Luminous Red Nova at 2019Zhd, a New Merger in M 31

arXiv:2011.10588v2 [astro-ph.SR] 18 Dec 2020 eu ls fgptaset Psoel rsr21) In 2019). heteroge- Fraser a & covers (Pastorello transients designation gap of (LRN) class nova neous red luminous The Introduction 1. .Lundqvist P. eebr2,2020 21, December Astronomy .Munday J. .Pastorello A. 3-R21 tr:idvda:V3 Mon V838 individual: stars: - M31-LRN2015 i words. system Key binary a for expectations with consistent is source nalbns tmxmm h pcr hwabu otnu wi continuum blue a show spectra the maximum, At bands. all in h oglsig lwpooercrs bevdbfr th before observed rise photometric slow long-lasting, The a,with mag, lta eln,so eyrdcnium( continuum no red lines very metal a absorption show narrow decline, of plateau forest a and detectable, hwamc edrcnium eebigta fa intermed an of that resembling continuum, redder observ much maximum a show red second the of reminiscent is feature this 1 h npcino rhvlHTiae ae 2yasbfr t before years 22 taken images HST archival of inspection subsequent The The ejection. common-envelope the of signature ie nti hs,teaslt antd agdfrom ranged magnitude absolute m five the about phase, (L for nova this surveys sky red In several luminous rise. by the followed was of object campaign The follow-up the present We 2020 21, December a eid T21zdeprecdamjrbiheig rea brightening, major a experienced 2019zhd AT period, day 21 20 19 18 17 16 15 13 14 12 11 10 . 9 8 7 6 5 4 3 2 1 4 × eateto hsc n srnm,JhsHpisUnivers Hopkins Johns Astronomy, and Physics of Department pc eecp cec nttt,30 a atnDie B Drive, Martin San 3700 Institute, Science Telescope Space nttt o srnm,Uiest fHwi,28 Woodla 2680 Hawaii, of University Astronomy, for Institute uoenSuhr bevtr,Aos eCroa30,Cas 3107, Córdova de Alonso Observatory, Southern European a-lnkIsiu ü srpyi,Karl-Schwarzschild Astrophysik, für Institut Max-Planck srpyisRsac nttt,LvrolJh orsUn Moores John Liverpool Institute, Research Astrophysics h sa li ete eateto srnm,Stockholm Astronomy, of Department Centre, Klein Oskar The Carrer UAB, Campus CSIC), (ICE, Sciences Space of Institute eateto hsc n srnm,Aru nvriy Ny University, Aarhus Astronomy, and Physics of Department Ph and Mathematics of School Centre, Research Astrophysics Kingdom United srpyisRsac ru,Fclyo niern n Ph and Engineering of Faculty Group, Research Astrophysics eatmnod srfsc,Uiesddd aLgn,E-3 Laguna, La de Universidad Astrofísica, de Departamento nttt eAtoíiad aais -80 aLgn,Te Laguna, La E-38205 Canarias, de Astrofísica de Instituto eateto hsc n srnm,Uiest fTru F Turku, of University Astronomy, and Physics of Department ins etefrAtooywt S FNA,Qatm Ves Quantum, (FINCA), ESO with Astronomy for Centre Finnish tgk srnmclOsraoy aaaa9029,Jap 990-2492, Yamagata Observatory, Astronomical Itagaki ilnimIsiueo srpyis(A) ucoMonsen Nuncio (MAS), Astrophysics of Institute Millennium eatmnod inisFscs nvria nrsBell Andres Universidad Fisicas, Ciencias de Departamento nvriádgiSuid aoa iatmnod iiaeA e Fisica di Dipartimento Padova, di Studi degli Universitá NF-Osraoi srnmc iPdv,Vcl dell’Oss Vicolo Padova, di Univer North, Science for Astronomico Centre O’Brien Physics, of School Osservatorio [email protected] - INAF 10 39 & uiosrdnv T21zd e egri 31 M in merger new a 2019zhd, AT nova red Luminous 9 15 , r s erg Astrophysics 11 F .A Mazzali A. P. , 555 1 .J Smartt J. S. , .Fraser M. , iais ls tr:wns uflw tr:individual stars: - outflows winds, stars: - close binaries: − 1 W fe atdcie h ih uv ete noashort-d a onto settled curve light the decline, fast a After . − F 814 aucitn.AT2019zhd_pastorello_arXivrev no. manuscript W 2 .Valerin G. , 12 16 = .W Smith W. K. , , 17 obde usi admctms-I - times pandemic in hugs Forbidden 2 . 96 .Munari U. , ± 0 . 2mg ttepsto fA 09h,wihi h otlikely most the is which 2019zhd, AT of position the at mag) 12 1 , 3 T .Reguitti A. , f f e 12 1 .Srivastav S. , .D Stritzinger D. M. , ≈ 00K ihbodmlclrbnso i,smlrt hs of those to similar TiO, of bands molecular broad with K) 3000 n .Rest A. and M cuigapeoiatM-yestar. M5-type predominant a ncluding r ekrsmlsta fLNV39So hc a interpreted was which Sco, V1309 LRN of that resembles peak e con- ABSTRACT ubrti ieydet h a uflwfloigaselrm stellar a following outflow gas the to due likely is outburst oiaetesetu.Telts pcr,otie durin obtained spectra, latest The spectrum. the dominate w Sr ,D871Grhn,Germany Garching, D-85741 1, -Str. 4 = , hn ekof peak a ching di te Re hspaewsfloe yarpdlna dec linear rapid a by followed was phase This LRNe. other in ed 5 an vriy c,16Bono il iepo 35F UK 5RF, L3 Liverpool Hill, Brownlow 146 ic2, iversity, ,FradzCnh 0,LsCne,Snig,Chile Santiago, Condes, Las 700, Concha Fernandez o, tooi,Vcl elOsraoi ,312Pdv,Ital Padova, 35122 2, dell’Osservatorio Vicolo stronomia, nDie oouu I982 USA 96822, HI Honolulu, Drive, wn eie Spain nerife, aetp tr nti hs,H phase, this In star. iate-type eLNdsoeyrvasafitrdsuc ( source red faint a reveals discovery LRN he − , lioe D228 USA 21218, MD altimore, nh eoeteotus,drn hc tsoe lwlumi slow a showed it which during outburst, the before onths iyCleeDbi,Bled uln4 Ireland 4, Dublin Belfield, Dublin, College sity 26L aua eeie Spain Tenerife, Laguna, La 8206 sclSine,Uiest fSre,Gidod Surrey, Guildford, Surrey, of University Sciences, ysical 1 rStr az10 rvdni,Snig,Chile Santiago, Providencia, 100, Sanz Sótero or sc,QensUiest efs,B71N UK 1NN, BT7 Belfast, University Queen’s ysics, 2 -01 uk,Finland Turku, I-20014 12 N T21zd h hr vn fti ls bevdi 31. M in observed class this of event third the 2019zhd, AT RN) .Itagaki K. , ukgd 2,80 ahsC Denmark C, Aarhus 8000 120, Munkegade t,Blioe D228 USA 21218, MD Baltimore, ity, la1,Snig,Chile Santiago, 19, illa . eCnMgass Magrans Can de hpoietBle msinlns h otmxmmspect post-maximum The lines. emission Balmer prominent th 8 nvriy laoa E161Sokom Sweden Stockholm, SE-10691 AlbaNova, University, rs ihohrfmle fgptaset,LN pna eno ( an transients span faint LRNe Intrinsically transients, range. gap luminosity of mous families other with trast to 03 n 10 c Msne l 00 yed ta.2011) al. et Tylenda 2010; al. et (Mason Sco V1309 and 2013) .Elias-Rosa N. , lnate5 nvriyo uk,F-01 uk,Finland Turku, FI-20014 Turku, of University 5, ilinnantie ± 20 T21zd-sas niiul 3-V-sas individual stars: - M31-RV individual: stars: - 2019zhd AT : − 0 14 . , 6 a to mag 2 rto lta nterdbns lhuhls pronounced, less Although bands. red the in plateau uration 21 . .Tomasella L. , a)sc sOL 02BG30(yed tal. et (Tylenda 2002-BLG-360 OGLE as such mag) 5 M 6 raoi ,I312Pdv,Iaye-mail: Italy Padova, I-35122 5, ervatorio r .Ochner P. , = M / − ,E013Breoa Spain Barcelona, E-08193 n, r 1 9 = , . 61 13 − .Kankare E. , 5 ± 1 . .P Anderson P. J. , 6 α 0 3 . ± .C Williams C. S. , 8mgada pia uioiyof luminosity optical an and mag 08 eoe eywa,H weak, very becomes 0 . a.Te,oe ort five to four a over Then, mag. 1 8 .Karamehmetoglu E. , uecn rcro.The precursor. quiescent M ril ubr ae1o 17 of 1 page number, Article 18 F 555 .C Chambers C. K. , 7 W , 8 .Jones D. , = β -yestars. M-type rigevent. erging sn longer no is 0 y U 7XH, GU2 h post- the g . 21 © ± nosity S 2020 ESO sthe as M 0 line . 14 9 V ra , : 10 − ≈ 14 , 19 , r- 4 , A&A proofs: manuscript no. AT2019zhd_pastorello_arXivrev are usually observed in our Galaxy, while much brighter events (MV . −12 mag) such as NGC4490-2011OT1 (Smith et al. 2016; Pastorello et al. 2019a), AT 2015dl in M 101 (Blagorodnova et al. 2017), AT 2017jfs (Pastorello et al. 2019b), and AT 2018hso (Cai et al. 2019) are occasionally discovered in other galaxies. This suggests that faint LRNe are common and can be observed with a rate of once per decade in a galaxy such as the Milky Way, while luminous events are intrinsically rare (Kochanek et al. 2014; Howitt et al. 2020). The detailed photometric and spectroscopic follow-up of V1309 Sco revealed the physical mechanisms powering at least most of LRN outbursts. For a few years before the outburst, V1309 Sco showed a short-period photometric modulation with a pseudo-period of about 1.4 d due to the inspiraling motion of the secondary star in a binary system. Later, the photometric period started to decline. The photometric modulation disappeared as soon as the light curvereached a minimumfollowed by a slow luminosity rise of ∼4 mag, lasting about six months. This was interpreted as the signature of the common-envelope (CE) ejection which obscured the binary system. The final LRN outburst was likely due to the gas outflow following the stellar coalescence. Recent theoretical studies (e.g. Pejcha et al. 2016, 2017; MacLeod et al. 2017, 2018) support this scenario. Starting from an alternative characterization of the sub- Fig. 1. AT 2019zhd in M 31: r-band image of the LRN field ob- families of gap transients, Soker & Kaplan (2020) recently pro- tained on 2020 February 14 with the 1.82m Asiago Copernico telescope, posed a jet-powered toy model to explain their observed prop- equipped with AFOSC. erties of different types of objects. Regardless of whether the physical scenario is a merging event or simple mass transfer that leaves the binary system intact, the accretion of mass through a disk may launch polar jets (Kashi& Soker 2016; Soker & Kashii 2016). The efficient conversion of kinetic energy into radiation produced by the jet colliding with pre- 2. The discovery of AT 2019zhd existing circum-stellar material may account for the light curves of LRNe and other gap transients (see, also, Soker 2020). The large range of observed peak luminosity of LRNe is re- lated to the different masses involved. Kochanek et al. (2014) AT 2019zhd1 was discovered by the Zwicky Transient Factory propose that the most luminous outbursts are produced by more (ZTF) on 2019 December 14.10 UT (Ho 2019) at an apparent massive mergers. This finding is supported by Pastorello et al. magnitude r = 20.35±0.20mag. The transient is observed at RA (2019a), who found a possible correlation between the luminos- = 00h40m37s.905 and Dec = +40◦34′52′′.85 (equinox J2000.0), ity of individual light curve features and the luminosity of the which is in the south-west outskirts of the M 31 disk. The object quiescent progenitor system. was then spectroscopically classified as a possible luminous blue In this paper, we present the results of our follow-up cam- variable (LBV) outburst or a LRN by Kawabata (2020). The as- paign of LRN AT 2019zhd, which was discovered in M 31. sociation of the transient with the galaxy is confirmed through its This is the third LRN that has been discovered so far in M 31, spectra, whose emission lines are slightly shifted towards bluer after M31-RV (Boschi & Munari 2004) and M31-LRN2015 wavelengths, consistent with the negative recessional velocity of (Kurtenkovet al. 2015; Williams et al. 2015; Lipunovet al. M 312. The region in M 31 with AT 2019zhd is shown in Fig. 1. 2017; Blagorodnova et al. 2020). Interestingly, all of them have rather similar observational properties. The follow-up campaign of AT 2019zhd has been carried out during the pandemic period The distance of M 31 is well constrained through a num- in part, and has been affected by a limited access to the observa- ber of methods. In this paper, we adopt the statistical estimate tional facilities. Despite these restrictions, the past few months from Tully et al. (2013), based on different methods (including have been prolific in terms of LRN discoveries as AT 2020hat cepheids, the tip of the red giant branch, and the surface bright- −1 −1 and AT 2020kog (Pastorello et al. 2020) were announced soon ness fluctuation) and scaled to H0 = 73 km s Mpc . The in- after AT 2019zhd. ferred distance is d = 0.785 ± 0.009 Mpc, which provides a The format of the paper is as follows. In Sect. 2, we report distance modulus µ = 24.47 ± 0.06 mag. information on the discovery of AT 2019zhd, along with its distance and interstellar reddening; in Sect. 3, we discuss the photometric properties during the different phases of the LRN evo- The Galactic line-of-sight extinction at the location of the lution, while in Sect. 4 we describe the spectroscopic evolution; transient is E(B−V) = 0.055 mag (Schlafly & Finkbeiner 2011). in Sect. 5 we analyse the bolometric light curve and the evolu- Our early spectra of AT 2019zhd (see Sect. 4) do not show evi- tion of the photospheric temperature and radius, compared with dent absorption features of interstellar Na I that can be attributed the other LRNe in M 31; in Sect. 6 we characterise the nature of to additional host galaxy reddening. For this reason, we assume the progenitor system, while a discussion and a short summary that the total reddening towards AT 2019zhd is consistent with follow in Sect. 7. the Galactic value.

