redder (Foley and et near-infrared al. 2010b), SN bands 2010X was even more common before the recognition of SN 2002cx (González-Gaitán et al. 2014) that characterize(Kasliwal et al. normal2010), and SN ultraluminous Ia. The second-maximum SNe I (e.g., Pastorello is as being distinct from SNe Ia and before a substantial set of et al. 2010;Quimbyetal.2011;Chomiuketal.2011). No known examples was assembled. We therefore do not believe that our especially strong in slowly-decliningcore-collapse SN 1999aa SN or 1991T-like passes these first SN two Ia; criteria. thus the The SN third Ia crite- that sample is complete, and it could be significantly incomplete are spectroscopically most similar torion SN Iaxexcludes have all quite “super-Chandrasekhar” a different photometric SNe Ia (e.g., behavior. Howell (especially for SNe discovered before 2002). However, the Nonetheless, similar to SN Ia, weeks afteret al. 2006 maximum), which light can have SN low Iax ejecta show velocities, only modest but have color high CfA and Berkeley Supernova Ia Program (BSNIP) spectral peak luminosities. samples have been searched for misclassified SNe Iax (Blondin Afinalcriterionisthatthespectraneedtobesimilarto et al. 2012; Silverman et al. 2012), with BSNIP identifying those of SN 2002cx at comparable epochs. This last criterion one new SN Iax. When determining membership in the class, is somewhat subjective, yet necessary. It is also a primary we first examine our own and published data to match the criterion, as the first two criteria listed above naturally result criteria described above. For some objects, we use data from from spectral similarity. We note that this criterion is no IAU Circulars and The Astronomer’s Telegrams. There is more subjective than the one used to distinguish between no definitively identified SN Iax for which we do not have

3 Type Iax Supernovae 5 evolution. Late-time colors may thus provide a useful diagnostic of host-galaxy redden- ing (Lira 1996; Foley et al. 2013), which is otherwise difficult to determine for SN Iax. The very late-time optical light curves of typical SN Iax continue to show a decline slower than SN Ia until about 300 to 400 days past maximum light, after which SN Ia light curves also slow to similar decline rates as SN Iax: 0.01−0.02 mag day−1 (McCully et al. 2014b). The peak optical luminosity of SN Iax is lower than typical SN Ia and it spans a much wider range, from MV ' −19 on the bright end to MV ' −13 for the faintest SN Iax known. Compared to the light curve decline rate, SN Iax fall well below the Phillips (1993) rela- tion, by anywhere between 0.5 and several magnitudes (Foley et al. 2013). Certainly, as seen in Figure 2, SN Iax do not show as tight a relation in this parameter space as nor- mal SN Ia (even including the SN 1991T/1999aa and SN 1991bg extremes of the normal SN Ia distribution). Magee et al. (2016, see their Figure 5) suggest a stronger correlation may exist between peak luminosity and rise time, rather than decline rate. Near-infrared light curves of SN Iax are limited, with SN 2005hk still providing the best data set (Phillips et al. 2007). Continuing the trend with wavelength in the optical, the YJH light curves of SN 2005hk show a broad, single peak that is delayed significantly (∼10 to 15 days) relative to the B peak. The NIR contribution to the quasi-bolometric “UVOIR” flux seems not too dissimilar to normal SN Ia (Stritzinger et al. 2015), and this fraction seems roughly consistent even for the faintest SN Iax like SN 2008ha and SN 2010ae (Stritzinger et al. 2014). A major surprise, however, is SN 2014dt, which showed a significant near- and mid-infrared excess beginning ∼100 days past maximum and lasting for a few hundred days at least (Fox et al. 2016). The near-UV photometric behavior of SN Iax is also interesting, with objects showing a faster evolution in near-UV minus optical color than SN Ia, and “crossing” the typically parallel tracks made by normal SN Ia in this space. SN Iax start bluer than normal SN Ia in the UV before maximum light but quickly redden (by ∼ 1.5 to 2 mag in Swift uvw1 − b) so that about ten days after maximum they are redder than normal SN Ia (Milne et al. 2010). As with other thermonuclear supernovae, no SN Iax has been definitively detected in the radio (Chomiuk et al. 2016) or X-ray (Margutti et al. 2014).

2.3 Spectroscopic properties

Beyond the defining spectroscopic features used for classification of these supernovae (Fe III dominated spectra near maximum light and low Si II velocity; see Sec. 2.1), SN Iax show quite homogeneous spectral evolution, which generally matches the evolution of SN Ia over the period of a few months from maximum light, except with lower line veloc- ities (Jha et al. 2006). Like SN Ia, the early-time spectra show Fe-group and intermediate mass elements (including Si, S, and Ca). This similarity extends to the near-UV (with Fe-group line blanketing). In their near-infrared maximum-light spectra, SN Iax show re- markable similarity to SN Ia, with Fe II and Si III lines, except at lower typical velocities, and most prominently, beautiful and unambiguous detections of Co II in the H and K 6M. D. Stritzinger et al.: The bright and energetic Saurabh Type W.Iax JhaSN 2012Z.

09ku

12bwh 08ge 14ck Fig. 6. MB vs. ∆m15(B)forasampleofCSP SNe Ia (black dots), a handful of SNe Iax (blue squares) that span their full range in luminos- 13dh ity, and SN 2012Z (red star). We note the error bars associated with SN 2012Z are smaller than its symbol size. The comparison SNe Iax plot- 16fnm ted are SN 2002cx (Li et al. 2003; Phillips et al. 2007), SN 2003gq (see Foley et al. 2013,and references therein), SN 2005hk (Phillips et al. 2007), SN 2007qd (McClelland et al. 2010), adapted from Stritzinger et al. (2015) SN 2008A (see Foley et al. 2013,andrefer- ences therein), SN 2008ge (Foley et al. 2013), A&A 589, A89 (2016) SN 2008ha (Stritzinger et al. 2014), SN 2010ae (Stritzinger et al. 2014), and SN 2011ay (Brown, Δm15(B) (mag) priv. comm.). -20

Normal Ia 4.2. Absolute magnitudes, UVOIR light curve, and light curve modeling -19 09ego 11hyh 09ku With estimates of peak apparentmagnitude for the ugriBV bands 12Z 11ay N5def in hand, peak absolute magnitudes were computed adopting an -18 08A 05hk PS15csdE(B V)tot = 0.11 0.03 mag and the direct distance mea- 08ae − ± N3defsurement µ = 32.59 0.09 mag. The resulting peak values are 02cx 3 ± 03gq listed in Table 6 ,withaccompanyinguncertaintiesthataccount -17 05cc 15H for both the error in the fit to the time of maximum, and the Magnitude N1def 09eoi 04cserror associated with the distance to the host galaxy. Reaching an absolute peak B-band magnitude (M ) 18.3 0.1with B ≈− ± -16 ∆m15(B) = 1.43 0.02 mag, SN 2012Z is among the brightest ±

Absolute 07qd 09J and slowest declining SN Iax yet observed. This is demonstrated in Fig. 6,wherethelocationofSN2012ZintheMB vs. ∆m15 diagram is shown compared to a handful of other well-observed

Peak -15 SNe Iax,and an extendedsample of SNe Ia observedby the CSP. Armed with the broadband photometry of SNe 2005hk -14 SNe Iax r R 10ae08ha and 2012Z, we proceeded to construct UVOIR bolometric light Absolute R (or r)-band Magnitude Normal SNe Ia curves. For SN 2005hk we adopted the early phase photometry adapted from Magee et al. (2016) Model presented in Appendix A, as well as the late phase photometry 0 0.2 0.4 0.6 0.8 1 published1.2 1.4 by Sahu et al. (2008). The early phase filtered light Decline Rate curves of both objects were fit with spline functions, which al- Δm15(R or r) (mag) lowed us to fill in missing gaps in the data based on interpola- tion. For SN 2012Z the u-band light curve was extended so that Fig. 6. Decline rate (m15) versus peak absolute r-band magnitude for Fig. 5. Possible linear correlation between the rise time to peak, andFig. 2 Absolute magnitude vs. decline rate relation for SN Iax (colored)its compared temporal coverageto normal matched that of the other optical filters. peak absolute magnitude for SNe Iax in the r-band. For comparison, weSN Ia (black)SNe Iax. showing As in the Fig. Phillips5, unfilled (1993) relation, circles arein B SNe-band Iax (above) in the andToRR-band, do(or so,r)-band we again resorted below. to extrapolation by adopting an average also show the region occupied by a few normal SNe Ia, using the esti-These plotswith are the adapted exception from Stritzinger of SN 2004cs, et al. (2015) which and is Magee based et al. on (2016). unfiltered(u B)colorderivedfromthelastthreeepochsinwhich imag- u-band mated explosion epochs and data from sources listed in Table A.8. There ing. Grey crosses denote the models discussed in Sect. 5photometry.− Black points was obtained. is no clear demarcation between the SNe Ia and the SNe Iax at the bright are values typical of normal SNe Ia, taken from CarnegieWith Supernova the definitive observed light curves in hand, the pho- end. Note that values shown for SN 2005cf and SN 2011fe are based on Project7 (Hamuy et al. 2006) fits to SNe Ia data. Notetometry objects was shown corrected for extinction, and then converted to flux Fig.here 7. (B thatV), are (V notr), and shown (V i)colorevolutionofSN2012Z(red in Fig. 5 as they lack goodat pre-maximum the effective wavelength of each passband. This allowed us to filter transformations (Jester et al. 2005). Unfilled circles denote mea- stars), compared− to− the Type Iax− SN 2005hk (blue dots) and the un- construct SEDs spanning from 300 to 2500 nm. The full series surements taken in the R-band, with the exception of SN 2004cs, which reddened,coverage normal are: Type SNe Ia 2003gq, SN 2006ax 2005cc, (solid line). 2008ae, The photometry 2011ay, of PTFof SEDs 11hyh, were and then summed over∼ wavelength using a trapezoidal is based on unfiltered imaging. The data sources for all objects are listed SNPS15csd. 2006ax is Distances taken from Contreras to PS15csd, et al. (2010 and). PTF The color 11hyh, curves 09ego, of integration and 09eoi technique, are assuming zero flux beyond the limits of in Table A.8. Grey crosses denote the models discussed in Sect. 5. SNederived 2005hk based and 2012Z on the have estimated been corrected redshift. for Milky The Way data and sources host integration. for all objects Owing to our limited NIR photometric coverage of extinction adopting E(B V) = 0.112 mag (Chornock et al. 2006; are listed in Table A.8− .tot SN 2012Z it was necessary to account for the NIR contribu- Phillips et al. 2007)andE(B V)tot = 0.105 mag, respectively, while tion of the flux. Given the close resemblance between the peak the color curves of SN 2006ax− have been corrected for Milky Way reddening. 3 To derive absolute magnitudes of SN 2005hk (and absolute lumi- nosity, see Sect. 4.2), we have adopted a distance modulus of µ = SNe Ia which has also been considered in previous studies (e.g. objectsand displays0.71 (p intrinsic-value colors0.02) or color if PTF evolution 09eoi which and could SN 2007qd33.46 0 are.27 mag ex- (Phillips et al. 2007)andthetotalcolorexcessvalue ⇠ ± Narayan et al. 2011; Foley et al. 2013; Stritzinger et al. 2014). allow an accurate estimation of the host extinction. E(B V)host = 0.11 mag. cluded. Thus, our findings are suggestive of the existence− of cor- We use the standard definition of decline rate: i.e., the change relations between the absolute (r-band) magnitude of SNe Iax A2, page 7 of 24 in observed magnitude in a given filter between maximum light with both rise time and decline rate. and 15 d thereafter. For SN 2015H, we find (m ) = 0.69 0.04 15 r Using the parameters derived from our light curve, we which is similar to that observed in some normal SNe Ia, despite± now seek to constrain the amount of 56Ni produced during being approximately two magnitudes fainter. It is also consistent the explosion, and the total amount of material ejected. Our with the roughly linear relation observed in SNe Iax (Fig. 6), al- maximum light coverage of SN 2015H unfortunately only in- though PTF 09eoi is again a clear outlier, as is SN 2007qd in this cludes one filter, so we are unable to construct a bolomet- instance. Together, Figs. 5 and 6 show that SNe Iax with longer ric light curve. We note however, that the colour and decline rise times show slower decline rates, indicating that the width of rates of SN 2015H and SN 2005hk, are very similar (see a SN Iax light curve is indeed linked with its absolute magnitude Figs. 3 and 6), and SN 2005hk has extensive pre-maximum and tied to 56Ni production. coverage in numerous filters. We therefore stretched the light We test whether the trends observed in Figs. 5 and 6 curves of SN 2005hk from Stritzinger et al. (2015) and scaled represent statistically significant correlations via the use of them to match SN 2015H, allowing us to use these values the Pearson correlation coecient, which is a measure of to construct a pseudo-bolometric light curve for SN 2015H whether a linear correlation exists between two variables. A across ugriJH filters. Applying Arnett’s law (Arnett 1982), and value of 1 represents a complete correlation, while 0 in- the descriptions provided by Stritzinger & Leibundgut (2005) dicates no± correlation. For the decline rate versus peak ab- and Ganeshalingam et al. (2012), we estimated the 56Ni and solute magnitude, we find a correlation coecient of 0.39 ejecta masses. By taking a peak bolometric luminosity of (p-value 0.09) when considering the entire sample. Exclud- ⇠ 8 log(L/L ) = 8.6, and a rise time to bolometric maximum ing SN 2007qd and PTF 09eoi we find this value increases of 14.5 days, we find that SN 2015H produced 0.06 M to 0.72 (p-value 0.001). For the rise time versus peak abso- of ⇠56Ni. Using an average ejecta velocity of 5500⇠ km s 1 lute magnitude distribution⇠ of SNe Iax, we find a correlation co- (Sect. 3.3, Fig. 9) we estimate an ejecta mass⇠ of 0.50 M . ecient of 0.52 (p-value 0.08) when including all objects, ⇠ ⇠ These values are based on simple scaling laws appropriate for normal SNe Ia. As a test of the validity of our estimates, we 6 http://csp.obs.carnegiescience.edu 8 used the light curve model described in Inserra et al. (2013) to The nature of PTF 09eoi may be uncertain: its limited spectral series fit our pseudo-bolometric light curve; we find that it is best shows some similarities to SNe Iax at early epochs, but the matches matched by 0.06 M of 56Ni and 0.54 M of ejecta. Thus, for worsen with time (White et al. 2015). SN 2007qd does not have spectra ⇠ ⇠ 56 beyond 15 d post B-band maximum (McClelland et al. 2010). Both SN 2015H, we find our values for the Ni mass and the ejecta ⇠ PTF 09eoi and SN 2007qd lie in a region of Mr vs. (m15)r separate mass to be consistent both with other estimates for SNe Iax (see from the rest of the sample. Given the lack of data for these objects, we Fig. 15 of McCully et al. 2014b) and the fact that it is of inter- are not able to make definitive statements about their nature. mediate brightness among this class.

