On the Progenitor System of the Type Iax Supernova 2014dt in M61
Ryan J. Foley, Schuyler D. Van Dyk, Saurabh W. Jha, Kelsey I. Clubb, Alexei V. Filippenko, Jon C. Mauerhan, Adam A. Miller, Nathan Smith
aa r X i v : . [ a s t r o - ph . H E ] D ec Draft version November 30, 2017
Preprint typeset using L A TEX style emulateapj v. 5/2/11
ON THE PROGENITOR SYSTEM OF THE TYPE Iax SUPERNOVA 2014dt IN M61
Ryan J. Foley , Schuyler D. Van Dyk , Saurabh W. Jha , Kelsey I. Clubb , Alexei V. Filippenko ,Jon C. Mauerhan , Adam A. Miller † , and Nathan Smith Draft version November 30, 2017
ABSTRACTWe present pre-explosion and post-explosion
Hubble Space Telescope images of the Type Iax su-pernova (SN Iax) 2014dt in M61. After astrometrically aligning these images, we do not detect anystellar sources at the position of the SN in the pre-explosion images to relatively deep limits (3 σ limits of M F438W > − . M F814W > − . M F435W = − . ± .
15 and M F814W = − . ± .
16 mag), theonly probable detected progenitor system in pre-explosion images of a SN Iax, and indeed, of anywhite dwarf supernova. SN 2014dt is consistent with having a C/O white-dwarf primary/helium-star companion progenitor system, as was suggested for SN 2012Z, although perhaps with a slightlysmaller or hotter donor. The data are also consistent with SN 2014dt having a low-mass red giant ormain-sequence star companion. The data rule out main-sequence stars with M init & M ⊙ and mostevolved stars with M init & M ⊙ as being the progenitor of SN 2014dt. Hot Wolf-Rayet stars are alsoallowed, but the lack of nearby bright sources makes this scenario unlikely. Because of its proximity( D = 12 Mpc), SN 2014dt is ideal for long-term monitoring, where images in ∼ Subject headings: galaxies—individual(M61), supernovae—general, supernovae—individual(SN 2014dt) INTRODUCTION
Type Iax supernovae (SNe Iax) are a recently definedclass of stellar explosion (Foley et al. 2013, hereafterF13). They are, in many ways, observationally similarto SNe Ia, having comparable spectra and thus compo-sitions (e.g., Li et al. 2003; Branch et al. 2005; Chornocket al. 2006; Jha et al. 2006; Foley et al. 2009, 2010a).However, SNe Iax are less energetic, with ejecta velocitiesnear maximum light 20–80% that of typical SNe Ia (e.g.,Narayan et al. 2011; F13; White et al. 2014). SNe Iaxalso tend to have lower luminosity than SNe Ia (e.g., Mc-Clelland et al. 2010; Stritzinger et al. 2014a,b), furtherindicating a low-energy explosion.One important difference between SNe Iax and SNe Iais that we have imaged the probable progenitor system ofa SN Iax (McCully et al. 2014a, hereafter, M14a), whileno progenitor system of a SN Ia has yet been directly ob-served. For SN 2012Z, M14a detected a luminous, bluesource in pre-explosion
Hubble Space Telescope ( HST ) Astronomy Department, University of Illinois at Urbana-Champaign, 1002 W. Green Street, Urbana, IL 61801, USA Department of Physics, University of Illinois Urbana-Champaign, 1110 W. Green Street, Urbana, IL 61801, USA IPAC/Caltech, Mail Code 100-22, Pasadena, CA 91125,USA Department of Physics and Astronomy, Rutgers, The StateUniversity of New Jersey, 136 Frelinghuysen Road, Piscataway,NJ 08854, USA Department of Astronomy, University of California, Berke-ley, CA 94720-3411, USA Jet Propulsion Laboratory, California Institute of Technol-ogy, 4800 Oak Grove Drive, MS 169-506, Pasadena, CA 91109,USA California Institute of Technology, Pasadena, CA 91125,USA Steward Observatory, University of Arizona, Tucson, AZ85721, USA † Hubble Fellow images at the SN position. Their favored interpretation isthat this source is the nondegenerate He companion starto a C/O white dwarf (WD), as originally predicted byF13 as the likely progenitor scenario for SNe Iax. How-ever, it is also possible that the light came from a massivestar that exploded to cause SN 2012Z or an accretion diskaround the exploding WD.An important prediction from both simple energeticarguments (Foley 2008; Foley et al. 2009, 2013; McCullyet al. 2014b) or from detailed explosion models (Jordanet al. 2012; Kromer et al. 2013; Fink et al. 2014) is thatat least some of the time, a SN Iax, if coming from a C/OWD, should leave behind a bound remnant star. Sincethe remnant retains a significant amount of radioactivematerial after the explosion (Kromer et al. 2013), thisstar will likely become quite luminous on the timescaleof years to decades (Bildsten et al., in preparation).Using