Article number, page 2 of 17 A. Pastorello et al.: Luminous red nova AT 2019zhd, a new merger in M 31

Fig. 2. Long-term light curve of AT 2019zhd, which includes both pre-outburst photometry and the LRN outburst. The phases are computed from the r-band maximum (MJD = 58892.0 ± 0.5). Only signiﬁcant detection limits are shown in the ﬁgure.

3. Photometric data bles A.1 and A.2. The comprehensive multi-band light curves of AT 2019zhd are shown in Fig. 2. Photometric data reduction was carried out using the SNOoPY pipeline (Cappellaro 2014). After correcting the science images for standard calibration images (bias and flat-fields), and 3.1. Pre-outburst phase after performing astrometric calibration, SNOoPY allows us to Although the object was discovered about two months before carry out the simultaneous PSF-fitting photometry of the tar- the r-band light-curve maximum (see Sect. 3.2), we collected get along with a number of stellar sources in the field of the data from the major public surveys to track its pre-outburst evo- transient. The instrumental photometry was then calibrated us- lution. Photometric data come from ZTF3 (Bellm et al. 2019; ing zero points and colour term corrections inferred, for each Graham et al. 2019), the Panoramic Survey telescope and Rapid night and instrumental configuration, using standard stars from Response System (Pan-STARRS, hereafter PS; Chambers et al. Smith et al. (2002). Finally, the Sloan-band photometry was 2019; Magnier et al. 2020), and the Asteroid Terrestrial-impact fine-tuned through a comparison of the magnitudes of a num- Last Alert System (ATLAS; Tonryet al. 2018; Smith et al. ber of stars in the field of AT 2019zhd with those of the SDSS 2020). While the ZTF photometrywas obtained with nearly stan- catalogue. dard Sloan-g and r filters, PS provided data in the i band (which A catalogue of comparison stars in the B and V Johnson fil- is close to Sloan-i) and the wide (w) filter (nearly covering the ters was obtained converting SDSS to Johnson-Bessell magni- g+r band). ATLAS provided magnitudes in the cyan (hereafter c, tudes using the transformation relations of Chronis & Gaskell which is approximately g+r) and orange (hereafter o, nearly r+i) (2008). The final magnitudes of AT 2019zhd are reported in Ta- filters. While the few ATLAS-c magnitudes have been converted to Sloan-g magnitudes following the prescriptions of Tonry et al. 1 Alternative survey designations are ZTF19adakuot and AT- (2018), the PS-w and the ATLAS-o magnitudes have been left in LAS19berq. their original photometric systems. 2 The redshift of AT 2019zhd measured from our early spectra (see Sect. 4) is consistent with that of the nearby emission line source 2035 3 The data have been obtained through the Lasair (Smith et al. 2019) (z = −0.001764) discussed by Merrett et al. (2006). and the ALeRCE brokers (Förster et al. 2020).

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The maximum is followed by an initially fast decline and then by a flattening, which is marginally visible in the blue bands, while it is more evident in the redder bands. The magnitude of the plateau in the r band is r = 16.55 ± 0.05 mag. A plateau or a broad second maximum have been observed in a number of LRNe (see, Pastorello et al. 2019a, and references therein). However, while the second red peak is prominent in the intrinsically luminous events, usually it is relatively shallow in fainter LRNe (see, e.g. Pastorello et al. 2019a, their figure 15). After the secondary peak (or the plateau) the light curve declines mono- tonically in all bands. The overall photometric evolution of the outburst in the different bands is shown in Fig. 3.

