A&A 546, A28 (2012) Astronomy DOI: 10.1051/0004-6361/201219528 & c ESO 2012 Astrophysics The progenitor mass of the Type IIP supernova SN 2004et from late-time spectral modeling A. Jerkstrand1,2, C. Fransson1, K. Maguire3, S. Smartt2, M. Ergon1, and J. Spyromilio4 1 Department of Astronomy, The Oskar Klein Centre, Stockholm University, 10601 Stockholm, Sweden e-mail: [email protected] 2 Astrophysics Research Centre, School of Maths and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK 3 Department of Physics (Astrophysics), University of Oxford, DWB, Keble Road, Oxford OX1 3RH, UK 4 ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany Received 3 May 2012 / Accepted 8 August 2012 ABSTRACT SN 2004et is one of the nearest and best-observed Type IIP supernovae, with a progenitor detection as well as good photometric and spectroscopic observational coverage well into the nebular phase. Based on nucleosynthesis from stellar evolution/explosion models we apply spectral modeling to analyze its 140−700 day evolution from ultraviolet to mid-infrared. We find a MZAMS = 15 M progen- itor star (with an oxygen mass of 0.8 M) to satisfactorily reproduce [O i] λλ6300, 6364 and other emission lines of carbon, sodium, magnesium, and silicon, while 12 M and 19 M models under- and overproduce most of these lines, respectively. This result is in fair agreement with the mass derived from the progenitor detection, but in disagreement with hydrodynamical modeling of the early-time light curve. From modeling of the mid-infrared iron-group emission lines, we determine the density of the “Ni-bubble” to ρ(t) 7 × 10−14 × (t/100 d)−3 gcm−3, corresponding to a filling factor of f = 0.15 in the metal core region (V = 1800 km s−1). We also confirm that silicate dust, CO, and SiO emission are all present in the spectra. Key words. supernovae: general – supernovae: individual: SN 2004et – line: formation – line: identification – radiative transfer 1. Introduction was verified by follow-up observations showing that the can- didate had disappeared (Maund & Smartt 2009). In the cases Stars with zero age main sequence (ZAMS) mass greater than where multiple-band detections were made, the progenitors have about 8 M end their lives as core-collapse supernovae (SNe). shown properties consistent with the RSG hypothesis, except for Over half of these events (per unit volume) are classified as SN 2008cn, where the progenitor was yellow1 (Elias-Rosa et al. Type IIP (Li et al. 2011), showing hydrogen lines, as well as 2009). a3−4 month plateau in the light curve, implying the presence of a massive hydrogen envelope. After the plateau phase, the core The estimated luminosity of the progenitor allows its ZAMS of the SN becomes visible, glowing from radioactive input by mass to be determined from stellar evolution models. The result- 56 ing values are consistently found to be below 20 M, with a sta- Co. As long as the ejecta remain opaque to the gamma-rays +1 +1.5 tistical analysis yielding a range of 8.5− −16.5− M for the emitted in the decay, the light curve follows the exponential de- 1.5 1.5 ff cay of 56Co, with an e-folding time of 111.4 days. The spectrum progenitor population (Smartt et al. 2009), although dust e ects may produce a somewhat higher upper boundary (Walmswell evolves from being dominated by absorption lines and lines ex- − hibiting P-Cygni profiles superimposed upon a blackbody-like & Eldridge 2012). The fate of stars in the 20 25/30 M range continuum to strong emission lines and a weaker continuum. As then remains unclear, as standard stellar evolution models pre- the temperature falls, thermal emission eventually shifts into the dict also these to evolve to RSGs and explode as Type IIP SNe infrared, but ultraviolet/optical features remain, caused by non- (e.g., Heger & Langer 2000; Meynet & Maeder 2003; Eldridge thermal ionizations and excitations. et al. 2008). Models including rotation show that stars with ini- tial mass greater than 20 M and time-averaged equatorial veloc- From hydrodynamical modeling, the progenitors of Type IIP − ities of 200 km s 1 could lose much of their hydrogen envelope SNe are believed to be red supergiants (RSGs; Chevalier 1976; and move bluewards in the HR diagram (e.g. Meynet & Maeder Falk & Arnett 1977). The rapidly growing image archives 2003). While this is immediately attractive as an explanation for have improved the prospects of confirming this by identifying the lack of luminous RSGs as Type IIP progenitors, one must ac- SN progenitors in imaging surveys. Such identifications have count for the broad observed distribution of equatorial velocities now been made for the Type IIP SNe SN 2003gd, SN 2004A, (e.g. Hunter et al. 2008) as well as the lack of detection of blue, SN 2004et, SN 