A89, page 6 of 18 Type Iax Supernovae 7 bands for SN 2010ae, 2012Z, and 2014ck (Stritzinger et al. 2014, 2015; Tomasella et al. 2016). It is at late times that the spectra of SN Iax radically diverge from SN Ia, and indeed, almost all other supernovae of any type (Jha et al. 2006; Sahu et al. 2008; Foley et al. 2010a, 2016). SN Iax never truly enter a fully “nebular” phase in which broad forbidden lines dominate the optical spectrum (Figure 3). Rather, in optical spectra taken more than a year past maximum light, SN Iax still show permitted lines of predominantly Fe II, often with low velocities < 2000 km s−1, plus Na I D and the Ca II IR triplet. Forbidden lines of [Fe II], [Ni II], and [Ca II] are also usually present, and in some cases with narrow widths down to < 500 km s−1 (McCully et al. 2014b; Stritzinger et al. 2015). The linewidths and relative strengths of the forbidden and permitted lines seem to vary significantly among different SN Iax (Yamanaka et al. 2015; Foley et al. 2016). The late-time linewidth varia- tion, for example, approaches nearly an order of magnitude, from a few hundred to ∼3000 km s−1. Spectra of SN Iax seem to show relatively little evolution from 200 to past 400 days after maximum (Foley et al. 2016). The faintest SN Iax, like SN 2008ha and SN 2010ae, show similar spectra to more luminous counterparts, perhaps with a more rapid spectral evolution to lower velocities in the few weeks after maximum light (Foley et al. 2009; Valenti et al. 2009; Stritzinger et al. 2014). Around 250 days past maximum, the spectra of SN 2002cx (a “bright” SN Iax; Jha et al. 2006) and SN 2010ae (one of the faintest) are nearly identical (see Figure 12 of Stritzinger et al. 2014). Chornock et al. (2006) and Maund et al. (2010) obtained spectropolarimetric observa- tions of SN 2005hk and report 0.2–0.4% continuum polarization, consistent with spec- tropolarimetry of normal SN Ia. Two objects, SN 2004cs and 2007J, have been classified as SN Iax by Foley et al. (2013) and show clear evidence of He I emission in their post-maximum spectra, some- thing never seen in normal SN Ia. White et al. (2015) argue that these objects may be type-IIb supernovae instead, though Foley et al. (2016) counter that claim and call PTF 09ego and PTF 09eiy into question as SN Iax. In the end, there are a handful of objects for which the classification may be ambiguous.

2.4 Photometric and spectroscopic correlations

SN Iax span a wide range of peak luminosities and line velocities, and it is natural to ask if these are related. McClelland et al. (2010) suggested a positive correlation between these two, with the lowest-velocity SN Iax also being the lowest luminosity. While such a correlation does seem to hold for the majority of SN Iax, there are clear counterexamples like SN 2009ku (Narayan et al. 2011), which had a low velocity (similar to SN 2008ha or SN 2010ae), but relatively high luminosity (like SN 2002cx or SN 2005hk). Tomasella et al. (2016) show that SN 2014ck is similarly an outlier to the velocity/luminosity corre- lation. Neither do the lowest-velocity SN Iax necessarily have the fastest optical decline rates: while SN 2008ha and SN 2010ae decline quickly, SN 2014ck has an intermediate The Astrophysical Journal,786:134(19pp),2014May10 8McCully et al. Saurabh W. Jha The Astrophysical Journal,786:134(19pp),2014May10 McCully et al. adapted from McCully et al. (2014) from McCully adapted

Late-time Spectra of SNe Iax 5

[Ca II] SN 2002cx +227 d [Ni II] SN 2008A +220 d 8 SN 2008ge +225 d λ 6 [Fe II]

4 Relative f

2

0 4000 5000 6000 7000 8000 9000 7000 7200 7400 7600 7800 Rest Wavelength (Å)

Figure 2. Late-time spectra of SNe 2002cx at a phase of +227 d, (red curve), 2008A at a phase of +220 d (black curve), and 2008ge at adapted from McCully et al. (2014)aphaseof+225d(bluecurve).Theleftpanelshowstheentireopticalregion,whiletherightpaneldisplaystheregioncontainingthe [Fe ii] 7155, [Ca ii] 7291, 7324, and [Ni ii] 7378 features (all labeled). The SN 2002cx spectrum has a relatively high signal-to-noise ratio (S/N), and the small-amplitude features in the SN 2002cx spectrum are mostly real (J06). This figure displays the heterogeneous late-time spectra of SNe Iax. adapted from et Foley al. (2016) adapted

02cx 05P 05hk 08A 08ge Figure 9. Late-time spectra8 of SN 2002cx ( + 277 days; top, green), SN 2008A ( + 283 days; top middle, blue), and SN 2005hk ( + 378 days; bottom middle, red; Sahu et al. 2008) compared to a synthetic model spectrum (black, bottom). The synthetic spectrum was created with Syn++ (Thomas et al. 2011b,similartoSYNOW relative flux + constant relative+ flux (Branch et al. 2005)). The synthetic6 spectrum assumes Boltzmann excitation and only models the permitted lines. Non-LTE effects and forbidden lines are important for the relative strengths of the4 lines, but Syn++ is useful for the identification of the lines. We have marked the strongest forbidden emission features, discussed in the text, with dashed lines. The signal-to-noise ratio is substantially better in SN 2005hk, so this was used for the primary fit and then compared to SN 2002cx and SN 2008A. Line identificationsλ 2 are included under the synthetic spectrum. (A color version of this figure0 is available in the online journal.) 10ae 11ay 11ce 12Z 14dt

unburned material atallRelative f 8 velocities, including the central regions continuum, as is predicted by the pure deflagration models that are revealed at late times. (Kozma et al. 2005), we would expect a strong r i and V r Jha et al. (2006a) tentatively6 identified permitted O i λ7774 color change as the SN transitions to a nebular phase.− − in SN 2002cx at 227 and4 277 days after B maximum. As can be To measure this color change, we examine two epochs of seen in Figure 9,thesamefeatureispresentinSN2008A,but2 HST photometry of SN 2008A: the first epoch is at +396 days, even stronger. If the identification is correct, this matches the at which we expect no contribution from [O i] λ6300 based model predictions of a0 pure deflagration explosion that never on our spectra of SN 2005hk at similar epochs. In our second transitioned to a detonation.7000 However,7250 7500 this7000 feature7250 is a 7500permitted7000 7250observation7500 7000 at7250 +5737500 days,7000 we do7250 see a7500 strong r i color change, transition, implying that the density of the ejecta is unexpectedlyRest Wavelengthbut V (Å) r remains relatively unchanged (see− the discussion − high out to rest280Figure days wavelength 3. pastLate-time maximum. spectra of By SNe (Å) thisIax (black). phase, Each SNe panel Ia displays thein spectrum Section of a3.5 di↵erent); this SN. The cannot red curve be corresponds easily explained to the by just the have transitioned∼ best-fitting to a nebular 10-parameter phase model dominated of the forbidden by lines.forbidden The blue dottedappearance curves and the gold of a dashed strong curves line correspond in the r toband. the individual Figure 6. Spectra of SN 2008A (this work) and SN 2005hk (Phillips et al. 2007, and this work) compared to those of SN 2002cx (Li ettransitions al. 2003;Jhaetal. of iron-peaknarrow2006a and elements. broad). components, These35 If respectively. the identification of Our photometry of SN 2008A at + 573 allows us to quan- three SNe Iax have remarkably homogeneous spectra throughout their evolution, and diverge dramatically from normal SNe Ia at lateO i times,λ7774 as is shown correct by and comparison there is unburned oxygen at low titatively constrain the amount of oxygen in the ejecta below Fig. 3 Late-time spectra of SN Iax (colored) compared to a normal SN Ia (black; left),6.5 3 showing spectra of SN 1991T (Filippenko et al. 1992b) and SN 1998bu (Jha et al. 1999;Lietal.2001;Silvermanetal.2012). velocity, [O i] λ6300tral should appearance be a between prominent +230 andfeature +403 in d the (Figure nebular4). Al- turethe of critical the photosphere. density ofAdditionally, 10 cm the− .Thecompletedeflagration [Ca ii] 7291, a clear divergence. Numerous linesspectra of that these objects.though look Weroughly searchedlike 6 months “noise” for has evidence passed between of are [O thesei] λ6300actually epochs, 7324models lines permitted of have Kozma a smaller et al.equivalent Fe (2005transitions, width)giveusarelationshipbetween (EW) in the (A color version of this figure is available in the online journal.) in spectra of SN 2005hkand the SN taken is nearly400 twice days as old after in the maximum, second epoch but as in laterthe oxygen spectrum mass (Figure and4). This the di[O↵erencei] λ6300 is caused line by flux. the If we unrealisti- the first and has∼ faded significantly, the spectra are nearly [Ca ii] lines becoming narrower, with the FWHM decreas- SN 2005hk did not enter a nebular phase even at these late cally assume that1 all of the observed1 flux in F625W is from an as seen with the Syn++ (Thomas et al. 2011)identical. spectrum synthesis modeling from 290 (black) km s to 230 kmof s , SN and moving 2005hk slightly epochs and there wasExamining no evidence the di↵ forerences [O i between] λ6300. the two spectra (Fig- tooxygen the red line, (as determined we would by derive simultaneously a mass fitting of 0.63 bothM of oxygen at ⊙ possibly O ii. SN 2008A has strong carbon features (even rel- 2003hv and has been used to measureSpectroscopy asymmetry isure no4 in), longer we the note feasible inner that there after is athese slight di late↵erence epochs in the lineslow with density. Gaussians), In reality, with the the velocity oxygen shift increasing line flux from is only part of the 1 1 λ (upper right). SN Iax show a diversity of line velocities and strengths in360the km s to [Fe180 km II] s (where7155, a negative velocity[Ca in- II] continuum strength, which may be the result of small er- ative to normal SNe Ia), but SN 2005hk shows no evidence layers of the ejecta (Leloudas et al. 2009because). the SN is too faint. Instead, we use photometry to dicatesobserved a blueshifted broad-band feature; F625W Figure 5 photometry.). Similar behaviour If we extrapolate just λλ7291,7324, and [Ni II] λ7378constrain lines the strength atrors late in flux of thecalibration times. [O i]lineat+573daysafter or aFigure slight change topanels the tempera- adaptedthe r-band photometry fromMcCully from earlier times et to dayal. 573, the data for carbon emission. One of the key differences between these We used Syn++ (Thomas et al. 2011bmaximum.)tomodelthepermitted [O i] λMNRAS6300 is000 near, 1–?? the(2015) center of the r band (F625W) allow for only 0.40 M of oxygen to contribute additional line two objects is that the velocities of the spectral features of (2014b)lines in and the Foley late-time et al.spectra. (2016). Ourand results is reasonably are also isolated shown from any in other spectral features flux. ⊙ 1 expectedtobepresent.Therefore,weusethe r-band photometric However, we can derive much more stringent oxygen mass SN 2008A are higher by 1500 km s− at maximum than those Figure 9.Becausethesignal-to-noiseratiowasthehighestinflux as a proxy for the flux in the oxygen line. If the [O i] λ6300 limits if we constrain the SN spectral energy distribution (SED) of SN 2005hk. The spectral features of SN 2005hk remain at the latest SN 2005hk spectrum, we fitline the began lines to in dominate this spectra other spectral and features and the nearby in this wavelength region. Based on the observed spectroscopy lower velocities that those of SN 2008A at all epochs, even out then matched them to the spectra of SNe 2002cx and 2008A. We at earlier times, we see that SNe Iax remain pseudo-continuum 35 Branch et al. (2008) argue that permitted lines dominate optical spectra of dominated in broad-band photometry to 400 days past maxi- to a year past maximum. find that Fe ii is necessary to fit manynormal of theSNe Ia lines as late as between a few months past 6000 maximum light, though by about mum, and the subsequent photometry does∼ not show dramatic decline rate similar to other higher-velocity160 days past maximum typical SNe SN Ia have nebular Iax, spectra while (SilvermanSN et al. 2009ku in fact has a slower Figure 9 compares the late-time spectra of SN 2002cx, and 6400 Å, as was found by Jha et2013 al.). (2006a)andconfirmed changes in late-time behavior among the different passbands. SN 2005hk, and SN 2008A. While these spectra are qualitatively declineby Sahu rate et al. than (2008 higher-velocity). While many lines are SN fit Iax. well with Fe ii, similar, there are key differences (also analyzed in detail by some had remained unidentified. We find that Fe i significantly 11 Foley et al. 2013). At all epochs the velocities of SN 2008A are improves the fit of the late-time SN 2002cx and SN 2005hk higher than those of SN 2005hk, similarly to the photospheric 2.5spectra. Environments There are strong lines and to the Rates red of the P Cygni profile spectra. SN 2002cx shows a stronger Ca near-IR triplet than of Na i λ5891 that can be seen in all three objects (SNe 2002cx, either SN 2005hk or SN 2008A. In the spectra of SN 2008A, 2005hk, and 2008A) that are well fit by Fe i with an excitation [Fe ii] λ7155 has an asymmetric profile. This may be caused SNtemperature Iax have near a 5000dramatically K. However, different there are also distribution [Fe i]lines of host galaxies than normal SN Ia. by contamination from another line; there is a [Co i]feature that match these features, making the identification of these lines at the wavelength in question, but no other [Co i]linesare In allambiguous. but a couple of cases (like SN 2008ge; Foley et al. 2010b), SN Iax are found in seen (including lines more easily excited), making this option star-forming,The velocity late-type structure of host the forbiddengalaxies lines (Figure relative 4). to The host galaxy distribution of SN Iax unlikely. the host galaxy is interesting. Figure 10 displays the [Ca ii] Sahu et al. (2008)identify[Feii] λ7389 in their spectra of is closestλλ7291, 7323 to those doublet of and SN [Fe IIPii] λ7155 or SN line referenced1991T/1999aa-like to the SN Ia (Foley et al. 2009; Valenti SN 2005hk. However, there is another strong feature, [Ni ii] et al.host-galaxy 2009; Perets(nucleus) et rest al. frame 2010). (as opposed Lyman the etrest al. frame (2013, of 2017) confirm and amplify this result λ7378, at almost the same wavelength. The feature near these the ejecta used elsewhere in this work). As the figure shows, the wavelengths is broader than the [Fe ii] λ7155 line, so we argue basedline velocitieson Hα areimaging largely consistent and integral-field over time, though spectroscopy the line of Iax locations and hosts: SN Iax that it is likely a blend of these two lines. There is a strong line mustwidths arise decrease from as athe relatively SNe evolve. Foley young et al. population. (2013)foundthat Qualitatively, based on SN Iax with high- that is near [Fe ii] λ8617 in SN 2008A that is not seen clearly the [Ca ii]and[Feii]featureswereshiftedinoppositedirections in SNe 2002cx or 2005hk, but is observed in the normal SN Ia resolutionrelative to theHubble host rest Space frame for Telescope the majority of like SNe SNIax, but 2008A, for SN 2012Z, and SN 2014dt, it seems as if SN Iax prefer the “outskirts” of their star-forming hosts, but this needs further quan- 8 Type Iax Supernovae 9 tification, given the selection bias against finding these fainter SN on a bright galaxy back- ground. SN Iax show no strong preference for high- or low-metallicity galaxies (Magee et al. 2017), though their explosion locations are more metal-poor than normal SN Ia (Lyman et al. 2017). The host reddening distribution of SN Iax is uncertain because of the difficulty in dis- entangling extinction from the intrinsic photometric diversity in the class, but most known SN Iax have low reddening, up to E(B −V) ' 0.5 mag for SN 2013en (Liu et al. 2015c). Again, selection biases work against finding heavily extinguished members of this already intrinsically faint class of supernovae. Because the luminosity function of SN Iax extends down to quite faint magnitudes, precisely estimating the rate of SN Iax is challenging. Foley et al. (2013) calculate the Iax +17 rate to be 31−13% of the SN Ia rate in a volume-limited sample. Consistent with this, Miller et al. (2017) found one SN Iax (iPTF16fnm; M ' −15 mag) and 4 SN Ia in a volume- limited survey. The overall SN Iax rate is dominated by the lower luminosity objects; the rate of brighter SN Iax (comparable in luminosity to SN 2002cx or SN 2005hk) is likely to be between 2 and 10% of the SN Ia rate (Li et al. 2011b; Foley et al. 2013; Graur et al. 2017). SN Iax are the most numerous “peculiar” cousins to normal SN Ia.