3.3. Comparison with other LRNe In Fig. 4, we compare the r-band absolute light curve, the B − V colour evolution, and the pseudo-bolometric light curve of AT 2019zhd with those of a sample of LRNe (see Pastorello et al. 2019a, and references therein). The top-left panel of Fig. 4 shows the pre-outburst light curves of a few objects whose fields were covered by survey observations, high- lighting the pre-outburst photometric evolution of the LRN pre- cursors. In particular, the historical light curve of V1309 Sco published by Tylenda et al. (2011) is considered as a milestone for our comprehension of the LRN phenomenon.V1309 Sco is a Fig. 3. Multi-band light curve of the AT 2019zhd outburst. Galactic LRN routinely observed by the OGLE project (Udalski 2003) from 2001 to late 2007, and showed a modulated light The earliest detections (from ∼ 150 to 100 days before max- curve with photometric period decreasing with time. This pho- imum) are provided by the PS survey. From Fig. 2, we note that tometric variability was likely produced by a binary system that a very faint source (at i = 22.25 ± 0.40 mag) was first observed was rapidly losing angular momentum. In early 2008, a broad at the location of AT 2019zhd on MJD = 58745.59, hence 146 d photometric minimum and a later brightening by 4 mag in about before the peak of the outburst (see Sect. 3.2). Then, the source six months were observed,while signatures of a light curvemod- experienceda moderate magnituderise of 1.6±0.3mag(100d)−1 ulation were no longer visible. This was interpreted as the re- in the g band and 2.6 ± 0.2 mag (100 d)−1 in the r band (these sult of the common envelope ejection, which embedded and ob- slopes have been obtained approximating ATLAS-o and PS-w scured the two stellar companions. A much more evident out- magnitudes to Sloan-r). Finally, a much more evident brighten- burst, characterised by a brightening by further 4 mags in about ing started on MJD ∼ 58887; the evolution of this outburst will one week, was later observed, and is believed to be due to gas be detailed in Sect. 3.2. outflow triggered by the coalescence of the two stellar cores. Unfortunately, V1309 Sco is the only LRN for which high signal to noise (S/N) and high cadence photome- 3.2. Multi-band light curves try is available. For this reason, the past history of binary variability is not detectable for other objects. However, The LRN outburst was well monitored by the surveys men- SNhunt248 (Kankare et al. 2015; Mauerhan et al. 2015, 2018), tioned in Sect. 3.1 and, in the g band, by the All-Sky Auto- AT 2015dl (Blagorodnova et al. 2017), and OGLE-2002-BLG- mated Survey for Supernovae (ASAS-SN; Shappee et al. 2014; 360 (Tylenda et al. 2013) showed a long period of slowly rising Kochanek et al. 2017). Supporting multi-band photometry has luminosity before the outburst onset, with in some cases minor been obtained at the 2.0m Liverpool Telescope equipped with to moderate luminosity fluctuations (see, e.g. M31-LRN2015; IO:O and the 2.56 m Nordic Optical Telescope (NOT) with AL- Blagorodnova et al. 2020). FOSC and StanCam4, hosted at the Roque de los Muchachos(La AT 2019zhd was observed for almost five months before the Palma Canary Islands, Spain); the 1.82m Copernico Telescope LRN outburst, during which its absolute magnitude increased with AFOSC, and the 67/92 cm Schmidt Telescope at Mt. Ekar from M ≈ −2.8 to M ≈ −5.6 mag. This relatively slow rise is (near Asiago, Italy). Additional unfiltered photometry (scaled to r r likely due to the photometric evolution of the stellar system af- Sloan r-band) has been taken using a 35 cm telescope of the Ita- ter the common envelope ejection, similarly to V1309 Sco. The gaki Astronomical Observatory (Yamagata, Japan). light-curve bump observed for a few months before the outburst The rise to the outburst peak is very fast, lasting 4-5 days is closely reminiscent that of V838 Mon (Munari et al. 2002; in the g and r bands. The peak parameters were determined Goranskij et al. 2002; Kimeswenger et al. 2002; Crause et al. through a low-order polynomial fit. We obtained that the light 2003, 2005, see Fig. 4, top-left panel) before the main peak. We curve reached the peak in the r-band on MJD = 58892.0 ± 0.5 also note that the absolute magnitude of AT 2019zhd, both be- (r = 15.00 ± 0.01mag), and in the g-band on MJD = 58991.5 max fore and during the outburst, is intermediate between luminous ± 0.7 (g = 15.38 ± 0.01 mag). Hereafter, the epoch of the r- max extra-galactic events such as SNhunt248 and AT 2015dl, and the band peak will be considered as the reference time in this paper. faint LRNe observed in the Milky Way, such as V1309 Sco and 4 The NOT data have been obtained in the framework of the Nordic OGLE-2002-BLG-360. Hence, its absolute magnitude at peak optical telescope Unbiased Transient Survey 2 (NUTS2) collaboration (Mr = −9.61 ± 0.08 mag) is somewhat comparable with that of (Holmbo et al. 2019); see https://nuts.sn.ie/. V838 Mon.

Article number, page 4 of 17 A. Pastorello et al.: Luminous red nova AT 2019zhd, a new merger in M 31

F814W

F555W

Fig. 4. Top-left: Comparison of absolute light curves for a sub-sample of LRNe, with emphasis on the pre-outburst phase. The data for the comparison objects, observed with Johnson-Cousins filters, are reported to the ABmag system by applying a shift of +0.21 mag for the R-band and +0.45 mag for the I-band observations, as indicated in Blanton & Roweis (2007). The short dotted lines to the left indicate the absolute magnitudes of the quiescent progenitor of AT 2019zhd (HST-F555W and F814 bands, in the ABmag system; see Sect. 6). Bottom-left: B − V colour curves of a few LRNe during the outburst. Right: pseudo-bolometric light curve of AT 2019zhd, along with those of a wide sample of LRNe. We used the same interstellar extinction and distance values as in Pastorello et al. (2019a, see their Table 3). The data of AT 2018hso and AT 2014ej are from Cai et al. (2019) and Stritzinger et al. (2020), respectively, while those of AT 2020kog and AT 2020hat are from Pastorello et al. (2020). In all panels, the curves for the different objects are identified with the symbols labelled in the right panel.

Fig. 4 (bottom-left panel) shows that the B − V colour evo- V1309 Sco. The short-duration plateau of AT 2019zhd is likely lution of AT 2019zhd during the first month after the outburst due to a smaller ejected mass than other LRNe. ranges from about 0 (at the blue peak) to 2 mag at the end of the plateau. This colour becomes redder much more rapidly than what has been observed in the extra-Galactic LRNe discussed 4. Spectral evolution in Pastorello et al. (2019a), and this evolution is comparable to The spectroscopic follow-up of AT 2019zhd lasted about one those shown by fainter Galactic objects, such as V1309 Sco. month. The monitoring campaign was limited by the short vis- Figure 4 (right panel) compares the quasi-bolometric evo- ibility window of the object, which was observable only soon lution of AT 2019zhd, obtained after integrating the flux over after sunset. Nonetheless, we obtained ten spectra using the the optical domain only, with those of a wide sample of LRNe. 2.0 m Liverpool Telescope equipped with SPRAT (Piascik et al. The luminosity of AT 2019zhd at peak, 1.4 × 1039 erg s−1, 2014), the 2.56 m NOT with ALFOSC, the 10.4 m Gran Telesco- is similar to that of the prototypical V838 Mon. The optical pio Canarias (GTC) with OSIRIS, the 1.82 m Copernico Tele- luminosity of AT 2019zhd is over two orders of magnitude scope with AFOSC, and the 1.22 m Galilei Telescope with a fainter than that of the luminous SNhunt248 (Kankare et al. B&C spectrograph. We monitored all crucial phases of the LRN 2015), AT 2014ej (Stritzinger et al. 2020), and AT 2017jfs evolution, from the initial blue peak to the end of the plateau. (Pastorelloet al. 2019b), and about 25 times brighter than The spectroscopic data were reduced through standard tasks in V1309 Sco. After maximum, the light curve decline is ini- IRAF5 or dedicated pipelines, such as the FOSCGUI6. Technical tially very fast, it flattens after one week, and increases again 5 IRAF is distributed by the National Optical Astronomy Observatory, at late phases. The figure also shows that fainter LRNe have which is operated by the Association of Universities for Research in light curves with shorter duration than those of the most lumi- Astronomy (AURA) under a cooperative agreement with the National nous counterparts. In general, AT 2019zhd has a luminosity sim- Science Foundation. ilar to V838 Mon and the other two LRNe in M 31, although 6 FOSCGUI is a graphic user interface developed by E. Cappellaro, with a faster-evolving light curve, which is reminiscent of that of and aimed at extracting supernova spectroscopy and photometry ob-

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Table 1. General information on the spectra of AT 2019zhd. The phases are from the r-band maximum (on MJD = 58892.0 ± 0.5).

Date MJD Phase Instrumentalconﬁguration Exptime Resol. Range (days) (s) (nm) (nm) 2020Feb13 58892.9 +0.9 LT + SPRAT 300 1.4 400 − 800 2020Feb14 58893.8 +1.8 Copernico + AFOSC + VPH7 + VPH6 1200 + 1200 1.5 + 1.5 330 − 930 2020Feb15 58894.8 +2.8 Copernico + AFOSC + VPH7 2000 1.5 340 − 730 2020Feb15 58894.8 +2.8 Galileo + B&C + 300tr 5400 0.6 330 − 780 2020Feb17 58896.9 +4.9 NOT + ALFOSC + gm4 1800 1.4 360 − 960 2020Feb18 58897.9 +5.9 Copernico + AFOSC + VPH6 + VPH7 1800 + 1500 1.5 + 1.5 350 − 920 2020Feb21 58900.8 +8.8 GTC + OSIRIS + R2500V + R2500R 360 + 360 0.21 + 0.25 443 − 768 2020Feb28 58907.8 +15.8 Copernico + AFOSC + VPH6 3600 1.5 500 − 920 2020Mar05 58913.9 +21.9 NOT + ALFOSC + gm4 2700 1.8 350 − 960 2020Mar12 58920.9 +28.9 NOT + ALFOSC + gm4 1200 1.8 420 − 960

Notes. LT = 2.0 m Liverpool Telescope (La Palma, Canary Islands, Spain); Copernico = 1.82 m Copernico Telescope (INAF - Padova Observatory; Mt. Ekar, Asiago, Italy); Galileo = 1.22 m Galileo Telescope (Padova University, Asiago - Pennar, Italy); GTC = 10.4 m Gran Telescopio Canarias (La Palma, Canary Islands, Spain); NOT = 2.56 m Nordic Optical Telescope (La Palma, Canary Islands, Spain).