2005cs, SN 2008bk, SN 2008cn, and SN 2009md luminous progenitors for Type IIn/Ib/Ic SNe (Smartt 2009). (Smartt 2009; Elias-Rosa et al. 2009; Fraser et al. 2011,and references therein). For SN 2003gd, the progenitor identification 1 There is, however, the possibility that the detected source is a blend Appendices are available in electronic form at of two or more stars, and SN 2008cn differs from normal Type IIP SNe http://www.aanda.org in its plateau length. Article published by EDP Sciences A28, page 1 of 21 A&A 546, A28 (2012) Another method to derive information about the exploded So far, SN 1987A is the only Type II SN for which de- star is through radiation-hydrodynamical modeling of the bolo- tailed spectral modeling has been undertaken. Despite being a metric light curve. Litvinova & Nadezhin (1983, 1985)presented Type IIpec rather than a Type IIP, it was probably similar to scaling relations from fits to a grid of Type IIP SN models allow- a Type IIP in the nebular phase, since the nucleosynthesis is ing for the determination of the explosion energy, ejecta mass, largely unaffected by the late-time evolution of the envelope. Xu and progenitor radius from the observed plateau length, V-band &McCray(1991)andKozma & Fransson (1992) obtained the magnitude, and photospheric velocity. However, the neglect of solutions for the ionization, excitation, and heating produced by 56Ni in these models renders them viable only for SNe with very the gamma-rays and positrons, which is the first step in the neb- small 56Ni masses. Eastman et al. (1994) found that a (typical) ular phase modeling. The evolution of individual lines were an- 56 Ni mass of 0.06 M extends the plateau by 40%, and Kasen alyzed in a series of papers by Xu et al. (1992) (hydrogen), Li & & Woosley (2009) found a prolongation by 20−30% for 56Ni- McCray (1992) (oxygen), Li et al. (1993) (iron, cobalt, nickel), masses of 0.05−0.1 M. Since the ejecta mass has a strong scal- Li & McCray (1993) (calcium), and Li & McCray (1995)(he- ing with the plateau length (to the power 2.9) in the Litvinova lium). The thermal evolution of the envelope was modeled in de- & Nadezhin (1985) relations, a significant overestimate of the tail by de Kool et al. (1998), and Kozma & Fransson (1998a,b, ejecta mass results. in the following KF98a,b) computed the spectra from detailed More reliable results can be obtained by fitting hydrodynam- explosion models to study the evolution of temperature, ioniza- ical models including 56Ni to the whole light curve evolution. tion, and line fluxes in the 200−800 day range. Kjær et al. (2010) Such modeling has been undertaken for SN 1999em (Baklanov and Jerkstrand et al. (2011, J11 hereafter) analyzed the spectrum et al. 2005; Utrobin 2007; Bersten et al. 2011), SN 2003Z in the 44Ti-powered phase (t 5 years), including the effects of (Utrobin et al. 2007), SN 2004et (Utrobin & Chugai 2009), multi-line radiative transfer. SN 2005cs (Utrobin & Chugai 2008), and SN 2009kf (Utrobin Modeling of other objects includes the work by Dessart & et al. 2010), obtaining ejecta masses Mej = 14−28 M.Asdis- Hillier (2011), who compared the emergent spectra of Type IIP cussed by Utrobin & Chugai (2009)andMaguire et al. (2010, explosion models to SN 1999em, assuming complete thermal- M10 hereafter), these ejecta masses are generally too high to be ization of the gamma-rays, but performing a detailed radiative consistent with the initial masses determined from direct obser- transfer calculation. vations of SN progenitors, as well as for what is expected from In this paper we undertake a detailed analysis of one of the stellar evolution in general. Recently, Inserra et al. (2011)and brightest and best-observed Type IIP SNe to date, SN 2004et. Inserra et al. (2012), using the radiation hydrodynamics code of This SN has been subject to progenitor analysis, hydrodynamical Pumo & Zampieri (2011), determined lower ejecta masses of modeling, and some qualitative spectral analysis. Li et al. (2005) Mej = 5−7.5andMej = 8−12 M for the Type IIP SNe 2007od identified a progenitor candidate in ground-based pre-explosion and SN 2009bw. images. However, Crockett et al. (2011) showed that this candi- A third method for diagnosing the progenitor mass is through date was still visible in 2007 (3 years after explosion), and that late-time spectral modeling. In the nebular phase (t 150 days), the source in the original images was a composite of two or three the inner ejecta become visible, and the various nuclear burning sources. They identified an excess flux in the pre-explosion im- zones can be analysed. Stellar evolution models predict the metal ages contributed by the true SN progenitor, and for two different core mass to strongly increase with progenitor ZAMS mass (e.g.