2.6 Progenitors and Remnants

A major breakthrough in understanding SN Iax came with the discovery of the progenitor system of SN 2012Z (McCully et al. 2014a). Nature was kind: SN 2012Z exploded in the nearby galaxy NGC 1309, which was also the host of the normal type-Ia SN 2002fk, a calibrator for the SN distance scale to measure H0 (Riess et al. 2011). As such, extremely deep, multi-epoch HST imaging of NGC 1309 (to observe Cepheids) covered the location of SN 2012Z before its explosion (Figure 5). This deep, high-resolution pre-explosion imaging revealed a source coincident with SN 2012Z, the first time a progenitor system has been discovered for a thermonuclear supernova. The detected source is luminous and blue (MV ' −5.3 mag; B −V ' −0.1 mag), and McCully et al. (2014a) argue that it is a helium-star companion (donor) to an exploding white dwarf. Further images taken after the supernova faded reveal the source has not disappeared, consistent with the compan- ion scenario (McCully et al., in preparation). This discovery marks a critical contrast for SN Iax compared to normal SN Ia: no such progenitor system has ever been seen for a SN Ia! Note, however, there are only two normal SN Ia, SN 2011fe (Li et al. 2011a) and SN 2014J (Kelly et al. 2014), with pre-explosion limits that are as deep as the data for SN 2012Z. Foley et al. (2014) detect a luminous (MI ' −5.4 mag), red (R − I ' 1.6 ± 0.6 mag) source consistent with the location of SN 2008ha in HST imaging taken about 4 years after the supernova explosion. At these epochs the SN ejecta flux should have faded well below this level, so Foley et al. (2014) suggest they may be observing a companion star to the supernova, or else a luminous “remnant” of the explosion. 10 Saurabh W. Jha

Fig. 4 Sloan Digital Sky Survey images centered at the locations of 25 SN Iax in the SDSS foot- print. Note the preponderance of late-type, star-forming host galaxies. Each image is 100 arcsec on a side. Type Iax Supernovae 11

Fig. 5 Discovery of the only known progenitor system for a white-dwarf (thermonuclear) super- nova. The left panel shows the deep Hubble Heritage image of NGC 1309 (made from observations to detect and monitor Cepheids) taken in 2005 and 2006 with HST ACS. Upper panels (b) and (c) zoom in on the region where the type Iax SN 2012Z was to explode, revealing the luminous, blue progenitor system S1, believed to be a helium star donor to an exploding white dwarf. Lower pan- els (d) and (e) show the region after the SN explosion, allowing for a precise measurement of its position with HST/WFC3, coincident with the progenitor. This figure is adapted from McCully et al. (2014a).

3 Models

In this section I discuss potential models for SN Iax, starting from general considera- tions and observational constraints, and then moving to specific scenarios that have been proposed in the literature.

3.1 SN Iax are likely thermonuclear, white dwarf supernovae

SN Iax show undeniable spectroscopic similarities to normal SN Ia, particularly near and in the few months after maximum light, with lower velocities being the primary distin- guishing factor. Given that spectroscopic observations probe different layers of supernova ejecta over time2, a natural starting point would be to suggest that SN Iax and SN Ia share commonalities in their progenitors and explosions. Conversely, the primarily star-forming environments of SN Iax may point to a core- collapse, massive-star supernova origin. Indeed Valenti et al. (2009) argue that SN 2008ha has similarities to some faint core-collapse SN, and suggest a “fallback” massive-star supernova (Moriya et al. 2010) or an electron-capture supernovae (Pumo et al. 2009) could explain SN 2008ha, though not without some difficulties (Eldridge et al. 2013). This core-collapse model does not seem to be able to account for higher luminosity SN Iax,

2 “Spectrum is truth.” –R. P. Kirshner 12 Saurabh W. Jha and so would require objects like SN 2008ha, 2010ae, and 2010el to be distinct from other SN Iax. The preponderance of evidence suggests that SN Iax are thermonuclear explosions of white dwarfs. Spectra near maximum light show carbon, intermediate mass elements like sulfur and silicon (weakly), and strong features of iron group elements. The Co II infrared lines (Stritzinger et al. 2015; Tomasella et al. 2016) clearly point to a fraternity with normal SN Ia. Iron lines are seen at a wide range of velocities, implying efficient mixing of fusion products rather than a highly layered structure. The lack of star formation or luminous massive stars in pre-explosion imaging of SN 2008ge (Foley et al. 2010b) and SN 2014dt (Foley et al. 2015), and the non-disappearance of the progenitor system flux in SN 2012Z (McCully et al. 2014a) argue against the explosion of massive luminous stars. The peak luminosities of SN Iax compared to their late-time photometry and modeling suggest a 56Ni → 56Co → 56Fe radioactively powered light curve (McCully et al. 2014b). Moreover, even fainter, lower velocity objects like SN 2008ha and 2010ae seem to connect to brighter SN Iax. Foley et al. (2010a) show an early spectrum of SN 2008ha that features sulfur lines similar to those seen in normal SN Ia. Stritzinger et al. (2014) show that SN 2010ae has strong Co II lines like other SN Iax and normal SN Ia, and its late-time spectrum is very similar to the SN Iax prototype, SN 2002cx.

3.2 General observational constraints

The environments of SN Iax do indeed suggest they come from a young population, but this does not require a core-collapse origin. SN 1991T-like SN Ia have a quite similar host galaxy preference (Foley et al. 2009; Perets et al. 2010) to SN Iax, and those SN Ia are still nearly-universally construed as white dwarf explosions. In fact, the problem can be turned around: the requirement for a young population favors certain binary systems that can produce and explode white dwarfs quickly. HST observations of nearby stars in the field of SN 2012Z yield ages of 10–50 Myr (McCully et al. 2014a), while for SN 2008ha the nearby population is ∼< 100 Myr (Foley et al. 2014). Though young, these are still significantly older than the expected lifetimes of, for example, Wolf-Rayet stars that might yield hydrogen-poor core-collapse supernovae (Groh et al. 2013). Short evolutionary times in a binary system suggest that SN Iax arise from more mas- sive white dwarfs. If the explosions are occurring at the Chandrasekhar mass (MCh), as supported by other evidence (see below), then the quickest binary channel for a car- bon/oxygen white dwarf (C/O WD) is to accrete helium from a He star companion (Hachisu et al. 1999; Postnov and Yungelson 2014). The stable mass transfer rate can be high for helium accretion, and Claeys et al. (2014) show that this channel dominates the thermonuclear SN rate between 40 Myr (with no younger systems) and 200 Myr (above which double-degenerate and hydrogen-accreting single degenerate systems dominate). This has been seen in several binary population synthesis studies; these generally find no problem for the He star + C/O WD channel to produce the required fraction of SN Iax rel- ative to normal SN Ia, but the total rates may not quite reach the observed values (Ruiter Type Iax Supernovae 13 et al. 2009, 2011; Wang et al. 2009a,b; Meng and Yang 2010; Piersanti et al. 2014; Liu et al. 2015a). Of course the WD+He star channel is in good accord with observations of SN 2012Z, for which the putative companion is consistent with a helium star. This scenario may also explain the helium observed in SN 2004cs and SN 2007J. In fact, Foley et al. (2013) predicted this type of system for SN Iax before the SN 2012Z progenitor discovery. Liu et al. (2010) present a model (though intended to explain a different kind of system) that starts with a 7 M + 4 M close binary that undergoes two phases of mass transfer and common envelope evolution and results in a 1 M C/O WD + 2 M He star. As the He star evolves, it can again fill its Roche lobe and begin stable mass transfer onto the white dwarf (at a high accretion rate, ∼ 10−5 M yr−1, for instance) that could lead to the SN Iax.

The low ejecta velocities of SN Iax imply lower kinetic energy compared to SN Ia (un- der the reasonable assumption that SN Iax do not have significantly higher ejecta mass). Their lower luminosity also points in this direction (though that depends specifically on how much 56Ni is synthesized). Moreover, there is a much larger variation in the kinetic energy in SN Iax. All of this points towards a deflagration (subsonic) explosion; pure de- flagration models of MCh C/O WDs show convoluted structure from the turbulent flame propagation (e.g., Gamezo et al. 2003; Townsley et al. 2007) that can produce a wide range of explosion energies. This contrasts with MCh detonation scenarios that lead to more uniform energy release and a layered structure (Gamezo et al. 2004, 2005). Deflagrations are thought to naturally occur in the onset of runaway carbon burning for MCh C/O WD progenitors (e.g., Zingale et al. 2011; Nonaka et al. 2012, and refer- ences therein). Indeed for years, a leading model to match observations of normal SN Ia has been the delayed detonation scenario, in which an initial deflagration transitions to a detonation after the WD has expanded to lower density (Khokhlov 1991; Gamezo et al. 2005). In SN Iax, one posits that this transition does not occur. Such a model matches many of the observations (Röpke et al. 2007; Ma et al. 2013): lower yet varied energy re- lease, well-mixed composition (this inhibits the secondary near-infrared maximum; Kasen 2006), unusual velocity structure (e.g., Ca interior of Fe, something not seen in normal SN Ia; Foley et al. 2013), and the strength of [Ni II] in late-time spectra implying sta- ble nickel, preferentially produced at high density (near MCh; Maeda and Terada 2016). Early on, Branch et al. (2004) suggested a pure deflagration model to match spectra of SN 2002cx. In these models, there is a prediction of significant unburned material (C/O), which may be confirmed: carbon features are nearly ubiquitous in SN Iax, more so than SN Ia (Foley et al. 2013), and there are hints of low-velocity oxygen in some late-time spectra of SN Iax (Jha et al. 2006). There remain challenges to a pure-deflagration model of SN Iax; for example, Fisher and Jumper (2015) find generically too-weak deflagra- tions for MCh progenitors because of non-central, buoyancy driven ignition. Furthermore, asymmetry in pure deflagrations may lead to polarization signatures in excess of what is observed for SN 2005hk (Chornock et al. 2006; Maund et al. 2010; Meng et al. 2017). The low-velocity late-time features, which have final velocities that can be much less than the typical escape velocity from the surface of a white dwarf, suggest that perhaps 14 Saurabh W. Jha not all of the material was unbound. Similarly, the range of energies that deflagrations produce could include outcomes less than the binding energy. Thus, in this model, though SN Iax would be MCh explosions, the ejecta mass could be significantly less, and the explosions could leave behind a bound remnant. The luminous remnant would be super- Eddington, and could be expected to drive an optically thick wind. This scenario might explain the high densities inferred at late times in SN Iax spectra and why they do not become completely nebular (n ∼ 109 cm−3; McCully et al. 2014b). Different amounts of ejecta versus wind material could furthermore explain the diversity in line widths and strengths between permitted and forbidden lines at late times (Foley et al. 2016). A wind “photosphere” may also explain the luminous, red source seen in post-explosion observa- tions of SN 2008ha (Foley et al. 2014) and perhaps is involved in producing the infrared excess in SN 2014dt (Fox et al. 2016). Shen and Schwab (2017) present an intriguing model for these radioactively powered winds and show good agreement with SN Iax ob- servations.