Fig. 5. Spectroscopic evolution of AT 2019zhd, from the light curve peak to late phases. No reddening and redshift corrections have been applied to the spectra. For clarity, the spectra have been shifted in ﬂux by an arbitrary amount. The phases are from the Sloan-r band luminosity peak. information on the spectra is reported in Table 1, while the entire emission component are also clearly detected, along with Ca II spectral sequence is shown in Fig. 5. H&K, which is instead observed in absorption. The continuum Early spectra, taken soon after the blue peak, show a blue temperature decreases from T = 9500 ± 700 to 7900 ± 500 K continuum with superposed prominent emission lines of the from 0.9 d to 2.8 d after maximum. The best resolution B&C Balmer series. A number of Fe II features with a predominant spectrum at +2.8 d (Munari et al. 2020) allows us to constrain an upper limit for the full-width at half-maximum (FWHM) ve- tained with FOSC-like instruments. A package description can be found locity for the Hα emission, which is about 280 km s−1. at http://sngroup.oapd.inaf.it/foscgui.html.

Article number, page 6 of 17 A. Pastorello et al.: Luminous red nova AT 2019zhd, a new merger in M 31

component which is blue-shifted by about 160 km s−1. During 6.4 10.4 the ﬁrst two weeks past maximum, the Balmer emission components become progressively weaker, and only Hα is barely vis- 9.6 5.6 ible at 15.8 d, with a slightly blueshifted peak (see Fig. 6). In 8.8 this phase, the spectrum is dominated by a forest of narrow absorption metal lines. Following Pastorello et al. (2019a, see their 4.8 8 ﬁgure 10), we identify the prominent Fe II multiplets in absorp- 7.2 tion, along with Sc II, Ba II, Ti II, Na I doublet, Ca II and O I. 4 6.4 This transitional spectrum is typical of LRNe during the second, redder peak (Smith et al. 2016; Blagorodnova et al. 2017; 5.6 3.2 Pastorello et al. 2019b; Cai et al. 2019; Stritzinger et al. 2020),

4.8 although here the transition is much more rapid, in agreement with the rapid light curve evolution. 2.4 4 When the light curve starts the fast post-plateau de- 3.2 cline, the spectral continuum temperature further decreases 1.6 to T = 3100 ± 400 K at 21.9 d, and T = 2600 ± 2.4 500 K at 28.9 d. In these later spectra, broad absorption 0.8 1.6 bands produced by molecules are clearly detected at about

0.8 5450 Å, 5900 Å, 6250 Å, 6750 Å, 7150 Å, 7700 Å and 0 8300 Å; in particular, TiO bands are becoming prominent -2000 -1000 0 1000 2000 -2000 -1000 0 1000 2000 (Valenti et al. 1998). Again, the presence of molecular bands is a common feature of late-time optical spectra of LRNe (Smith et al. 2016; Blagorodnovaet al. 2017; Pastorello et al. Fig. 6. Evolution of the Hβ (left) and Hα (right) proﬁles in the spectra 2019a,b; Caietal. 2019), although it happens earlier in of AT 2019zhd. The adopted redshift, z = −0.001764, it that of the faint, fast-evolving LRNe observed in the Milky Way (e.g. emission line source 2035 of Merrett et al. (2006, see Sect. 2), close to Martini et al. 1999; Munari et al. 2007; Goranskij & Barsukova the LRN site. 2007; Kaminski´ et al. 2009; Mason et al. 2010; Barsukova et al. 2014). A comparison of the latest spectrum of AT 2019zhd with late spectra of other LRNe is shown in Fig. 7.

5. Bolometric light curve and evolution of the temperature and the radius As noted in Sect. 3.2 (see Fig. 4, right panel), AT 2019zhd belongs to the LRN sample having intermediate luminosity at maximum (MV ∼ −10 mag). This group also includes V838 Mon, and two other LRNe discovered in M 31: M31-LRN2015 (Kurtenkovet al. 2015; Williams et al. 2015; Lipunovet al. 2017; Blagorodnova et al. 2020) and M31-RV (Rich et al. 1989; Mould et al. 1990; Bryan & Royer 1992; Tomaney & Shafter 1992; Boschi & Munari 2004). The fact that only intermediate- brightness LRNe have been discovered in M 31 is somewhat puzzling. While it is not surprising that very luminous LRN events have not been observed so far in M 31 as they are intrinsically rare (Kochanek et al. 2014), faint events similar to V1309 Sco are expected to be common, and they should have been discovered in M 31. One may invoke selection effects to explain the lack of faint red nova discoveries in this galaxy. Ob- jects similar to V1309 Sco, at the distance of M 31, would have a peak apparent magnitude of about 18, hence they should be comfortably observed, unless they occur in crowded or dusty re- Fig. 7. Comparison of late-time spectra of LRNe, with the most prominent molecular bands highlighted. The sample includes AT 2019zhd gions. In this respect, we note that a few tens of nova candidates (phase ∼ 29 d), AT 2020hat (∼ 76 d) and AT 2020kog (∼ 100 d; are discovered every year in M 31 in the magnitude range 17-21 Pastorello et al. 2020), NGC4490-2011OT1 (∼ 218 d; Pastorello et al. mag, and a large fraction of them (nearly 50 %, from the on-line 2019a), AT 2015dl (∼ 252 d; Blagorodnova et al. 2017), and V1309 Sco database of M 31 novae7; see Pietsch 2010) remain unclassi- (∼ 62 d; Mason et al. 2010). fied. Hence, it is plausible that low-luminosity events similar to V1309 Sco can be found among these unclassified candidates. To best characterise the three LRNe in M 31, we now com- The following spectra show a remarkable transition, with the pare their parameters inferred from the available photomet- continuum becoming rapidly redder (T = 5400 ± 400 Kat4.9 d, ric data, adopting a similar approach as MacLeod et al. (2017, T = 4700 ± 400 K at 5.9 d, and T = 4100 ± 600 K at 15.8 d). see their figure 2). For M31-LRN2015, we adopt a moderate- In a moderate resolution GTC spectrum obtained at 8.8 d after maximum, Hα shows a P-Cygni profile with an absorption 7 https://www.mpe.mpg.de/~m31novae/opt/m31

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Fig. 8. Comparison of the physical parameters of the three LRNe in M 31 (adopting µ = 24.47 ± 0.08 mag for M 31): AT 2019zhd, M31-LRN2015 and M31-RV. Top-left: Bolometric light curves. Top-right: B − V colour evolution. Bottom-left: Evolution of the blackbody temperature evolution. Bottom-right: Evolution of the radius. The data of M31-RV are from Boschi & Munari (2004), those of M31-LRN2015 are from Kurtenkov et al. (2015); Williams et al. (2015); Lipunov et al. (2017); Blagorodnova et al. (2020). For the total reddening of the comparison objects, we adopt E(B − V) = 0.37 mag for M31-LRN2015 (from Kurtenkov et al. 2015), and E(B − V) = 0.12 mag for M31-RV (Boschi & Munari 2004).

reddening scenario (E(B − V) = 0.37 mag, Kurtenkov et al. The total flux (Fbb) and Tbb are estimated for each epoch 2015). through a Monte Carlo simulation. Firstly, flux values at different wavelengths (Fλ) are generated with a Gaussian distribution B−V In Fig. 8 (top-rightpanel), we show the colour curve as centred at the measured Fλ, and with a standard deviation equal in Fig. 4 (bottom-left), but we limit the sample to the three LRNe to the measured Fλ error. Then, we fitted a black-body to the in M 31. The colour evolution of the three objects is remarkably randomly generated Fλ,i values, obtaining an i-th estimate of the B − V similar, with growing from about 0.1 mag at maximum to total flux Fbb,i and the temperature Tbb,i. over 1.4-2 mag (depending on the object) after one month. The B − V colour increases more rapidly in AT 2019zhd than in the other two LRNe, reaching B − V ∼ 2 mag at about 15 d after After reiterating this procedure for 200 times, we adopt the maximum. The late evolution, up to +82 d, is observed only for median values of Tbb,i and Fbb,i as best estimates of Fbb and Tbb M31-RV, and the colour seems to flatten to B − V ≈ 1.8 mag. for that epoch, and the errors are given by the standard deviation of the 200 Tbb,i and Fbb,i estimates. Adopting the distance In order to determine the evolution of the bolometric lumi- given in Sect. 2 and assuming spherical symmetry for the emit- nosity (Lbol), the temperature (Tbb) and the photospheric radius ting source, from Fbb we infer Lbol for that epoch. Finally, Rph (Rph), we first construct the spectral energy distribution (SED) corresponing to the above Lbol is obtained through the Stefan- for each object, and for any epochs with available multi-band, Boltzmann law for an emitting sphere. Trivial error propagation reddening-corrected photometric data. When the measurement provides the uncertainties on the Lbol and Rph estimates. The evo- in a determined band is missing, its flux contribution can be esti- lution of Lbol, Tbb and Rph are obtained by repeating the full pro- mated through an interpolation using the magnitudes at adjacent cedure for all available epochs. We remark that, while in gen- epochs, or extrapolating the missing magnitude by assuming a eral a temperature inferred through black-body fits to the SED constant colour from the closest epoch. This assumption is very is not a good proxy for the photospheric temperature in spectra crude, and is critical mainly in the pre-maximum phases, when dominated by deep TiO features (e.g. Fluks et al. 1994), this is the object is expected to have a very rapid colour evolution. acceptable in the case of AT 2019zhd, as the TiO bands become

Article number, page 8 of 17 A. Pastorello et al.: Luminous red nova AT 2019zhd, a new merger in M 31

F439W F555W F814W

0.5 B N 0 A C Dec [arcesec]Dec -0.5 E -0.5 0 0.5 RA [arcsec]

Fig. 9. 2′′×2′′cutouts from the 1997 HST+WFPC2 images covering the site of AT 2019zhd. The 0.31′′positional uncertainty on AT 2019zhd is indicated with a red dashed circle. Sources detected by dolphot at > 5σ significance in each filter are marked with coloured points. Sources A, B and C as discussed in the text are labelled. relatively prominent only at late epochs (∼30 days after maxi- ature is lower by a factor of two) and has a larger radius at mum, see Sect. 4). maximum. This behaviour is reminiscent of those of AT 2015dl The resulting bolometric light curves of the three objects (Blagorodnovaet al. 2017) and AT 2020hat (Pastorello et al. are shown in Fig. 8 (top-left panel). AT 2019zhd peaks at L ≈ 2020), which do not show a prominent early, hot peak. The vari- 39 −1 5 2.1 × 10 erg s (hence, L ∼ 5.5 × 10 L⊙), about two times ety in early-time temperatures and radii of LRNe could be a con- fainter than M31-LRN2015 (L ≈ 4 × 1039 erg s−1). We note that sequence of the different geometry of the expelled gas, the radii the bolometric light curve of M31-RV8 shown here is the most of the two stellar components, the total masses involved, and luminous in the M 31 LRN sample, peaking at L ≈ 5.1 × 1039 the final outcome of the dynamic interaction process. Detailed erg s−1. This is due to the large NIR contribution to the bolomet- modelling is thus necessary to infer whether the heterogeneous ric luminosity, obtained through the NIR observations originally evolution of the above parameters brings some information on presented by Mould et al. (1990). the fate of the system. In particular, the LRN observables are The evolution of Tbb is shown in Fig. 8 (bottom-left panel). expected to be different when the post common-envelope evolu- While the pre-maximum evolution is very uncertain for all ob- tion has led to coalescence, or eventually to a new stable binary jects due to the incomplete colour information, from the avail- configuration. able data we note that M31-LRN2015 peaks at almost 8000 K, AT 2019zhd peaks at about 7000 K, while M31-RV seems to peak at much lower temperatures (Tbb ≈ 3800 K). Soon af- 6. The quiescent progenitor ter maximum, their temperatures undergo a fast decline during the first week, and then the decline rates become slower. In order to characterise the progenitor of AT 2019zhd, we At one month after maximum, the temperature measured for searched in the Mikulski Archive for Space Telescopes for Hub- AT 2019zhd was T ≈ 2500 K, while both M31-RV and M31- ble Space Telescope (HST) images covering the site of the tran- bb sient. While M 31 has been observed extensively with HST, the LRN2015 have hotter blackbody temperatures, i.e. Tbb ≈ 3200 K and 3600 K, respectively. site of AT 2019zhd was only covered by one epoch of multi- band imaging, due to its small field of view. These images were Finally, Fig. 8 (bottom-right panel) shows the evolution of taken with WFPC2 on 1997 December 9, with exposure times R . Again, the evolution of R is very uncertain in the pre- ph ph of 2×350 s, 2×260 s and 2×260 s in the F439W, F555W and peak phases for the reasons mentioned above. At maximum, F814W filters respectively. The site of AT 2019zhd falls on the R ∼ 500 R in both AT 2019zhd and M31-LRN2015. After ph ⊙ WF4 chip, which has a pixel scale of 0.1′′ pix−1, and is unfortu- a short-lasting decline soon after peak, R increases almost lin- ph nately <10 pixels from the edge of the detector. early in AT 2019zhd until phase ∼35d(R ∼ 1500 R ). At the ph ⊙ To precisely localise the position of AT 2019zhd on the same epoch, Rph ∼ 1800 R⊙ in M31-LRN2015, while M31-RV appears to be much more expanded, with R ∼ 3000 R . WFPC2 images, we performed differential astrometry between ph ⊙ the F814W imagesanda35s r-band image of the transient taken We note that in AT 2019zhd and M31-LRN2015 the evolu- on 2020 February 17 with the NOT plus ALFOSC. Seven point tion of Tbb and Rph follows a similar trend as more luminous LRNe, such as AT 2020kog (Pastorello et al. 2020), AT 2018hso sources common to both images were used to determine a geometric transformation. From this, we find the pixel coordinates (Cai et al. 2019), AT 2017jfs (Pastorelloet al. 2019b), and F W AT 2014ej (Stritzinger et al. 2020), that have higher tempera- of AT 2019zhd to be (792.67,522.15) on the 814 image. The ∼ ture (T > 7000 K) and a more compact radius at the early associated uncertainty on this position is 103 mas ( 1 WF pixel) bb from the rms scatter of the sources used in deriving the trans- maximum. In contrast, M31-RV is much redder (the temper- formation. However, due to systematic effects, this is likely an 8 We assume the epoch of the early peak is MJD = 47355.5 underestimate of the true uncertainty on the position. Firstly, the (Mould et al. 1990). Unfortunately, no precise colour information is only sources visible in common to both the NOT and HST im- available at that epoch, and this uncertainty affects the accuracy of the ages were bright foreground stars. These will potentially exhibit bolometric correction. The first epoch with multi-band photometry was proper motion at &mas level over the 22 years between the HST obtained at about 36 d days after the peak. and NOT images. As AT 2019zhd lies at the extreme edge of

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of sources in the field, and this difference with the local stellar population makes source A as an appealing progenitor candidate for AT 2019zhd. For Source B, we measure F555W = 25.93 ± 0.12 mag; F814W = 24.74 ± 0.18 mag; while for Source C, we find F555W = 24.83 ± 0.12 mag; F814W = 24.36 ± 0.19 mag. As these are further from the location of AT 2019zhd, and appear very similar to other sources in the field, we regard them as un- likely to be the progenitor. Turning back to Source A, after correcting for foreground reddening (E(B−V)= 0.055 mag), and adopting a distance mod- ulusforM 31of µ = 24.47±0.08mag, we find an absolute magnitude of MF555W = 0.21 ± 0.14 mag and a F555W − F814W colour of 2.96 ± 0.12 mag. While this candidate progenitor is much redder than those of most LRNe studied so far (e.g. Pastorello et al. 2019a), its red colour is somewhat reminiscent of that of the progenitor of AT 2020hat (Pastorello et al. 2020). The progenitor of M31-LRN2015 has been also detected in archive images, showing a brighter absolute magnitude (MV = −1.50 ± 0.23 mag) and a bluer colour (V − I = 1.05 ± 0.15 mag; Williams et al. 2015). We remark that, as most (if not all) LRNe are closely interacting binary systems, the two stellar components contribute to the magnitudes and the colours inferred here. An alternative scenario is that none of the three sources mentioned above is the progenitor of AT 2019zhd, hence source A is merely a chance alignment with the location of the LRN. Accounting for the detection limits of the HST images, a non- detected progenitor would have F439W > 25.6 mag, F555W > 25.7 mag and F814W > 25.4 mag, hence MF439W > 0.9 mag, MF555W > 1.1 mag and MF814W > 0.8 mag. Such a faint source would not comfortably match the correlations between progenitor versus light curve absolute magnitudes (Pastorello et al. Fig. 10. Top: histogram of the F555W−F814W colour for all sources on 2019a), as it would fainter than the expected absolute magni- the WF4 chip, showing how Source A is clearly redder than the vast ma- tude of the progenitor of a LRN whose light curve peaks at jority of these. Bottom: colour-magnitude diagram of the sources found MV ≈ −9.1 mag (see, also, figure 9 of Pastorello et al. 2020). by dolphot in the vicinity of AT 2019zhd. The grey density plot shows Moreover, the exceptionally red colour of Source A is markedly all sources detected at > 5σ on the WF4 chip, while the red points show different from other sources in the field, and may be explained sources within the 2′′×2′′region shown in Fig. 9. Sources A, B and C as byapuffed-up star that has formed a common-envelope.For this discussed in the text are labelled. reason, while we believe that source A remains the most plausible progenitor of AT 2019zhd, a bluer progenitor below the detection threshold of the HST images cannot be definitely ruled the WF4 chip we must extrapolate the geometric transformation out. (since the reference sources used for the alignment all lie to one side of the transient). Finally, our geometric transformation has six free parameters, while we are only using seven sources to 7. Discussion and conclusions derive the transformation. All things considered, we adopt three ′′ In order to infer the V-band magnitudes of AT 2019zhd at a few times the transform uncertainty (i.e. 0.31 ) as a conservative un- critical phases of its evolution, we apply the conversion relations certainty on the location of AT 2019zhd in the HST images. between photometric systems from Jordi et al. (2006). During In order to identify sources on the HST images, we used the slow magnitude rise before the LRN outburst, AT 2019zhd the dolphot photometry package (Dolphin 2000). All sources reaches a maximum V-band absolute magnitude MV = −5.74 ± detected by dolphot at > 3σ confidence are plotted in Fig. 0.28 mag. Superposed to the global rise, the light curve shows 9. One source was found to be consistent with the position of some fluctuations, similar to those observed for instance in M31- AT 2019zhd (designated as Source A), along with two sources LRN2015. just outside our adopted positional uncertainty (Sources B and As discussed by Pejcha et al. (2017), the dynamical mass C). While we regard Source A as a credible counterpart to transfer in a binary system, which happens when one star has AT2019zhdon its location, we notethat it also has markedlydif- filled its Roche lobe, may cause a systemic mass loss with a dou- ferent colour with respect to the other sources in the field. PSF- ble effect: the decrease of the angular momentum (which may fitting photometry was performed for Source A using dolphot, eventually lead the binary to merge) and the formation of an op- and we find magnitudes for the source (in the Vegamag system) tically thick expanding cocoon (the CE) that engulfs the stel- of F555W = 24.85 ± 0.12 and F814W = 21.82 ± 0.03 mag, lar system (Paczynski 1976). This phase is characterised by a while it is not detected in the F439 filter, down to ∼ 25.6 mag. substantial luminosity decline. Shocks between material ejected Source A is extremely red, hence it is not surprising it is un- in more recent mass-loss events and the CE may cause a re- detected in the F439W filter. A comparison to other sources in newed luminosity rise, as observed before the outburst of M31- the field in Fig. 10 shows it to be far redder than the majority 2015LRN and other LRNe (Blagorodnova et al. 2020). The non-