3.3 Specific models for SN Iax

In some sense, the pure-deflagration MCh model is a “failed” SN Ia; indeed the primary reason these models were first explored was to explain normal SN Ia. However, first in 1-d and later in 3-d, it was shown these were unlikely to match observations of SN Ia (see references listed above). After the identification and rapid observational growth in the SN Iax class, it became clear that these “failures” might be successfully applied to SN Iax. Jordan et al. (2012) and Kromer et al. (2013) presented 3-d simulations of pure- deflagration MCh C/O WD explosions that did not fully unbind the star (Figure 6), and connected these to observed properties of SN Iax, as described above. Fink et al. (2014) explore a range of initial conditions in this scenario varying the number and location of ignition spots and find that they can yield a wide range of total energy and ejecta masses from 0.1 M to MCh (i.e., complete disruption). This provides a natural way to explain the diversity observed in SN Iax, and Magee et al. (2016) show some success in matching a particular realization of this explosion model to observations of SN 2015H. Long et al. (2014) also show qualitatively similar results in being able to reproduce SN Iax proper- ties, but find the opposite sense in the relation between energy yield and the number of ignition spots, with high luminosity objects resulting from fewer ignition points. A new wrinkle on the pure-deflagration MCh scenario is based the idea of “hybrid” C/O/Ne white dwarfs (Denissenkov et al. 2013; Chen et al. 2014). Uncertainties in con- vective mixing and carbon-flame quenching may allow for central carbon to exist in WDs as massive as 1.3 M , rather than the traditional ∼1.05 M boundary between C/O and

O/Ne white dwarfs (Nomoto 1984; Doherty et al. 2017). Such massive white dwarfs come from more massive progenitors and require less accreted material to reach MCh: both of these lead to shorter delay time between formation and explosion (perhaps as low as 30 Myr), and thus could be particularly relevant to SN Iax (Meng and Podsiadlowski 2014; Type Iax Supernovae 15 2002cx-like SNe from deflagrations with bound remnants4 M.3 Kromer et al.

Tableadapted 2. Yields from of select Kromer species et for al. model (2013) N5def. hydrodynamic simulations (by which point homologous ex- pansion is a good approximation) to a 503 Cartesian grid 8 Bound remnant Ejecta and followed the propagation of 10 photon packets for 111 (M )(M) logarithmically-spaced time steps between 2 and 120d after explosion. To speed up the initial phase, a grey approxima- Total 1.028 0.372 tion, as discussed by Kromer & Sim (2009), was used in C0.4220.043 optically thick cells, and the initial 10 time steps (i.e. the O0.4840.060 Ne 0.054 0.005 initial three days after the explosion) were treated in local Mg 0.004 0.013 thermodynamic equilibrium (LTE). For our simulation we Si 0.015 0.025 used the ‘big_gf-4’ atomic data set of Kromer & Sim (2009) 6 S0.0040.009 with a total of 8.2 10 bound–bound transitions. The ⇠ ⇥ Ca 0.0003 0.001 resulting broad-band light curves and a spectral time series Fe 0.004 0.031 are shown in Figs. 4 and 5 and compared to SN 2005hk as Ni 0.025 0.187 a proxy for SN 2002cx-like objects. 56Ni 0.022 0.158

(Travaglio et al. 2004; Seitenzahl et al. 2010). A compilation Figure 2. Snapshots of the hydrodynamic evolution of our model N5def. Shown are volume renderings of the mean atomicof number the masses of important species is given in Table 2. To de- 3.1 Broad-band light curves calculated from the reduced set of species in the hydrodynamic simulation (see colour bar). To allow a view to the central parttermine of the the mass that stays bound in the remnant and that Fig.ejecta, a 6 wedgeSnapshots was carved out from of the a front partial of the ejecta. 3-d (i) At deflagration 0.75s a one-sided deflagration of a plumeChandrasekhar-mass rises towards the WD surface carbon-oxygen white As reported by Jha et al. (2006) and Phillips et al. (2007), of the unbound ejecta, we calculated the asymptotic specific and fragments due to Rayleigh-Taylor and Kelvin-Helmholtz instabilities. (ii) At 1.5s the expansion of the WD quenches the burning SN 2002cx-like objects are characterised by distinct spectral dwarf that leaves a bound remnant. Note the lack of burned materialkinetic in energy the✏kin white,a = ✏kin,f + dwarf✏grav,f forcore all tracer particles. features and light curves. In the following, we will investi- and the explosion ashes wrap around the unburned core. (iii) Finally, at 100 s the unburned core is completely engulfed by the explosion 2 ashes which are ejected into space. The small triads at the bottom right corner of each panel indicate the scaling at the originHere, of each✏kin,f = vf /2and✏grav,f are the specific kinetic and gate to which extent our model is capable to reproduce these inplot: the from left middle to right the panel legs of the (1.5 triads represent sec into 1000 km, the 4000 km explosion). and 500 000 km, respectively. While the thermonucleargravitational runaway binding energies in at thet =100s,i.e.attheendof outer features. From the broad-band light curves in Fig. 4 it is our simulation, respectively. Only for positive values of ✏kin,a immediately obvious that our model matches the low peak parts of the white dwarf surrounds and engulfs the core at later times,a theparticle explosion will be able to escapeenergy the gravitational is not potential. luminosities of 2002cx-like SNe quite well. This, of course, is Table 1. Position of the ignition kernels of model N5def. Given The hydrodynamic evolution of our model isOtherwise, shown it will stay bound. not so surprising, since we picked a model with a 56Ni mass sufficientare the x, y and toz coordinates unbind of the the centre star. of the The individual modelin predicts Fig. 2. Sincethe a deflagration composition flame cannot of burn the againstFinally, ejecta we and mapped remnant the unbound in tracer the particles to close to that observationally derived for SN 2005hk. How- ignition kernels and their distance d to the centre of the WD. the density gradient, our asymmetric ignition configurationa Cartesian grid to reconstruct the chemical composition ever, we do not only get the correct peak luminosities but table shown. This figure is adapted from Kromer et al. (2013). 2 leads to the formation of a one-sided deflagration plumeof the that explosion ejecta in the asymptotic velocity space. also approximately the right colours at maximum. Moreover, # xyzd fragments due to Rayleigh-Taylor and Kelvin-HelmholtzTo this in- end, we used an SPH like algorithm as described our model naturally explains the absence of secondary max- (in km) stabilities. Once the deflagration front reaches the outerin Kromer lay- et al. (2010). The resulting ejecta structure is ima in the NIR bands. This is a consequence of the turbulent ers of the WD, the burning quenches due to the expansionshown of in Fig. 3. In contrast to the simulation by Jordan 165.5 15.524.071.5 burning, which leads to an almost completely mixed ejecta et al. (2012b), our model shows no pronounced large-scale 238.6 22.767.380.9 the WD and the ashes wrap around the still unburned core structure with a roughly constant iron-group element (IGE) Wang et313 al..08 2014)..215 Furthermore,.121.6 a rangeuntil they finally of masses engulf it completely. for the A similar C/O evolutioncomposition core of could asymmetry. play There a are, role however, in small scale mass fraction at all velocities (see Figs. 2 and 3). Kasen 4 5.0 51.3 2.651.7 the deflagration flame was already described for single-spotanisotropies due to the turbulent evolution of the deflagra- (2006) has shown that SN ejecta with such a homogenized 1 SN Iax diversity55.62. (Denissenkov90.66.3 et al.o 2015;↵-centre ignitions Kromer by e.g. etPlewa, al. Calder, 2015). & Lamb Bravotion (2004) flame. et The al. density (2016) jump at aboutsuggest 2000kms is pro- composition are not expected to show secondary maxima in and R¨opke, Woosley, & Hillebrandt (2007). Whileduced Plewa during the strongest pulsation of the bound core (at the NIR. et al. (2004) found an ensuing detonation to be triggeredt 12s): After significant initial expansion due to burn- that even delayed detonations of hybrid white dwarfs could explain⇠ SN Iax, though Will- Other peculiarities of 2002cx-like SNe are their slowly when the ashes collide on the far side of the star (seeing, also the mostly unburned inner parts of the ejecta start declining light curves in R and redder bands and conse- lowed the flame evolution up to 100s after ignition when the Seitenzahl et al. 2009a), we – similarly to R¨opke et al.falling (2007) back inwards. This leads to the formation of a central quently also in UVOIR bolometric. In this respect our model coxejecta reach et al. homologous (2016) expansion. find Tothe this end detonation we used the phase makes these explosions more similar to normal and Jordan et al. (2012b) – do not find high enoughcompact densi- core, which, after maximum compression, starts to has some shortcomings. While it nicely reproduces the post- LEAFS code, a three-dimensional finite-volume discretiza- ties and temperatures for such a detonation to occurexpand due to again. At the same time, outer layers are still in- maximum decline in the U, B and V bands, our post- SNtion of Ia. the reactive Liu Euler et al. equations (2015b) which is based argue on the that from a binary evolution point of view the compan- a significant expansion of the WD during the deflagrationfalling and an accretion shock forms at the edge of the dense maximum light curve evolution is too fast particularly in R PROMETHEUS implementation (Fryxell, M¨uller, & Arnett phase. core (cf. Bravo & Garc´ıa-Senz 2009). In our model, maxi- and redder bands, but also in UVOIR bolometric. This could ion1989) of star the ‘piecewise to SN parabolic 2012Z method’ is (PPM) best by Colella explained in a system with a C/O/Ne white dwarf primary. 9 Since only a moderate fraction of the core of themum WD temperatures in the accretion shock just reach 10 K be a consequence of our low ejecta mass of only 0.37M , & Woodward (1984). Deflagration fronts are modelled as dis- 5 3 at densities of 5 10 gcm , therefore no detonation will which is not able to trap enough -radiation after maximum. Dohertycontinuities between et al. carbon–oxygen (2017) fuel note and nuclear some ash, concernsis burned, the about nuclear energy the release viability in our simulation of the hybrid⇥ white dwarf 50 be triggered (for critical conditions see R¨opke et al. 2007 We note, however, that the synthetic light curves in Fig. 4 and their propagation is tracked with a level-set scheme (Enuc =4.9 10 erg) is less than the binding energy of ⇥ 50 and Seitenzahl et al. 2009b). are only powered by radionuclides in the ejecta. Thereby scenario,(Reinecke et al. 1999; including Osher & Sethian whether 1988; Smiljanovski, the carbonthe initial flameWD (Ebind =5 can.2 10 beerg). successfully Nevertheless, about quenched (Lecoanet ⇥ we neglect any possible influence from the bound remnant, Moser, & Klein 1997). All material crossed by these fronts is 0.37 M of the WD are accelerated to escape velocity and apu↵ed-up stellar object heated by the explosion, and also etconverted al. 2016)to nuclear ash or with if a thecomposition central and energy C/O re- regionejected into can the ambient survive medium without with a kinetic being energy of mixed into the O/Ne 50 from the 56Ni that falls back onto the remnant (see Table 2). lease depending on fuel density. Composition and energy re- 1.34 10 erg. The remainder of the mass of the initial3SYNTHETICOBSERVABLES WD ⇥ After maximum light, when the ejecta start to become opti- layerlease are interpolated above from (Brooks tables which et have al. been 2017). calibrated is left behind and forms a bound remnant.Similarfindings To obtain synthetic spectra and light curves for our model, cally thin, the bound remnant will be exposed and it could using our full 384-isotope network. After a very short phase were already reported in 2D (e.g. Livne, Asida, & H¨oflich we used the time-dependent 3D Monte Carlo radiative trans- kick in as an additional luminosity source. We will discuss of laminarAnother burning followingclass ignition, of SN the propagation Iax models of 2005) explores and recently in the the context recent of a failed resurgence gravitationally- in sub-M double- fer code artis (Kromer & SimCh 2009; Sim 2007). For the ra- this intriguing possibility in Section 4.3. deflagrations is dominated by buoyancy- and shear-induced confined detonation (Jordan et al. 2012b). diative transfer simulation we mapped the abundance and However, looking at the overall shape of the early light instabilities and interactions with a complex turbulent flow To obtain detailed nucleosynthesis yields of the explo- detonation scenarios for normal SN Ia. In this case varying WDdensity mass structure at of the explosion unbound ejecta can at the end of the curves, an insucient trapping of radiation in the ejecta field. The unresolved acceleration of the flame due to turbu- sion, we performed a post-processing calculation with a 384 and thus too low ejecta mass seems to be more likely. With lence is accounted for by a sub-grid scale model (Schmidt, isotope network for 106 Lagrangian tracer particles which lead to diversity, and perhaps explain both prompt SN Ia and the full range of SN Iax a B-band rise time of 11.2d, our model evolves somewhat Niemeyer, & Hillebrandt 2006a; Schmidt et al. 2006b). Self- were passively advected during the hydrodynamic simula- 2 too fast compared to the rise time of SN 2005hk, for which (Wanggravity is dealt et with al. by a 2013; monopole gravity Zhou solver. et al. 2014;tion to Neunteufel record thermodynamic et trajectories al. 2016). of mass elementsFor Stritzinger unbound tracer particles, et al. the asymptotic (2015) velocity is deter- mined as va = 2✏ . (Phillips et al. 2007) find a value of 15 1d. kin,a ± c 2012 RAS, MNRAS 000,1–12 argue that the ejecta mass for SN 2012Z is consistent with MCh and advocatep a pulsational c 2012 RAS, MNRAS 000,1–12 delayed detonation model (Höflich et al. 1995) for bright SN Iax. Metzger (2012) and Fernández and Metzger (2013) explore the potential of SN Iax to result from white dwarf plus neutron star mergers.

4 Conclusion

Taken together, observations and theory point to a leading model that needs to be tested: a type Iax supernova results from a C/O or (hybrid C/O/Ne) white dwarf that accretes helium from a He-star companion, approaches the Chandrasekhar mass, and explodes as a deflagration that does not necessarily completely disrupt the star. It is quite possible, 16 Saurabh W. Jha even likely, that one or more aspects of this model is wrong, but it nonetheless gives observers something to directly test and modelers a general framework to explore and pick apart. Even within this model, there are important questions: is MCh always required? Is it always a deflagration? Does varying the ejecta/remnant mass explain the diversity? Does the WD + He-star channel always lead to a SN Iax? In addition to testing this and other models with a broad range of observations, we can look forward to some novel possibilities. For example, what should happen to the He-star companion of SN 2012Z (e.g., Shappee et al. 2013; Liu et al. 2013)? What might we expect to observe from SN Iax bound remnants and what happens to them? Will future extremely-large-telescopes be able to spectroscopically confirm that the companion to SN 2012Z (m ≈ 27.5) was actually a helium star? What are the broader impacts of our understanding of SN Iax? Is there a connection to systems like the Galactic “helium nova” V445 Pup, a near MCh white dwarf accreting from a helium star (Kato et al. 2008; Woudt et al. 2009)? Is it a SN Iax precursor, or is some parameter different (e.g., the accretion rate) that leads to the nova outcome? Is there a connection between SN Iax and 2002es-like SN (Ganeshalingam et al. 2012)? Those are found in older environments, but also have a detection of a likely single-degenerate progenitor (Cao et al. 2015). What is the relation of SN Iax the population of Ca-rich transients (Perets et al. 2010)? Are those C/O + He WD mergers? Of course, one of the key reasons why understanding SN Iax is important is the insight that gives us about normal SN Ia. But what is that insight telling us? Does it point to sub-MCh or double degenerate progenitors for SN Ia? Is some form of detonation required in SN Ia? Does the single degenerate channel always lead to peculiar supernovae? One factor in explaining why the SN Ia progenitor/explosion problem has been with us for decades is the vast array of possibilities to explode white dwarfs. Given the enormous effort that has gone into explaining normal SN Ia, it is astounding to think that we may have a better understanding of their peculiar cousins, SN Iax.

Cross-References

Combustion in Thermonuclear Supernova Explosions (Röpke 2017) Evolution of Accreting White Dwarfs to the Thermonuclear Runaway (Starrfield 2016) Explosion Physics of Thermonuclear Supernovae and Their Signatures (Hoeflich 2017) Light Curves of Type I Supernovae (Bersten and Mazzali 2017) Low- and Intermediate-Mass Stars (Karakas 2017) Nucleosynthesis in Thermonuclear Supernovae (Seitenzahl and Townsley 2017) Observational and Physical Classification of Supernovae (Gal-Yam 2017) Population Synthesis of Massive Close Binary Evolution (Eldridge 2017) Spectra of Supernovae During the Photospheric Phase (Sim 2017) Supernova Progenitors Observed with HST (Van Dyk 2016) The Extremes of Thermonuclear Supernovae (Taubenberger 2016) Type Ia Supernovae (Maguire 2016) Type Iax Supernovae 17

Unusual Supernovae and Alternative Power Sources (Kasen 2017)

Acknowledgements I thank Ryan Foley and Curtis McCully for our close collaboration working on SN Iax and their assistance with this manuscript. I am also grateful to Mark Sullivan, Daniela Graf, and Kerstin Beckert, for their seemingly inexhaustible patience. This work was supported in part by US National Science Foundation award 161545 and benefited greatly from discussions at the Munich Institute for Astro- and Particle Physics Scientific Program “The Physics of Super- novae” and associated Topical Workshop “Supernovae: The Outliers.” I dedicate this review to the memory of my friend and colleague, Weidong Li, who started it all.

References

Antilogus P, Garavini G, Gilles S, Pain R, Aldering G, Bailey S, Lee BC, Loken S, Nugent P, Perlmutter S, Scalzo R, Thomas RC, Wang L, Weaver B, Bonnaud C, Pecontal E, Blanc N, Bongard S, Copin Y, Gangler E, Sauge L, Smadja G, Kessler R, Baltay C, Rabinowitz D, Bauer A (2005) Type Determination for SN 2005cc. The Astronomer’s Telegram 502 Balam D (2017) Transient Classification Report for 2017-03-31. Transient Name Server Classification Report 381 Bersier D, Dennefeld M, Lyman J, Maund JR, Pastorello A, De Cia A, Benetti S, Inserra C, Smartt S, Smith K, Young D, Sullivan M, Taubenberger S, Valenti S, Fraser M, Yaron O, Gal-Yam A, Manulis I, Scalzo R, Yuan F, Childress M, Tucker B, Schmidt B, Knapic C, Smareglia R, Molinaro M, Baltay C, Ellman N, Hadjiyska E, McKinnon R, Rabinowitz D, Walker ES, Feindt U, Kowalski M, Nugent P, Wyrzykowski L (2013) PESSTO spectroscopic classification of optical transients. The Astronomer’s Telegram 5620 Bersten MC, Mazzali PA (2017) Light curves of type i supernovae. In: Alsabti AW, Mur- din P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1– 13, DOI 10.1007/978-3-319-20794-0_25-1 Bessell MS, Schmidt BP (2010) Supernova 2010el in NGC 1566. Central Bureau Elec- tronic Telegrams 2337 Blondin S, Calkins M (2008) Supernova 2008ae in IC 577. Central Bureau Electronic Telegrams 1250 Branch D, Baron E, Thomas RC, Kasen D, Li W, Filippenko AV (2004) Read- ing the Spectra of the Most Peculiar Type Ia Supernova 2002cx. Publications of the Astronomical Society of the Pacific 116:903–908, DOI 10.1086/425081, arXiv:astro-ph/0408130 Bravo E, Gil-Pons P, Gutiérrez JL, Doherty CL (2016) Explosion of white dwarfs har- boring hybrid CONe cores. Astronomy & Astrophysics 589:A38, DOI 10.1051/0004- 6361/201527861, arXiv:1603.00641 18 Saurabh W. Jha

Brooks J, Schwab J, Bildsten L, Quataert E, Paxton B (2017) Convection Destroys the Core/Mantle Structure in Hybrid C/O/Ne White Dwarfs. The Astrophysical Journal, Letters 834:L9, DOI 10.3847/2041-8213/834/2/L9, arXiv:1611.03061 Cao Y, Kulkarni SR, Howell DA, Gal-Yam A, Kasliwal MM, Valenti S, Johansson J, Amanullah R, Goobar A, Sollerman J, Taddia F, Horesh A, Sagiv I, Cenko SB, Nugent PE, Arcavi I, Surace J, Wo´zniakPR, Moody DI, Rebbapragada UD, Bue BD, Gehrels N (2015) A strong ultraviolet pulse from a newborn type Ia supernova. Nature 521:328– 331, DOI 10.1038/nature14440, arXiv:1505.05158 Cao Y, Kulkarni SR, Gal-Yam A, Papadogiannakis S, Nugent PE, Masci FJ, Bue BD (2016) SN2002es-like Supernovae from Different Viewing Angles. The Astrophysical Journal 832:86, DOI 10.3847/0004-637X/832/1/86, arXiv:1606.05655 Chen MC, Herwig F, Denissenkov PA, Paxton B (2014) The dependence of the evolution of Type Ia SN progenitors on the C-burning rate uncertainty and parameters of convec- tive boundary mixing. Monthly Notices of the Royal Astronomical Society 440:1274– 1280, DOI 10.1093/mnras/stu108, arXiv:1310.1898 Childress M, Scalzo R, Yuan F, Zhang B, Ruiter A, Seitenzahl I, Schmidt B, Tucker B (2014) Classification of seven supernovae with WiFeS. The Astronomer’s Telegram 6302 Chomiuk L, Soderberg AM, Chevalier RA, Bruzewski S, Foley RJ, Parrent J, Strader J, Badenes C, Fransson C, Kamble A, Margutti R, Rupen MP, Simon JD (2016) A Deep Search for Prompt Radio Emission from Thermonuclear Supernovae with the Very Large Array. The Astrophysical Journal 821:119, DOI 10.3847/0004-637X/821/2/119, arXiv:1510.07662 Chornock R, Filippenko AV, Branch D, Foley RJ, Jha S, Li W (2006) Spectropolarimetry of the Peculiar Type Ia Supernova 2005hk. Publications of the Astronomical Society of the Pacific 118:722–732, DOI 10.1086/504117, arXiv:astro-ph/0603083 Claeys JSW, Pols OR, Izzard RG, Vink J, Verbunt FWM (2014) Theoretical uncer- tainties of the Type Ia supernova rate. Astronomy & Astrophysics 563:A83, DOI 10.1051/0004-6361/201322714, arXiv:1401.2895 Copin Y, Gangler E, Pereira R, Rigault M, Smadja G, Aldering G, Birchall D, Childress M, Fakhouri H, Kim A, Nordin J, Nugent P, Perlmutter S, Runge K, Saunders C, Suzuki N, Thomas RC, Pecontal E, Buton C, Feindt U, Kerschhaggl M, Kowalski M, Benitez S, Hillebrandt W, Kromer M, Sasdelli M, Sternberg A, Taubenberger S, Baugh D, Chen J, Chotard N, Tao CWC, Fouchez D, Tilquin A, Hadjiyska E, Rabinowitz D, Baltay C, Ellman N, McKinnon R, Cellier-Holzem AEF, Canto A, Antilogus P, Bongard S, Pain R (2012) Spectroscopic classification of the peculiar Type Ia SN LSQ12fhs by the Nearby Supernova Factory II. The Astronomer’s Telegram 4476 Denissenkov PA, Herwig F, Truran JW, Paxton B (2013) The C-flame Quenching by Convective Boundary Mixing in Super-AGB Stars and the Formation of Hybrid C/O/Ne White Dwarfs and SN Progenitors. The Astrophysical Journal 772:37, DOI 10.1088/0004-637X/772/1/37, arXiv:1305.2649 Type Iax Supernovae 19

Denissenkov PA, Truran JW, Herwig F, Jones S, Paxton B, Nomoto K, Suzuki T, Toki H (2015) Hybrid C-O-Ne white dwarfs as progenitors of Type Ia supernovae: dependence on Urca process and mixing assumptions. Monthly Notices of the Royal Astronomical Society 447:2696–2705, DOI 10.1093/mnras/stu2589, arXiv:1407.0248 Dimitriadis G, Pursiainen M, Smith M, Cartier R, Firth R, Prajs S, Chen TW, Fraser M, Inserra C, Kankare E, Maguire K, Smartt SJ, Smith KW, Sullivan M, Valenti S, Yaron O, Young D, Manulis I, Wyrzykowski L, Tonry J, Stalder B, Denneau L, Heinze A, Sherstyuk A, Rest A, Wright D, Chambers K, Flewelling H, Huber M, Magnier E, Wa- ters C, Wainscoat RJ (2016) PESSTO spectroscopic classification of optical transients. The Astronomer’s Telegram 9660 Doherty CL, Gil-Pons P, Siess L, Lattanzio JC (2017) Super-AGB Stars and their role as Electron Capture Supernova progenitors. ArXiv e-prints arXiv:1703.06895 Eldridge J (2017) Population synthesis of massive close binary evolution. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1–22, DOI 10.1007/978-3-319-20794-0_125-1 Eldridge JJ, Fraser M, Smartt SJ, Maund JR, Crockett RM (2013) The death of massive stars - II. Observational constraints on the progenitors of Type Ibc supernovae. Monthly Notices of the Royal Astronomical Society 436:774–795, DOI 10.1093/mnras/stt1612, arXiv:1301.1975 Elias-Rosa N, Tartaglia L, Morales-Garoffolo LTA, Pastorello A, Botticella MT, Inserra C, Maguire K, Smartt S, Smith KW, Sullivan M, Valenti S, Yaron O, Young D, Manulis I, Baltay C, Ellman N, Hadjiyska E, McKinnon R, Rabinowitz D, Walker ES, Feindt U, Kowalski M, Nugent P, Wyrzykowski L (2014) PESSTO spectroscopic classification of optical transients. The Astronomer’s Telegram 6398 Fernández R, Metzger BD (2013) Nuclear Dominated Accretion Flows in Two Dimen- sions. I. Torus Evolution with Parametric Microphysics. The Astrophysical Journal 763:108, DOI 10.1088/0004-637X/763/2/108, arXiv:1209.2712 Filippenko AV, Foley RJ, Silverman JM, Chornock R, Li W, Blondin S, Matheson T (2007) Supernova 2007J in UGC 1778. Central Bureau Electronic Telegrams 926 Fink M, Kromer M, Seitenzahl IR, Ciaraldi-Schoolmann F, Röpke FK, Sim SA, Pak- mor R, Ruiter AJ, Hillebrandt W (2014) Three-dimensional pure deflagration models with nucleosynthesis and synthetic observables for Type Ia supernovae. Monthly No- tices of the Royal Astronomical Society 438:1762–1783, DOI 10.1093/mnras/stt2315, arXiv:1308.3257 Fisher R, Jumper K (2015) Single-degenerate Type Ia Supernovae Are Prefer- entially Overluminous. The Astrophysical Journal 805:150, DOI 10.1088/0004- 637X/805/2/150, arXiv:1504.00014 Foley RJ, Chornock R, Filippenko AV, Ganeshalingam M, Kirshner RP, Li W, Cenko SB, Challis PJ, Friedman AS, Modjaz M, Silverman JM, Wood-Vasey WM (2009) SN 2008ha: An Extremely Low Luminosity and Exceptionally Low Energy Super- nova. The Astronomical Journal 138:376–391, DOI 10.1088/0004-6256/138/2/376, arXiv:0902.2794 20 Saurabh W. Jha