Article number, page 10 of 17 A. Pastorello et al.: Luminous red nova AT 2019zhd, a new merger in M 31 monotonic light curve of AT 2019zhd before the main outburst similar photometric characteristics, and the progenitor belongs (see Fig. 2) can be explained with a similar mechanism as pro- to a population of old red giants in the M 31 halo. While no posed for M31-2015LRN. information is available on the system before the M31-RV out- The subsequent luminous blue peak in the light curve of burst, an in-depth analysis of the LRN location years after the AT 2019zhd at MV = −9.08 ± 0.12 mag is produced by a vi- outbursts was performed by some authors, although the results olent gas ejection during the merging process, which then inter- do not give firm constraints on the identification of the survivor. acts with the pre-existing circumstellar material (MacLeod et al. Most likely, the merger faded below the detection limit due to 2017). The average magnitude of the plateau (which is con- dust obscuration, or the survivor is eventually one of the red sidered analogous of the red peak in more luminous events) giants in the field9 (Bond & Siegel 2006; Bond 2011, 2018). is MV = −7.59 ± 0.32 mag. This feature in the LRN light In the case of AT 2019zhd, a stellar source at the expected curve, characterised by a roughly constant effective temperature, position of the quiescent progenitor was tentatively identified has been explained as due to a hydrogen recombination front in pre-explosion HST archive images, with absolute magnitude which propagates through the expanding gas (Ivanovaet al. MF555W = 0.21 ± 0.14 mag, and a reddening-corrected colour 2013; Lipunov et al. 2017), similar to that observed during the F555W − F814W = 2.96 ± 0.12 mag, which is consistent with plateau phase in type IIP supernovae (Popov 1993). This sce- an intermediate M-type source. nario is also confirmed by the Balmer decrement, which ranges In this context, while the V838 Mon LRN event from 2.4 to 2.8 in the first two spectra, consistent with the expec- was relatively similar to the above objects, its progeni- tations for Case B recombination, along with the decrease of the tor system was more massive and bluer, and hosted in a Balmer emission lines intensity. young stellar population environment (Barsukova et al. 2017; While some observational properties (e.g. the light curve Goranskij et al. 2020, and references therein), somewhat more shape with an early, sharp peak followed by a broader max- similar to the environments of the brightest extra-galactic LRNe imum or a plateau, and the late spectra reminiscent of those (Smith et al. 2016; Mauerhan et al. 2015; Goranskij et al. 2016; of intermediate-to-late M spectral types) are shared among all Blagorodnovaet al. 2017; Pastorello et al. 2019a; Cai et al. LRNe, we have seen in Sect. 5 that other parameters (peak lu- 2019). minosity, effective temperature, and photospheric radius) show Recent studies on LRNe have confirmed a large heterogene- a wide diversity which could be due to the dynamical processes ity in their properties, which is expected since binary interac- occurring in the binary system, and the physical properties of tion leading to merging events may involve stars spanning an the individual stellar components. Exploring the diversity in the enormous range of masses. Further observational efforts and in- observed parameters through hydro-dynamical models can give vestments in developing theoretical models are necessary to pro- some constraints on the main parameters of the binary system vide robust correlations between the observational parameters and, eventually, the final merger. and those of the binary progenitor system. Kochanek et al. (2014) proposed a tight relation between the Acknowledgements. We acknowledge with thanks the variable star observations luminosity of the outburst and the total mass of the binary. This from the AAVSO International Database contributed by observers worldwide is also supported by the correlations of the LRN luminosity with and used in this research. that of the progenitorsystem before the outburst (Pastorello et al. MF gratefully acknowledges the support of a Royal Society – Science Foun- 2019a), and with the outburst duration (Pastorello et al. 2020). dation Ireland University Research Fellowship. EK and MS are supported by 41 −1 generous grants from Villum FONDEN (13261,28021) and by a project grant Objects with Lbol at maximum exceeding 10 erg s have (8021-00170B) from the Independent Research Fund Denmark. SJS and KWS been discovered, along with transients whose Lbol is 3-4 orders acknowledge funding from STFC Grants ST/P000312/1, ST/T000198/1 and of magnitude smaller. In the luminosity distribution of LRNe, ST/S006109/1. DJ acknowledges support from the State Research Agency AT 2019zhd lies in the middle (see figure 9 in Pastorello et al. (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU) and the European Regional Development Fund (FEDER) under grant AYA2017- 2020), sharing similar photometric parameters with objects such 83383-P. DJ also acknowledges support under grant P/308614 financed by funds as V838 Mon (e.g. Goranskij et al. 2002; Kimeswenger et al. transferred from the Spanish Ministry of Science, Innovation and Universities, 2002; Munari et al. 2002; Crause et al. 2003; Goranskij et al. charged to the General State Budgets and with funds transferred from the 2004; Crauseet al. 2005; Goranskijet al. 2020), M31-RV General Budgets of the Autonomous Community of the Canary Islands by the Ministry of Economy, Industry, Trade and Knowledge. (Bond 2003; Boschi& Munari 2004), and M31-LRN2015 This research is based on observations made with the Nordic Optical Tele- (Kurtenkovet al. 2015; Williams et al. 2015; Lipunovet al. scope, operated by the Nordic Optical Telescope Scientific Association at the 2017; Blagorodnova et al. 2020). This implies that these tran- Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de sients were likely produced by stellar systems with relatively Astrofisica de Canarias; the Gran Telescopio Canarias, installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de similar masses, intermediate between those of the two groups Canarias, in the island of La Palma; the Liverpool Telescope operated on mentioned above. the island of La Palma by Liverpool John Moores University at the Spanish The system leading to the M31-LRN2015 event was ob- Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de served over a decade before the LRN outburst in HST im- Canarias with financial support from the UK Science and Technology Facilities Council; the 1.82 m Copernico Telescope and the 67/92 cm Schmidt Telescope ages at MV = −1.50 ± 0.23 mag and V − I = 1.05 ± 0.15 of INAF-Osservatorio Astronomico di Padova at Mt. Ekar; and the 1.22 Galileo mag (Williams et al. 2015) (see, also, Dong et al. 2015), which Galilei Telescope of the Padova University in the Asiago site. are typical of a late-G to early K-type stars. The progenitor This work has made use of data from the Asteroid Terrestrial-impact Last of M31-LRN2015 was somewhat redder than those discussed Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and in Pastorello et al. (2019a). It is important to note, however, 80NSSC18K1575; byproducts of the NEO search include images and catalogues that MacLeod et al. (2017) estimated a somewhat bluer intrinsic from the survey area. The ATLAS science products have been made possible colour adopting a higher reddening scenario (up to V − I ∼ 0.6 through the contributions of the University of Hawaii Institute for Astronomy, mag, giving an F spectral type source, see their figure 4). The the Queen’s University Belfast, the Space Telescope Science Institute, and the M31-LRN2015 precursor was likely formed by a primary sub- South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile. giant of 3-5.5 M⊙ with a radius of ∼35 R⊙, and a lower-mass The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have (0.1-0.6 M⊙) main sequence companion (MacLeod et al. 2017; Lipunov et al. 2017). The M31-RV LRN event has somewhat 9 We note that this conclusion was questioned by Shara et al. (2010).

Article number, page 11 of 17 A&A proofs: manuscript no. AT2019zhd_pastorello_arXivrev been made possible through contributions by the Institute for Astronomy, Howitt, G., Stevenson, S., Vigna-Gómez, A., et al. 2020, MNRAS, 492, 3229 the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Ivanova, N., Justham, S., Avendano Nandez, J. L., & Lombardi, J. C. 2013, Sci- Society and its participating institutes, the Max Planck Institute for Astron- ence, 339, 433 omy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Kafka, S. 2020, Observations from the AAVSO International Database, Garching, The Johns Hopkins University, Durham University, the University https://www.aavso.org of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center Kaminski,´ T., Schmidt, M., Tylenda, R., Konacki, M., & Gromadzki, M. 2009, for Astrophysics, the Las Cumbres Observatory Global Telescope Network ApJS, 182, 33 Incorporated, the National Central University of Taiwan, the Space Telescope Kankare, E., Kotak, R., Pastorello, A., et al. 2015, A&A, 581, L4 Science Institute, the National Aeronautics and Space Administration under Kashi, A., & Soker, N. 2016, Res. Astron. Astrophys., 16, 99 Grant No. NNX08AR22G issued through the Planetary Science Division of the Kawabata, M. 2020, TNSCR, 499, 1 NASA Science Mission Directorate, the National Science Foundation Grant Kimeswengen, S. Lederle, C., Schmeja, S., & Armsdorfer, B. 2002, MNRAS, No. AST-1238877, the University of Maryland, Eotvos Lorand University 336, L43 (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Kochanek, C. S., Adams, S. M., & Belczynski, K. 2014, MNRAS, 443, 1319 Foundation. Kochanek, C. S., Shappee, B. J., Stanek, K. Z., et al. 2017, PASP, 129, 104502 Lasair is supported by the UKRI Science and Technology Facilities Council and Kurtenkov, A., Pessev, P., Tomov, T., et al. 2015, A&A, 578, L10 is a collaboration between the University of Edinburgh (grant ST/N002512/1) Lipunov, V. 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Article number, page 12 of 17 A. Pastorello et al.: Luminous red nova AT 2019zhd, a new merger in M 31

Appendix A: Supplementary material

Table A.1. Johnson-Bessell B, V photometry of AT 2019zhd and associated errors (in Vega magnitude scale).

Date MJD B V Instrument 2019 − 12 − 17 58834.19 >19.58 20.114 (0.296) 0 2020 − 02 − 14 58893.18 −− 15.56 (0.10) 1 2020 − 02 − 14 58893.73 −− 15.806 (0.032) 2 2020 − 02 − 14 58893.74 −− 15.809 (0.047) 2 2020 − 02 − 14 58893.75 16.185 (0.011) 15.896 (0.011) 3 2020 − 02 − 14 58893.80 16.142 (0.016) 15.893 (0.019) 4 2020 − 02 − 14 58893.85 −− 15.953 (0.026) 2 2020 − 02 − 14 58893.86 −− 15.963 (0.020) 2 2020 − 02 − 14 58893.86 −− 15.965 (0.020) 2 2020 − 02 − 14 58893.88 −− 15.953 (0.029) 2 2020 − 02 − 14 58893.88 −− 15.986 (0.028) 2 2020 − 02 − 15 58894.74 16.822 (0.049) 16.437 (0.044) 5 2020 − 02 − 15 58894.79 16.824 (0.014) 16.440 (0.012) 4 2020 − 02 − 15 58894.82 −− 16.446 (0.048) 2 2020 − 02 − 15 58894.84 −− 16.434 (0.034) 2 2020 − 02 − 15 58894.85 −− 16.429 (0.039) 2 2020 − 02 − 15 58894.86 −− 16.424 (0.037) 2 2020 − 02 − 15 58894.87 −− 16.457 (0.035) 2 2020 − 02 − 15 58894.88 −− 16.453 (0.042) 2 2020 − 02 − 15 58894.89 −− 16.463 (0.036) 2 2020 − 02 − 15 58894.87 16.898 (0.015) 16.472 (0.011) 6 2020 − 02 − 16 58895.82 −− 16.629 (0.033) 2 2020 − 02 − 16 58895.86 17.320 (0.015) 16.705 (0.011) 6 2020 − 02 − 17 58896.74 −− 16.694 (0.036) 2 2020 − 02 − 17 58896.83 17.547 (0.022) 16.776 (0.016) 6 2020 − 02 − 17 58896.84 17.553 (0.011) 16.781 (0.011) 7 2020 − 02 − 18 58897.82 −− 16.657 (0.054) 2 2020 − 02 − 19 58898.92 −− 16.816 (0.134) 2 2020 − 02 − 22 58901.01 −− 16.942 (0.057) 8 2020 − 02 − 28 58907.75 19.444 (0.227) 17.357 (0.028) 4 2020 − 03 − 03 58911.83 >19.27 17.814 (0.079) 6 2020 − 03 − 07 58915.77 −− 18.150 (0.129) 5 2020 − 03 − 07 58915.83 >19.83 −− 6 2020 − 03 − 08 58916.79 −− 18.356 (0.132) 5 2020 − 03 − 16 58924.77 −− 20.055 (0.336) 5

Notes. 0 = 2.0m Faulkees North Telescope + fa05 camera (Hawaii Isl., USA); 1 = CBAT Transient Object Followup Reports (http://www.cbat.eps.harvard.edu/unconf/followups/J00403785+4034529.html); 2 = AAVSO Observations from the AAVSO In- ternational Database (Kafka 2020); 3 = ANS Collaboration telescopes ID 310 and 2202 (Munari et al. 2020); 4 = 1.82 m Copernico Telescope + AFOSC (Cima Ekar, Asiago, Italy); 5 = 67/92 cm Schmidt Telescope + Moravian G4-16000LC + KAF-16803 CCD CCD (Cima Ekar, Asiago, Italy); 6 = 2.0 m Liverpool Telescope (LT) + IO:O (La Palma, Canary Islands, Spain); 7 = 2.56 m Nordic Optical Telescope (NOT) + ALFOSC (La Palma, Canary Islands, Spain); 8 = 0.61 m Planewave CDK24 telescope + Apogee CG-16M Camera (Burke-Gaﬀney Observatory, Saint Marys´ University, Canada).

Article number, page 13 of 17 A&A proofs: manuscript no. AT2019zhd_pastorello_arXivrev ‡ † ‡ ‡ Inst. o / 18.76 6 19.80 5 21.92 3 18.81 5 19.41 6 > > > > > ) photometric systems (in column 8). All data are the AB o -ATLAS ( orange 21.84 – – 3 22.38 – – 2 21.39 – – 3 20.57 – – 3 20.97 – – 3 > > > > > -PanSTARRS and w 19.91 – – – 4 19.93 – – – 4 20.15 – – – 4 20.82 – – – 4 18.89 – – – 4 20.24 – – – 4 20.32 – – – 1 18.64 – – – 4 20.02 – – – 4 > > > > > > > > > 20.71 – – – – 5 19.41 – – – – 4 18.54 – – – – 4 18.66 – – – – 4 19.79 – – – – 4 19.10 – – – – 4 19.08 – – – – 4 19.41 – – – – 4 19.84 – – – – 4 > > > > > > > > > and magnitudes in the natural z , i , r , u g r i zw g , u MJD Date 2019-11-3058818.41 – – – – – 2019-11-2758814.302019-11-29 58816.32 – – – – – – 21.066(0.078) 3 2019-12-0358820.36 – – – – – 2019-11-27 58814.19 – – 2019-12-03 58820.16 – 2019-11-27 58814.15 – – 2019-12-02 58819.19 – 2019-11-27 58814.11 – 2019-12-02 58819.16 – – 2018-12-1258464.23 – – – – – 2019-11-25 58812.24 – – 2019-12-02 58819.12 – – 2017-12-25 58122.24 – – – 2019-11-24 58811.19 – 2019-12-0558822.292019-12-0758824.332019-12-0958826.342019-12-1058827.40 – – – – – – – – – – – – – – – – – – – – 20.232(0.519) 19.854(0.259) 6 20.008(0.501) 5 6 2017-06-17 57921.14 – – – 2019-09-1958745.592019-10-1358769.422019-10-2258778.44 – – – – – – – – – 22.254(0.396) 21.500(0.429) – – – – – – 3 21.847(0.108) 3 3 2019-11-0558792.432019-11-24 58811.15 – – – – – 21.176(0.100) – – 3 2007-08-13 54325.61 – – 2019-08-24 58719.54 – – – 2019-08-20 58715.57 – – – 2019-12-1158828.34 – – – – – 2018-12-15 58467.32 – – – 2019-12-11 58828.19 – – 2019-12-1958836.35 – – – – – 19.964(0.321) 5 2019-12-11 58828.12 – 2019-12-1658833.362019-12-1758834.192019-12-18 58835.14 – – – – – 19.921(0.192) – – – – – – 20.178(0.494) 6 7 2019-12-1358830.292019-12-1458831.102019-12-14 58831.12 – – – – – 20.345(0.243) – – – – – – 20.298(0.451) 6 4 2019-12-1358830.222019-12-13 58830.28 – – – – – 19.590(0.117) – – 3 2019-12-1258829.312019-12-13 58830.18 – – – – – – 20.130(0.474) 5 Optical photometry of AT 2019zhd: Sloan magnitude scale. Table A.2.