Foley RJ, Brown PJ, Rest A, Challis PJ, Kirshner RP, Wood-Vasey WM (2010a) Early- and Late-Time Observations of SN 2008ha: Additional Constraints for the Progenitor and Explosion. The Astrophysical Journal, Letters 708:L61–L65, DOI 10.1088/2041- 8205/708/1/L61, arXiv:0912.0732 Foley RJ, Rest A, Stritzinger M, Pignata G, Anderson JP, Hamuy M, Morrell NI, Phillips MM, Salgado F (2010b) On the Progenitor and Supernova of the SN 2002cx-like Supernova 2008ge. The Astronomical Journal 140:1321–1328, DOI 10.1088/0004- 6256/140/5/1321, arXiv:1008.0635 Foley RJ, Challis PJ, Chornock R, Ganeshalingam M, Li W, Marion GH, Morrell NI, Pignata G, Stritzinger MD, Silverman JM, Wang X, Anderson JP, Filippenko AV, Freedman WL, Hamuy M, Jha SW, Kirshner RP, McCully C, Persson SE, Phillips MM, Reichart DE, Soderberg AM (2013) Type Iax Supernovae: A New Class of Stel- lar Explosion. The Astrophysical Journal 767:57, DOI 10.1088/0004-637X/767/1/57, arXiv:1212.2209 Foley RJ, McCully C, Jha SW, Bildsten L, Fong Wf, Narayan G, Rest A, Stritzinger MD (2014) Possible Detection of the Stellar Donor or Remnant for the Type Iax Super- nova 2008ha. The Astrophysical Journal 792:29, DOI 10.1088/0004-637X/792/1/29, arXiv:1408.1091 Foley RJ, Van Dyk SD, Jha SW, Clubb KI, Filippenko AV, Mauerhan JC, Miller AA, Smith N (2015) On the Progenitor System of the Type Iax Supernova 2014dt in M61. The Astrophysical Journal, Letters 798:L37, DOI 10.1088/2041-8205/798/2/L37, arXiv:1412.1088 Foley RJ, Jha SW, Pan YC, Zheng WK, Bildsten L, Filippenko AV, Kasen D (2016) Late- time spectroscopy of Type Iax Supernovae. Monthly Notices of the Royal Astronomical Society 461:433–457, DOI 10.1093/mnras/stw1320, arXiv:1601.05955 Fox OD, Johansson J, Kasliwal M, Andrews J, Bally J, Bond HE, Boyer ML, Gehrz RD, Helou G, Hsiao EY, Masci FJ, Parthasarathy M, Smith N, Tinyanont S, Van Dyk SD (2016) An Excess of Mid-infrared Emission from the Type Iax SN 2014dt. The Astrophysical Journal, Letters 816:L13, DOI 10.3847/2041-8205/816/1/L13, arXiv:1510.08070 Gal-Yam A (2017) Observational and physical classification of supernovae. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1–43, DOI 10.1007/978-3-319-20794-0_35-1, arXiv:1611.09353 Gamezo VN, Khokhlov AM, Oran ES, Chtchelkanova AY, Rosenberg RO (2003) Thermonuclear Supernovae: Simulations of the Deflagration Stage and Their Implications. Science 299:77–81, DOI 10.1126/science.299.5603.77, arXiv:astro-ph/0212054 Gamezo VN, Khokhlov AM, Oran ES (2004) Deflagrations and Detonations in Thermonuclear Supernovae. Physical Review Letters 92(21):211,102–+, DOI 10.1103/PhysRevLett.92.211102, arXiv:astro-ph/0406101 Gamezo VN, Khokhlov AM, Oran ES (2005) Three-dimensional Delayed-Detonation Model of Type Ia Supernovae. The Astrophysical Journal 623:337–346, DOI Type Iax Supernovae 21

10.1086/428767, arXiv:astro-ph/0409598 Ganeshalingam M, Li W, Filippenko AV, Silverman JM, Chornock R, Foley RJ, Matheson T, Kirshner RP, Milne P, Calkins M, Shen KJ (2012) The Low-velocity, Rapidly Fading Type Ia Supernova 2002es. The Astrophysical Journal 751:142, DOI 10.1088/0004- 637X/751/2/142, arXiv:1202.3140 González-Gaitán S, Hsiao EY, Pignata G, Förster F, Gutiérrez CP, Bufano F, Galbany L, Folatelli G, Phillips MM, Hamuy M, Anderson JP, de Jaeger T (2014) Defining Photometric Peculiar Type Ia Supernovae. The Astrophysical Journal 795:142, DOI 10.1088/0004-637X/795/2/142, arXiv:1409.4811 Graur O, Bianco FB, Modjaz M, Shivvers I, Filippenko AV, Li W, Smith N (2017) LOSS Revisited. II. The Relative Rates of Different Types of Supernovae Vary between Low- and High-mass Galaxies. The Astrophysical Journal 837:121, DOI 10.3847/1538- 4357/aa5eb7, arXiv:1609.02923 Groh JH, Meynet G, Georgy C, Ekström S (2013) Fundamental properties of core- collapse supernova and GRB progenitors: predicting the look of massive stars before death. Astronomy & Astrophysics 558:A131, DOI 10.1051/0004-6361/201321906, arXiv:1308.4681 Guillochon J, Parrent J, Kelley LZ, Margutti R (2017) An Open Catalog for Su- pernova Data. The Astrophysical Journal 835:64, DOI 10.3847/1538-4357/835/1/64, arXiv:1605.01054 Hachisu I, Kato M, Nomoto K, Umeda H (1999) A New Evolutionary Path to Type IA Su- pernovae: A Helium-rich Supersoft X-Ray Source Channel. The Astrophysical Journal 519:314–323, DOI 10.1086/307370, arXiv:astro-ph/9902303 Harmanen J, Dennefeld M, Mattila S, Kangas T, Maguire K, Galbany L, Inserra C, Kankare E, Manulis I, Smartt SJ, Smith KW, Sullivan M, Valenti S, Yaron O, Young D, Wright D, Chambers K, Flewelling H, Huber M, Magnier E, Tonry J, Waters C, Wainscoat RJ (2015) PESSTO spectroscopic classification of optical transients. The Astronomer’s Telegram 8264 Hoeflich P (2017) Explosion physics of thermonuclear supernovae and their signatures. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Pub- lishing, Cham, pp 1–34, DOI 10.1007/978-3-319-20794-0_56-1 Höflich P, Khokhlov AM, Wheeler JC (1995) Delayed detonation models for normal and subluminous type IA sueprnovae: Absolute brightness, light curves, and molecule for- mation. The Astrophysical Journal 444:831–847, DOI 10.1086/175656 Hsiao EY, Marion GH, Marples P, Diamond T, Phillips MM, Morrell N, Stritzinger MD, Contreras C, Kirshner RP (2013a) PSN J01462790-5840238 is a probable SN Iax, Clas- sification by FIRE NIR spectrum. The Astronomer’s Telegram 5612 Hsiao EY, Morrell N, Phillips MM, Marion GH, Diamond T, Stritzinger MD, Contreras C, Kirshner RP, Dennefeld M, Bersier D, Lyman J, Maund JR, Pastorello A, De Cia A, Benetti S, Inserra C, Smartt S, Smith K, Young D, Sullivan M, Taubenberger S, Valenti S, Fraser M, Yaron O, Gal-Yam A (2013b) Supernova 2013gr in ESO 114-G7 = Psn J01462790-5840238. Central Bureau Electronic Telegrams 3733 22 Saurabh W. Jha

Jha S, Branch D, Chornock R, Foley RJ, Li W, Swift BJ, Casebeer D, Filippenko AV (2006) Late-Time Spectroscopy of SN 2002cx: The Prototype of a New Subclass of Type Ia Supernovae. The Astronomical Journal 132:189–196, DOI 10.1086/504599, arXiv:astro-ph/0602250 Jha SW, McCully C, Foley RJ, Cenko SB, Zheng W, Clubb KI, Shivvers I, Filippenko AV, Tucker BE, Garnavich PM (2013) SN 2013dh in NGC 5936 is probably a type Iax supernova. The Astronomer’s Telegram 5143 Jordan GC IV, Perets HB, Fisher RT, van Rossum DR (2012) Failed-detonation Super- novae: Subluminous Low-velocity Ia Supernovae and their Kicked Remnant White Dwarfs with Iron-rich Cores. The Astrophysical Journal, Letters 761:L23, DOI 10.1088/2041-8205/761/2/L23, arXiv:1208.5069 Karakas AI (2017) Low- and intermediate-mass stars. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1–21, DOI 10.1007/978-3-319-20794-0_117-1 Kasen D (2006) Secondary Maximum in the Near-Infrared Light Curves of Type Ia Supernovae. The Astrophysical Journal 649:939–953, DOI 10.1086/506588, arXiv:astro-ph/0606449 Kasen D (2017) Unusual supernovae and alternative power sources. In: Alsabti AW, Mur- din P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1– 27, DOI 10.1007/978-3-319-20794-0_32-1 Kato M, Hachisu I, Kiyota S, Saio H (2008) Helium Nova on a Very Massive White Dwarf: A Revised Light-Curve Model of V445 Puppis (2000). The Astrophysical Jour- nal 684:1366–1373, DOI 10.1086/590329, arXiv:0805.2540 Kelly PL, Fox OD, Filippenko AV, Cenko SB, Prato L, Schaefer G, Shen KJ, Zheng W, Graham ML, Tucker BE (2014) Constraints on the Progenitor System of the Type Ia Supernova 2014J from Pre-explosion Hubble Space Telescope Imaging. The Astro- physical Journal 790:3, DOI 10.1088/0004-637X/790/1/3, arXiv:1403.4250 Khokhlov AM (1991) Delayed detonation model for type IA supernovae. Astronomy & Astrophysics 245:114–128 Kromer M, Fink M, Stanishev V, Taubenberger S, Ciaraldi-Schoolman F, Pakmor R, Röpke FK, Ruiter AJ, Seitenzahl IR, Sim SA, Blanc G, Elias-Rosa N, Hillebrandt W (2013) 3D deflagration simulations leaving bound remnants: a model for 2002cx-like Type Ia supernovae. Monthly Notices of the Royal Astronomical Society 429:2287– 2297, DOI 10.1093/mnras/sts498, arXiv:1210.5243 Kromer M, Ohlmann ST, Pakmor R, Ruiter AJ, Hillebrandt W, Marquardt KS, Röpke FK, Seitenzahl IR, Sim SA, Taubenberger S (2015) Deflagrations in hybrid CONe white dwarfs: a route to explain the faint Type Iax supernova 2008ha. Monthly No- tices of the Royal Astronomical Society 450:3045–3053, DOI 10.1093/mnras/stv886, arXiv:1503.04292 Le Guillou L, Fleury M, Leget PF, Balland C, Baumont S, Pastorello A, Benetti S, Inserra C, Smartt S, Smith K, Young D, Sullivan M, Taubenberger S, Valenti S, Fraser M, Yaron O, Manulis I, Gal-Yam A, Knapic C, Smareglia R, Molinaro M, Wyrzykowski Type Iax Supernovae 23

L (2013) Pessto spectroscopic classification of optical transients. The Astronomer’s Telegram 5689 Lecoanet D, Schwab J, Quataert E, Bildsten L, Timmes FX, Burns KJ, Vasil GM, Oishi JS, Brown BP (2016) Turbulent Chemical Diffusion in Convectively Bounded Car- bon Flames. The Astrophysical Journal 832:71, DOI 10.3847/0004-637X/832/1/71, arXiv:1603.08921 Li W, Filippenko AV, Treffers RR, Riess AG, Hu J, Qiu Y (2001) A High Intrinsic Pe- culiarity Rate among Type IA Supernovae. The Astrophysical Journal 546:734–743, DOI 10.1086/318299, arXiv:astro-ph/0006292 Li W, Filippenko AV, Chornock R, Berger E, Berlind P, Calkins ML, Challis P, Fass- nacht C, Jha S, Kirshner RP, Matheson T, Sargent WLW, Simcoe RA, Smith GH, Squires G (2003) SN 2002cx: The Most Peculiar Known Type Ia Supernova. Publi- cations of the Astronomical Society of the Pacific 115:453–473, DOI 10.1086/374200, arXiv:astro-ph/0301428 Li W, Bloom JS, Podsiadlowski P, Miller AA, Cenko SB, Jha SW, Sullivan M, Howell DA, Nugent PE, Butler NR, Ofek EO, Kasliwal MM, Richards JW, Stockton A, Shih HY, Bildsten L, Shara MM, Bibby J, Filippenko AV, Ganeshalingam M, Silverman JM, Kulkarni SR, Law NM, Poznanski D, Quimby RM, McCully C, Patel B, Maguire K, Shen KJ (2011a) Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe. Nature 480:348–350, DOI 10.1038/nature10646, arXiv:1109.1593 Li W, Leaman J, Chornock R, Filippenko AV, Poznanski D, Ganeshalingam M, Wang X, Modjaz M, Jha S, Foley RJ, Smith N (2011b) Nearby supernova rates from the Lick Observatory Supernova Search - II. The observed luminosity functions and fractions of supernovae in a complete sample. Monthly Notices of the Royal Astronomical Society 412:1441–1472, DOI 10.1111/j.1365-2966.2011.18160.x, arXiv:1006.4612 Lira P (1996) Light curves of the supernovae 1990N and 1990T. Master’s thesis, MS thesis. Univ. Chile (1996) Liu WM, Chen WC, Wang B, Han ZW (2010) Helium-star evolutionary channel to super- Chandrasekhar mass type Ia supernovae. Astronomy & Astrophysics 523:A3, DOI 10.1051/0004-6361/201014180, arXiv:1007.4751 Liu ZW, Kromer M, Fink M, Pakmor R, Röpke FK, Chen XF, Wang B, Han ZW (2013) Predicting the Amount of Hydrogen Stripped by the SN Explosion for SN 2002cx- like SNe Ia. The Astrophysical Journal 778:121, DOI 10.1088/0004-637X/778/2/121, arXiv:1310.3884 Liu ZW, Moriya TJ, Stancliffe RJ, Wang B (2015a) Constraints on single-degenerate Chandrasekhar mass progenitors of Type Iax supernovae. Astronomy & Astrophysics 574:A12, DOI 10.1051/0004-6361/201424532, arXiv:1412.0820 Liu ZW, Stancliffe RJ, Abate C, Wang B (2015b) Pre-explosion Companion Stars in Type Iax Supernovae. The Astrophysical Journal 808:138, DOI 10.1088/0004- 637X/808/2/138, arXiv:1506.04903 24 Saurabh W. Jha