Article number, page 14 of 17 A. Pastorello et al.: Luminous red nova AT 2019zhd, a new merger in M 31 † † Inst. o / 19.01 – – – 4 > 19.25 – – – – 4 19.13 – – – – 4 19.52 – – – – 4 20.55 – – – – 4 > > > > u g r i zw Date MJD 2020-01-2258870.222020-01-2358871.092020-01-2358871.162020-01-2458872.232020-01-2658874.12 –2020-01-2658874.26 –2020-01-2858876.26 –2020-01-2958877.12 –2020-01-2958877.18 –2020-02-0158880.15 –2020-02-0358882.12 19.657(0.198) – –2020-02-0358882.24 19.636(0.367) – –2020-02-0558884.13 –2020-02-0558884.28 – –2020-02-0658885.41 – – –2020-02-0758886.38 19.356(0.141) – –2020-02-0858887.11 19.502(0.141) – – –2020-02-0858887.18 – – 19.601(0.166) 19.314(0.083) – – – – – – – – 19.133(0.128) – – – – – – – – 18.954(0.097) – – – – 19.183(0.269) – – – 18.813(0.187) – – – – – – – 18.779(0.400) – – – 18.755(0.233) – – 18.717(0.091) – – – – – 19.135(0.195) – – – – – – – – 6 – – 4 – – 4 5 – 19.158(0.087) – – 4 19.063(0.194) – – 6 – – – 5 – 4 – – 4 4 18.736(0.170) – 4 – 6 18.842(0.209) – 4 – 5 1 1 4 4 2020-01-2058868.27 – 19.665(0.264) – – – – 5 2020-01-0658854.142020-01-0658854.252020-01-0758855.162020-01-0758855.192020-01-1158859.10 –2020-01-1158859.18 –2020-01-1258860.18 –2020-01-1458862.11 –2020-01-1658864.11 –2020-01-2258866.31 –2020-01-19 19.758(0.236) – – 58867.14 – – 19.674(0.254) – – – – 19.312(0.200) – 19.845(0.124) – – 19.701(0.129) – 19.248(0.166) – – – 19.149(0.159) – – – 18.921(0.162) – – – – – – – – – – – – – – – – – – – – – 18.889(0.079) – 4 – 6 – 4 – 4 – – 4 4 4 4 19.050(0.087) 4 6 2020-01-0458852.112020-01-0458852.162020-01-05 58853.08 – – – 19.693(0.315) – – – 19.480(0.116) – – – – – – 4 4 2019-12-2158838.272019-12-2958846.122019-12-2958846.262020-01-0158849.082020-01-0158849.09 –2020-01-0158849.09 –2020-01-0158849.15 –2020-01-0258850.30 –2020-01-03 – 58851.13 – – – – – 20.084(0.138) – – 20.058(0.178) – 19.756(0.114) – – – – 19.562(0.135) – – – 19.547(0.121) – – – – – – – – – – – – – – – – 20.335(0.329) – – 6 19.471(0.208) – 4 – – 6 4 4 4 19.157(0.192) 4 6 2019-12-21 58838.14 – 2019-12-2058837.082019-12-20 58837.14 – – – 20.116(0.169) – – – 4 continued. Table A.2.

Article number, page 15 of 17 A&A proofs: manuscript no. AT2019zhd_pastorello_arXivrev ⋆ ⋆ ⋆ † Inst. o / 0(0.026) – 10 (0.012) –(0.085)(0.009)(0.010) – – – – – 9 – – 11 2 2 (0.019) 16.180(0.027)(0.022) 16.202(0.019) –(0.038) – – 10 2 – 11 0.016) 15.817(0.017) 15.949(0.022)0.015) 16.291(0.016) – 16.312(0.014)0.011)0.009) 16.381(0.010) 16.389(0.008) 16.361(0.014) 10 – 16.364(0.014) – 10 – 2 12 u g r i zw Date MJD 2020-02-142020-02-1558894.13 58893.812020-02-1558894.16 16.646(0.021)2020-02-15 15.953(0.011)2020-02-15 58894.74 15.766(2020-02-15 58894.79 –2020-02-16 58894.88 17.680(0.025) –2020-02-17 58895.86 17.726(0.039) 16.578(0.011) –2020-02-17 58896.83 18.502(0.049) 16.343(2020-02-1858897.42 58896.84 18.724(0.099)2020-02-1858897.83 18.767(0.045) 16.247(0.034) 17.129(0.015) –2020-02-1958898.42 16.568(0.037) 17.136(0.009) – 16.522( –2020-02-2058899.42 16.349(0.059) 16.527(2020-02-2158900.16 – 16.289 –2020-02-2158900.23 16.368(0.010) –2020-02-2158900.42 16.128(0.032) 16.496(0.012) – 16.294 2020-02-2158900.85 – 16.371 2020-02-2258901.42 –2020-02-2358902.43 – –2020-02-2458903.42 – – –2020-02-2558904.8320.667(0.467) – –2020-02-2658905.12 – –2020-02-2658905.14 17.262(0.096) – –2020-02-2658905.46 16.538(0.063) – –2020-02-27 16.544(0.020)2020-02-28 – 58906.79 16.563(0.063) – – – –2020-03-0258910.43 – 58907.74 16.561(0.067) – – –2020-03-03 16.571(0.047) – – –2020-03-0558913.84 58911.83 – – –2020-03-0658914.44 16.566(0.081) – – –2020-03-07 16.568(0.021) 17.865(0.095 – – – 58915.78 16.579(0.080) – – – – – 16.601(0.067) – – – 18.259(0.050) 16.646(0.069) – – – – – 16.786(0.021) – – – – 4 18.818(0.117) 16.538(0.038) 16.306 – – 4 17.030(0.033) – – – 16.738(0.265) 16.441 – – 19.283(0.205) 16.686(0.019) – – – – 17.384(0.072) 16.282(0.019) – – – 16.977(0.105) – 16.629 16.14 – – – – – 17.174(0.066) – – 1 17.265(0.211) 10 – – – – – 1 – – 1 – – – – 4 5 – – 1 12 – – – 1 – 1 – – – 1 12 4 – 4 1 – – 1 12 1 2020-02-0858887.412020-02-0958888.412020-02-1158890.412020-02-1258891.112020-02-1258891.16 –2020-02-1258891.39 –2020-02-1258892.11 –2020-02-1358892.41 –2020-02-1358892.86 –2020-02-1458893.41 –2020-02-1458893.09 – –2020-02-1458893.14 – –2020-02-1458893.73 15.389(0.026) – –2020-02-14 – – 58893.75 15.422(0.081) 18.509(0.416) – 18.210(0.281) – – – 16.253(0.052) – 15.437(0.018) – – – – 15.836(0.122) – – 15.110(0.028) – – – 15.954(0.012) 15.062(0.055) – – – 15.206(0.012) 15.731(0.012) – – 15.654(0.057) 15.767 – – – – – – 15.670(0.060) – – – – – – – – – – – 15.621(0.168) – – – – – – 1 – – 1 – – 1 – 4 – 4 – 1 – 8 – 1 – 2 1 8 9 9 continued. Table A.2.

Article number, page 16 of 17 A. Pastorello et al.: Luminous red nova AT 2019zhd, a new merger in M 31 1.8 = ASAS-SN = ACAM2 (Mauna + ALFOSC (La Palma, ⋆ + Inst. AFOSC (Cima Ekar, Asiago, + photometry. r o / 0.5 m ATLAS Telescope = IO:O (La Palma, Canary Islands, Spain); 3 fa05 camera (Hawaii Isl., USA); 8 + + Johnson-Bessell-Cousins observations from AAVSO, I , R , V , 1.82 m Copernico Telescope B = = 2.56 m Nordic Optical Telescope (NOT) = 2006); 10 Unﬁltered pointing data, scaled to Sloan- (0.048)(0.033) – – – – 11 11 ⋆ ZTF-Cam (Mt. Palomar, USA); 5 2.0 m Liverpool Telescope (LT) + 2.0m Faulkees North Telescope = = a, Japan); 2 (Tonry et al. 2018); A; see Kochanek et al. (2017)); 9 g 1.2 m S. Oschin Telescope = 21.14 – – – – 11 > StanCam (La Palma, Canary Islands, Spain). KAF-16803 CCD CCD (Cima Ekar, Asiago, Italy); 12 + + band data, converted to Sloan- c d in Sloan bands following the prescriptions of Jordi et al. ( ACAM1 (Haleakala, Hawaii Islands, USA); 7 + u g r i zw ATLAS- † Moravian G4-16000LC + -band data; w KAF-1001E CCD (Itagaki Astronomical Observatory, Yamagat 0.5 m ATLAS Telescope + = GPC cameras (Haleakala, Hawaii Islands, USA); 4 FLI ProLine PL230 CCD (LCOGT - Haleakala, Hawaii Islands, US 2.56 m Nordic Optical Telescope (NOT) + + = Pan-STARRS Date MJD ‡ 2020-03-112020-03-1258920.85 58919.782020-03-1658924.762020-03-172020-03-18 58925.84 –2020-03-18 58926.77 – 58926.77 – – 19.972(0.373) – 17.985(0.061) – 16.944 – – – – 18.198(0.185) 19.046(0.100) 19.329(0.402) – – 17.906(0.570) – 17.570(0.273) – 17.087(0.050) – – – – – – 13 11 12 11 2020-03-0758915.832020-03-0858916.452020-03-08 58916.76 – – – 19.408(0.283) – 17.543(0.036) – 16.729 17.527(0.242) – – – – 16.277(0.015) – – 2 1 11 telescope / 92 cm Schmidt Telescope / 67 continued. 35 cm F = = 1 0.16 m Brutus Telescope × Italy); 11 Additional information: TOCP and ANS collaboration (see, also, Table A.1), converte Canary Islands, Spain); 13 Loa, Hawaii Islands, USA); 6 4 m Pan-STARRS Telescopes Table A.2. Notes.

Article number, page 17 of 17