Liu ZW, Zhang JJ, Ciabattari F, Tomasella L, Wang XF, Zhao XL, Zhang TM, Xin YX, Wang CJ, Chang L (2015c) Optical observations of an SN 2002cx-like peculiar super- nova SN 2013en in UGC 11369. Monthly Notices of the Royal Astronomical Society 452:838–844, DOI 10.1093/mnras/stv1303, arXiv:1506.02845 Long M, Jordan GC IV, van Rossum DR, Diemer B, Graziani C, Kessler R, Meyer B, Rich P, Lamb DQ (2014) Three-dimensional Simulations of Pure Deflagration Models for Thermonuclear Supernovae. The Astrophysical Journal 789:103, DOI 10.1088/0004- 637X/789/2/103, arXiv:1307.8221 Lyman JD, James PA, Perets HB, Anderson JP, Gal-Yam A, Mazzali P, Percival SM (2013) Environment-derived constraints on the progenitors of low-luminosity Type I supernovae. Monthly Notices of the Royal Astronomical Society 434:527–541, DOI 10.1093/mnras/stt1038, arXiv:1306.2474 Lyman JD, Taddia F, Stritzinger MD, Galbany L, Leloudas G, Anderson JP, Eldridge JJ, James PA, Krühler T, Levan AJ, Pignata G, Stanway ER (2017) Investigating the diversity of supernovae type Iax: A MUSE and NOT spectroscopic study of their envi- ronments. ArXiv e-prints arXiv:1707.04270 Ma H, Woosley SE, Malone CM, Almgren A, Bell J (2013) Carbon Deflagration in Type Ia Supernova. I. Centrally Ignited Models. The Astrophysical Journal 771:58, DOI 10.1088/0004-637X/771/1/58, arXiv:1305.2433 Maeda K, Terada Y (2016) Progenitors of type Ia supernovae. International Journal of Modern Physics D 25:1630024, DOI 10.1142/S021827181630024X, arXiv:1609.03639 Magee MR, Kotak R, Sim SA, Kromer M, Rabinowitz D, Smartt SJ, Baltay C, Campbell HC, Chen TW, Fink M, Gal-Yam A, Galbany L, Hillebrandt W, Inserra C, Kankare E, Le Guillou L, Lyman JD, Maguire K, Pakmor R, Röpke FK, Ruiter AJ, Seitenzahl IR, Sullivan M, Valenti S, Young DR (2016) The type Iax supernova, SN 2015H. A white dwarf deflagration candidate. Astronomy & Astrophysics 589:A89, DOI 10.1051/0004- 6361/201528036, arXiv:1603.04728 Magee MR, Kotak R, Sim SA, Wright D, Smartt SJ, Berger E, Chornock R, Foley RJ, Howell DA, Kaiser N, Magnier EA, Wainscoat R, Waters C (2017) Growing evidence that SNe Iax are not a one-parameter family. The case of PS1-12bwh. Astronomy & Astrophysics 601:A62, DOI 10.1051/0004-6361/201629643, arXiv:1701.05459 Maguire K (2016) Type ia supernovae. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1–24, DOI 10.1007/978-3- 319-20794-0_36-1 Margutti R, Parrent J, Kamble A, Soderberg AM, Foley RJ, Milisavljevic D, Drout MR, Kirshner R (2014) No X-Rays from the Very Nearby Type Ia SN 2014J: Con- straints on Its Environment. The Astrophysical Journal 790:52, DOI 10.1088/0004- 637X/790/1/52, arXiv:1405.1488 Maund JR, Wheeler JC, Wang L, Baade D, Clocchiatti A, Patat F, Höflich P, Quinn J, Zelaya P (2010) A Spectropolarimetric View on the Nature of the Peculiar Type Iax Supernovae 25

Type I SN 2005hk. The Astrophysical Journal 722:1162–1174, DOI 10.1088/0004- 637X/722/2/1162, arXiv:1008.3985 Maza J, Hamuy M, Antezana R, Gonzalez L, Cartier R, Forster F, Silva S, Carrasco F, Sanchez P, Hervias C, Iturra D, Pignata G, Cifuentes M, Farias C, Conuel B, Folatelli G, Reichart D, Ivarsen K, Haislip J, Crain A, Foster D, Nysewander M, Lacluyze A, An- derson J, Morrell N (2011) Supernova 2011ce in NGC 6708 = Psn J18553580-5343290. Central Bureau Electronic Telegrams 2715 McClelland CM, Garnavich PM, Galbany L, Miquel R, Foley RJ, Filippenko AV, Bas- sett B, Wheeler JC, Goobar A, Jha SW, Sako M, Frieman JA, Sollerman J, Vinko J, Schneider DP (2010) The Subluminous Supernova 2007qd: A Missing Link in a Fam- ily of Low-luminosity Type Ia Supernovae. The Astrophysical Journal 720:704–716, DOI 10.1088/0004-637X/720/1/704, arXiv:1007.2850 McCully C, Jha SW, Foley RJ, Bildsten L, Fong Wf, Kirshner RP, Marion GH, Riess AG, Stritzinger MD (2014a) A luminous, blue progenitor system for the type Iax supernova 2012Z. Nature 512:54–56, DOI 10.1038/nature13615, arXiv:1408.1089 McCully C, Jha SW, Foley RJ, Chornock R, Holtzman JA, Balam DD, Branch D, Filip- penko AV, Frieman J, Fynbo J, Galbany L, Ganeshalingam M, Garnavich PM, Graham ML, Hsiao EY, Leloudas G, Leonard DC, Li W, Riess AG, Sako M, Schneider DP, Silverman JM, Sollerman J, Steele TN, Thomas RC, Wheeler JC, Zheng C (2014b) Hubble Space Telescope and Ground-based Observations of the Type Iax Supernovae SN 2005hk and SN 2008A. The Astrophysical Journal 786:134, DOI 10.1088/0004- 637X/786/2/134, arXiv:1309.4457 Meng X, Podsiadlowski P (2014) The Birth Rate of SNe Ia from Hybrid CONe White Dwarfs. The Astrophysical Journal, Letters 789:L45, DOI 10.1088/2041- 8205/789/2/L45, arXiv:1406.4208 Meng X, Yang W (2010) A Comprehensive Progenitor Model for SNe Ia. The Astrophysical Journal 710:1310–1323, DOI 10.1088/0004-637X/710/2/1310, arXiv:0910.4992 Meng X, Zhang J, Han Z (2017) A Polarization Sequence for Type Ia Su- pernovae? The Astrophysical Journal 841:62, DOI 10.3847/1538-4357/aa6f18, arXiv:1704.06386 Metzger BD (2012) Nuclear-dominated accretion and subluminous supernovae from the merger of a white dwarf with a neutron star or black hole. Monthly Notices of the Royal Astronomical Society 419:827–840, DOI 10.1111/j.1365-2966.2011.19747.x, arXiv:1105.6096 Miller AA, Kasliwal MM, Cao Y, Goobar A, Kneževic´ S, Laher RR, Lunnan R, Masci FJ, Nugent PE, Perley DA, Petrushevska T, Quimby RM, Rebbapragada UD, Sollerman J, Taddia F, Kulkarni SR (2017) Color Me Intrigued: the Discovery of iPTF 16fnm, a Supernova 2002cx-like Object. ArXiv e-prints arXiv:1703.07449 Milne PA, Brown PJ, Roming PWA, Holland ST, Immler S, Filippenko AV, Gane- shalingam M, Li W, Stritzinger M, Phillips MM, Hicken M, Kirshner RP, Chal- lis PJ, Mazzali P, Schmidt BP, Bufano F, Gehrels N, Vanden Berk D (2010) Near- 26 Saurabh W. Jha

ultraviolet Properties of a Large Sample of Type Ia Supernovae as Observed with the Swift UVOT. The Astrophysical Journal 721:1627–1655, DOI 10.1088/0004- 637X/721/2/1627, arXiv:1007.5279 Moriya T, Tominaga N, Tanaka M, Nomoto K, Sauer DN, Mazzali PA, Maeda K, Suzuki T (2010) Fallback Supernovae: A Possible Origin of Peculiar Supernovae with Ex- tremely Low Explosion Energies. The Astrophysical Journal 719:1445–1453, DOI 10.1088/0004-637X/719/2/1445, arXiv:1006.5336 Narayan G, Foley RJ, Berger E, Botticella MT, Chornock R, Huber ME, Rest A, Scolnic D, Smartt S, Valenti S, Soderberg AM, Burgett WS, Chambers KC, Flewelling HA, Gates G, Grav T, Kaiser N, Kirshner RP, Magnier EA, Morgan JS, Price PA, Riess AG, Stubbs CW, Sweeney WE, Tonry JL, Wainscoat RJ, Waters C, Wood-Vasey WM (2011) Displaying the Heterogeneity of the SN 2002cx-like Subclass of Type Ia Supernovae with Observations of the Pan-STARRS-1 Discovered SN 2009ku. The Astrophysical Journal, Letters 731:L11, DOI 10.1088/2041-8205/731/1/L11, arXiv:1008.4353 Neunteufel P, Yoon SC, Langer N (2016) Models for the evolution of close bina- ries with He-star and white dwarf components towards Type Ia supernova explo- sions. Astronomy & Astrophysics 589:A43, DOI 10.1051/0004-6361/201527845, arXiv:1603.00768 Nomoto K (1984) Evolution of 8-10 solar mass stars toward electron capture supernovae. I - Formation of electron-degenerate O + Ne + Mg cores. The Astrophysical Journal 277:791–805, DOI 10.1086/161749 Nonaka A, Aspden AJ, Zingale M, Almgren AS, Bell JB, Woosley SE (2012) High- resolution Simulations of Convection Preceding Ignition in Type Ia Supernovae Using Adaptive Mesh Refinement. The Astrophysical Journal 745:73, DOI 10.1088/0004- 637X/745/1/73, arXiv:1111.3086 Nugent P, Phillips M, Baron E, Branch D, Hauschildt P (1995) Evidence for a Spec- troscopic Sequence among Type 1a Supernovae. The Astrophysical Journal, Letters 455:L147+, DOI 10.1086/309846, arXiv:astro-ph/9510004 Östman L, Nordin J, Goobar A, Amanullah R, Smith M, Sollerman J, Stanishev V, Stritzinger MD, Bassett BA, Davis TM, Edmondson E, Frieman JA, Garnavich PM, Lampeitl H, Leloudas G, Marriner J, Nichol RC, Romer K, Sako M, Schneider DP, Zheng C (2011) NTT and NOT spectroscopy of SDSS-II supernovae. Astronomy & Astrophysics 526:A28, DOI 10.1051/0004-6361/201015704, arXiv:1011.5869 Pan YC, Foley RJ, Downing S, Jha SW, Rest A, Scolnic D, Smith KW, Wright D, Smartt SJ, Huber M, Chambers KC, Flewelling H, Willman M, Primak N, Schultz A, Gibson B, Magnier E, Waters C, Tonry J, Wainscoat RJ (2015) ATel 7534: Spectroscopic Clas- sifications of Optical Transients with Mayall/KOSMOS. The Astronomer’s Telegram 7534 Pan YC, Miller JA, Hounsell RA, Foley RJ, Jha SW, Rest A, Scolnic D, Smith KW, Wright D, Smartt SJ, Huber M, Chambers KC, Flewelling H, Willman M, Primak N, Schultz A, Gibson B, Magnier E, Waters C, Tonry J, Wainscoat RJ (2016) Spectro- Type Iax Supernovae 27

scopic Classifications of Optical Transients with SOAR. The Astronomer’s Telegram 8810 Perets HB, Gal-Yam A, Mazzali PA, Arnett D, Kagan D, Filippenko AV, Li W, Arcavi I, Cenko SB, Fox DB, Leonard DC, Moon DS, Sand DJ, Soderberg AM, Anderson JP, James PA, Foley RJ, Ganeshalingam M, Ofek EO, Bildsten L, Nelemans G, Shen KJ, Weinberg NN, Metzger BD, Piro AL, Quataert E, Kiewe M, Poznanski D (2010) A faint type of supernova from a white dwarf with a helium-rich companion. Nature 465:322–325, DOI 10.1038/nature09056, arXiv:0906.2003 Phillips MM (1993) The absolute magnitudes of Type IA supernovae. The Astrophysical Journal, Letters 413:L105–L108, DOI 10.1086/186970 Phillips MM, Li W, Frieman JA, Blinnikov SI, DePoy D, Prieto JL, Milne P, Contreras C, Folatelli G, Morrell N, Hamuy M, Suntzeff NB, Roth M, González S, Krzeminski W, Filippenko AV, Freedman WL, Chornock R, Jha S, Madore BF, Persson SE, Burns CR, Wyatt P, Murphy D, Foley RJ, Ganeshalingam M, Serduke FJD, Krisciunas K, Bassett B, Becker A, Dilday B, Eastman J, Garnavich PM, Holtzman J, Kessler R, Lampeitl H, Marriner J, Frank S, Marshall JL, Miknaitis G, Sako M, Schneider DP, van der Heyden K, Yasuda N (2007) The Peculiar SN 2005hk: Do Some Type Ia Supernovae Explode as Deflagrations? Publications of the Astronomical Society of the Pacific 119:360–387, DOI 10.1086/518372, arXiv:astro-ph/0611295 Piersanti L, Tornambé A, Yungelson LR (2014) He-accreting white dwarfs: accretion regimes and final outcomes. Monthly Notices of the Royal Astronomical Society 445:3239–3262, DOI 10.1093/mnras/stu1885, arXiv:1409.3589 Postnov KA, Yungelson LR (2014) The Evolution of Compact Binary Star Systems. Liv- ing Reviews in Relativity 17:3, DOI 10.12942/lrr-2014-3, arXiv:1403.4754 Pumo ML, Turatto M, Botticella MT, Pastorello A, Valenti S, Zampieri L, Benetti S, Cappellaro E, Patat F (2009) EC-SNe from Super-Asymptotic Giant Branch Progen- itors: Theoretical Models Versus Observations. The Astrophysical Journal, Letters 705:L138–L142, DOI 10.1088/0004-637X/705/2/L138, arXiv:0910.0640 Riess AG, Macri L, Casertano S, Lampeitl H, Ferguson HC, Filippenko AV, Jha SW, Li W, Chornock R, Silverman JM (2011) A 3% Solution: Determination of the Hubble Con- stant with the Hubble Space Telescope and Wide Field Camera 3. The Astrophysical Journal 730:119, DOI 10.1088/0004-637X/730/2/119, arXiv:1103.2976 Röpke FK (2017) Combustion in thermonuclear supernova explosions. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1–25, DOI 10.1007/978-3-319-20794-0_58-2, arXiv:1703.09274 Röpke FK, Hillebrandt W, Schmidt W, Niemeyer JC, Blinnikov SI, Mazzali PA (2007) A Three-Dimensional Deflagration Model for Type Ia Supernovae Compared with Observations. The Astrophysical Journal 668:1132–1139, DOI 10.1086/521347, arXiv:0707.1024 Ruiter AJ, Belczynski K, Fryer C (2009) Rates and Delay Times of Type Ia Supernovae. The Astrophysical Journal 699:2026–2036, DOI 10.1088/0004-637X/699/2/2026, arXiv:0904.3108 28 Saurabh W. Jha

Ruiter AJ, Belczynski K, Sim SA, Hillebrandt W, Fryer CL, Fink M, Kromer M (2011) Delay times and rates for Type Ia supernovae and thermonuclear explosions from double-detonation sub-Chandrasekhar mass models. Monthly Notices of the Royal Astronomical Society 417:408–419, DOI 10.1111/j.1365-2966.2011.19276.x, arXiv:1011.1407 Sahu DK, Tanaka M, Anupama GC, Kawabata KS, Maeda K, Tominaga N, Nomoto K, Mazzali PA, Prabhu TP (2008) The Evolution of the Peculiar Type Ia Supernova SN 2005hk over 400 Days. The Astrophysical Journal 680:580–592, DOI 10.1086/587772, arXiv:0710.3636 Seitenzahl IR, Townsley DM (2017) Nucleosynthesis in thermonuclear supernovae. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Publish- ing, Cham, pp 1–24, DOI 10.1007/978-3-319-20794-0_87-1, arXiv:1704.00415 Shappee BJ, Kochanek CS, Stanek KZ (2013) Type Ia Single Degenerate Survivors must be Overluminous. The Astrophysical Journal 765:150, DOI 10.1088/0004- 637X/765/2/150, arXiv:1205.5028 Shen KJ, Schwab J (2017) Wait for It: Post-supernova Winds Driven by Delayed Radioac- tive Decays. The Astrophysical Journal 834:180, DOI 10.3847/1538-4357/834/2/180, arXiv:1610.06573 Silverman JM, Foley RJ, Filippenko AV, Ganeshalingam M, Barth AJ, Chornock R, Grif- fith CV, Kong JJ, Lee N, Leonard DC, Matheson T, Miller EG, Steele TN, Barris BJ, Bloom JS, Cobb BE, Coil AL, Desroches LB, Gates EL, Ho LC, Jha SW, Kandrashoff MT, Li W, Mandel KS, Modjaz M, Moore MR, Mostardi RE, Papenkova MS, Park S, Perley DA, Poznanski D, Reuter CA, Scala J, Serduke FJD, Shields JC, Swift BJ, Tonry JL, Van Dyk SD, Wang X, Wong DS (2012) Berkeley Supernova Ia Program - I. Observations, data reduction and spectroscopic sample of 582 low-redshift Type Ia supernovae. Monthly Notices of the Royal Astronomical Society 425:1789–1818, DOI 10.1111/j.1365-2966.2012.21270.x, arXiv:1202.2128 Sim SA (2017) Spectra of supernovae during the photospheric phase. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1–25, DOI 10.1007/978-3-319-20794-0_28-1 Stanishev V, Taubenberger S, Blanc G, Anupama GC, Benetti S, Cappellaro E, Elias-Rosa N, Féron C, Goobar A, Krisciunas K, Pastorello A, Sahu DK, Salvo ME, Schmidt BP, Sollerman J, Thöne CC, Turatto M, Hillebrandt W (2007) The Peculiar Type Ia Super- nova 2005hk. In: T di Salvo, G L Israel, L Piersant, L Burderi, G Matt, A Tornambe, & M T Menna (ed) The Multicolored Landscape of Compact Objects and Their Explo- sive Origins, American Institute of Physics Conference Series, vol 924, pp 336–341, DOI 10.1063/1.2774878, arXiv:astro-ph/0611354 Starrfield S (2016) Evolution of accreting white dwarfs to the thermonuclear runaway. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Publish- ing, Cham, pp 1–26, DOI 10.1007/978-3-319-20794-0_59-1 Steele TN, Cobb BE, Li W, Silverman JM, Chornock R, Filippenko AV (2009) Super- novae 2009hn, 2009ho, and 2009hp. Central Bureau Electronic Telegrams 1889 Type Iax Supernovae 29

Stritzinger M (2009) Supernovae 2009I, 2009J, and 2009K. Central Bureau Electronic Telegrams 1665 Stritzinger MD, Hsiao E, Valenti S, Taddia F, Rivera-Thorsen TJ, Leloudas G, Maeda K, Pastorello A, Phillips MM, Pignata G, Baron E, Burns CR, Contreras C, Folatelli G, Hamuy M, Höflich P, Morrell N, Prieto JL, Benetti S, Campillay A, Haislip JB, LaClutze AP, Moore JP, Reichart DE (2014) Optical and near-IR observations of the faint and fast 2008ha-like supernova 2010ae. Astronomy & Astrophysics 561:A146, DOI 10.1051/0004-6361/201322889, arXiv:1311.4525 Stritzinger MD, Valenti S, Hoeflich P, Baron E, Phillips MM, Taddia F, Foley RJ, Hsiao EY, Jha SW, McCully C, Pandya V, Simon JD, Benetti S, Brown PJ, Burns CR, Campil- lay A, Contreras C, Förster F, Holmbo S, Marion GH, Morrell N, Pignata G (2015) Comprehensive observations of the bright and energetic Type Iax SN 2012Z: Inter- pretation as a Chandrasekhar mass white dwarf explosion. Astronomy & Astrophysics 573:A2, DOI 10.1051/0004-6361/201424168, arXiv:1408.1093 Szalai T, Vinkó J, Sárneczky K, Takáts K, Benko˝ JM, Kelemen J, Kuli Z, Silverman JM, Marion GH, Wheeler JC (2015) The early phases of the Type Iax supernova SN 2011ay. Monthly Notices of the Royal Astronomical Society 453:2103–2114, DOI 10.1093/mnras/stv1776, arXiv:1508.00602 Taubenberger S (2016) The extremes of thermonuclear supernovae. In: Alsabti AW, Mur- din P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1– 57, DOI 10.1007/978-3-319-20794-0_37-1, arXiv:1703.00528 Thomas RC, Nugent PE, Meza JC (2011) SYNAPPS: Data-Driven Analysis for Super- nova Spectroscopy. Publications of the Astronomical Society of the Pacific 123:237– 248, DOI 10.1086/658673 Tomasella L, Cappellaro E, Benetti S, Pastorello A, Hsiao EY, Sand DJ, Stritzinger M, Valenti S, McCully C, Arcavi I, Elias-Rosa N, Harmanen J, Harutyunyan A, Hossein- zadeh G, Howell DA, Kankare E, Morales-Garoffolo A, Taddia F, Tartaglia L, Terreran G, Turatto M (2016) Optical and near-infrared observations of SN 2014ck: an outlier among the Type Iax supernovae. Monthly Notices of the Royal Astronomical Society 459:1018–1038, DOI 10.1093/mnras/stw696, arXiv:1603.07084 Townsley DM, Calder AC, Asida SM, Seitenzahl IR, Peng F, Vladimirova N, Lamb DQ, Truran JW (2007) Flame Evolution During Type Ia Supernovae and the Deflagration Phase in the Gravitationally Confined Detonation Scenario. The Astrophysical Journal 668:1118–1131, DOI 10.1086/521013, arXiv:0706.1094 Valenti S, Pastorello A, Cappellaro E, Benetti S, Mazzali PA, Manteca J, Tauben- berger S, Elias-Rosa N, Ferrando R, Harutyunyan A, Hentunen VP, Nissinen M, Pian E, Turatto M, Zampieri L, Smartt SJ (2009) A low-energy core-collapse super- nova without a hydrogen envelope. Nature 459:674–677, DOI 10.1038/nature08023, arXiv:0901.2074 Van Dyk SD (2016) Supernova progenitors observed with hst. In: Alsabti AW, Murdin P (eds) Handbook of Supernovae, Springer International Publishing, Cham, pp 1–27, DOI 10.1007/978-3-319-20794-0_126-1 30 Saurabh W. Jha

Wang B, Chen X, Meng X, Han Z (2009a) Evolving to Type Ia Supernovae with Short Delay Times. The Astrophysical Journal 701:1540–1546, DOI 10.1088/0004- 637X/701/2/1540, arXiv:0906.4148 Wang B, Meng X, Chen X, Han Z (2009b) The helium star donor channel for the pro- genitors of Type Ia supernovae. Monthly Notices of the Royal Astronomical Society 395:847–854, DOI 10.1111/j.1365-2966.2009.14545.x, arXiv:0901.3496 Wang B, Justham S, Han Z (2013) Producing Type Iax supernovae from a specific class of helium-ignited WD explosions. Astronomy & Astrophysics 559:A94, DOI 10.1051/0004-6361/201322298, arXiv:1310.2297 Wang B, Meng X, Liu DD, Liu ZW, Han Z (2014) The Hybrid CONe WD + He Star Scenario for the Progenitors of Type Ia Supernovae. The Astrophysical Journal, Letters 794:L28, DOI 10.1088/2041-8205/794/2/L28, arXiv:1409.7759 White CJ, Kasliwal MM, Nugent PE, Gal-Yam A, Howell DA, Sullivan M, Goobar A, Piro AL, Bloom JS, Kulkarni SR, Laher RR, Masci F, Ofek EO, Surace J, Ben- Ami S, Cao Y, Cenko SB, Hook IM, Jönsson J, Matheson T, Sternberg A, Quimby RM, Yaron O (2015) Slow-speed Supernovae from the Palomar Transient Factory: Two Channels. The Astrophysical Journal 799:52, DOI 10.1088/0004-637X/799/1/52, arXiv:1405.7409 Willcox DE, Townsley DM, Calder AC, Denissenkov PA, Herwig F (2016) Type Ia Supernova Explosions from Hybrid Carbon-Oxygen-Neon White Dwarf Pro- genitors. The Astrophysical Journal 832:13, DOI 10.3847/0004-637X/832/1/13, arXiv:1602.06356 Woudt PA, Steeghs D, Karovska M, Warner B, Groot PJ, Nelemans G, Roelofs GHA, Marsh TR, Nagayama T, Smits DP, O’Brien T (2009) The Expanding Bipolar Shell of the Helium Nova V445 Puppis. The Astrophysical Journal 706:738–746, DOI 10.1088/0004-637X/706/1/738, arXiv:0910.1069 Yamanaka M, Maeda K, Kawabata KS, Tanaka M, Tominaga N, Akitaya H, Nagayama T, Kuroda D, Takahashi J, Saito Y, Yanagisawa K, Fukui A, Miyanoshita R, Watanabe M, Arai A, Isogai M, Hattori T, Hanayama H, Itoh R, Ui T, Takaki K, Ueno I, Yoshida M, Ali GB, Essam A, Ozaki A, Nakao H, Hamamoto K, Nogami D, Morokuma T, Oasa Y, Izumiura H, Sekiguchi K (2015) OISTER Optical and Near-Infrared Observations of Type Iax Supernova 2012Z. The Astrophysical Journal 806:191, DOI 10.1088/0004- 637X/806/2/191, arXiv:1505.01593 Zhang J, Wang X (2014) Spectroscopic Classification of PSN J23560655+2922423 as a Peculiar Type Ia Supernova. The Astronomer’s Telegram 6611 Zhang J, Chang L, Wang X, Li W, Rui L, Xiang D, Zhang T, Xu Z, Tan H (2016) Spec- troscopic Classification of SN 2016ilf as a Peculiar SN 2002cx-like Type Ia Supernova. The Astronomer’s Telegram 9795 Zhou WH, Wang B, Zhao G (2014) Double-detonation model of type Ia supernovae with a variable helium layer ignition mass. Research in Astronomy and Astrophysics 14:1146- 1156, DOI 10.1088/1674-4527/14/9/005 Type Iax Supernovae 31

Zingale M, Nonaka A, Almgren AS, Bell JB, Malone CM, Woosley SE (2011) The Con- vective Phase Preceding Type Ia Supernovae. The Astrophysical Journal 740:8, DOI 10.1088/0004-637X/740/1/8