HST images that include the location ofSN 2008ha, a low-luminosity SN Iax (Foley et al. 2009,2010a; Valenti et al. 2009), from 4 years after the explo-sion, Foley et al. (2014) detected a luminous, red sourceat the position of the SN. Since these images came afterthe explosion, the source could be a thermally pulsatingasymptotic giant branch (TP-AGB) companion star or abound remnant star. If the sources detected at the posi-tions of SNe 2008ha and 2012Z are both companion stars,SNe Iax must have a diverse set of progenitor systems.Until recently, SN 2008ge was the only other SN Iaxwith pre-explosion
HST imaging (Foley et al. 2010b).These data are not particularly deep (3 σ limit of M V > − . Letter , we present deep pre- and post-explosion
HST images of SN 2014dt, a SN Iax discovered in M61at a distance of only 12.3 Mpc. By comparing the pre-and post-explosion images, we precisely determine theposition of the SN in the pre-explosion images. We donot detect any star at that position with a 3 σ limit of M F450W > − . OBSERVATIONS AND DATA REDUCTION
SN 2014dt was detected in M61 on 2014 October 29.8(all times are UT) at 13.6 mag by Nakano & Itagaki(2014) and promptly classified as a SN Iax by Ochneret al. (2014) from a spectrum obtained 2014 October31.2. The SN was past peak at discovery and there areno recent nondetections which constrain the date of ex-plosion.M61 is in the Virgo cluster and has a distance of12.3 Mpc ( µ = 30 . ± .
10 mag) as determined by withthe expanding photosphere method (EPM) applied toSN 2008in, also in M61 (Bose & Kumar 2014). Thisis consistent with the Tully-Fisher distance of 11.0 Mpc( µ = 30 . ± .
70 mag; Schoeniger & Sofue 1997), andthe redshift-derived distance (corrected for Virgo infall)of 13.1 Mpc (30 . ± .
16 mag). Here we assume theEPM distance modulus, but increase the uncertainty to0.24 mag, corresponding to the offset between the EPMand Tully-Fisher measurements; this range also includesthe redshift-derived distance.On 18.6 November 2014, we obtained a low-resolutionspectrum of SN 2014dt with the Kast double spectro-graph (Miller & Stone 1993) on the Shane 3 m telescopeat Lick Observatory. Standard CCD processing and spec-trum extraction were accomplished with IRAF . Thedata were extracted using the optimal algorithm of Horne(1986). Low-order polynomial fits to calibration-lampspectra were used to establish the wavelength scale, andsmall adjustments derived from night-sky lines in the ob-ject frames were applied. We employed our own IDL rou-tines to flux calibrate the data and remove telluric linesusing the well-exposed continua of spectrophotometricstandards (Wade & Horne 1988; Foley et al. 2003; Silver-man et al. 2012). The spectrum is presented in Figure 1.The position of SN 2014dt was imaged by HST /WFPC2 on 2001 July 27.10 (Program GO–9042; PI Smartt) in F450W (roughly B ) and F814W(roughly I ), each for 460 s. We obtained drizzledmosaics from the Hubble Legacy Archive.We also imaged SN 2014dt with HST /WFC3/UVIS inF438W (roughly B and well matched to the pre-explosionF450W image) on 2014 November 18.89 (Program GO–13683; PI Van Dyk). We took 20 short (20 s) exposuresso as to not saturate the SN.We combined exposures (including cosmic ray rejec-tion) using AstroDrizzle. We drizzled the images to thenative scale of WFC3, 0 . ′′
04 pixel − . We also ran theindividual flt images of the SN through Dolphot, an ex-tension of HSTPhot (Dolphin 2000), and measured thebrightness to be m F438W = 16 . ± .
001 mag. IRAF: the Image Reduction and Analysis Facility is dis-tributed by the National Optical Astronomy Observatory, which isoperated by the Association of Universities for Research in Astron-omy, Inc. (AURA) under cooperative agreement with the NationalScience Foundation (NSF). R e l a ti v e f λ SN 2014dtSN 2002cx +17 d
Fig. 1.—
Optical spectrum of SN 2014dt (black) compared tothat of SN 2002cx at +17 d (red).
Using 30 stars in common between the F450W pre-explosion and F438W post-explosion images, we com-puted a relative astrometric solution between the two im-ages and precisely determined the position of SN 2014dtin the pre-explosion image. The position of SN 2014dthas uncertainties of 0.13 and 0.10 pixels (0 . ′′
006 and0 . ′′ HST flt frames, usingthe suggested Dolphot parameters for WFPC2. We onlyconsider photometry with a “flag” of 0 and an “objecttype” of 1 (point sources) in our results.There is no source detected at the position ofSN 2014dt in either pre-explosion image. The closestsource, detected by Dolphot at 3 . σ with 25.61 magin F450W only, is 1 . ′′
64 (97 pc) to the southwest ofthe SN position. The next closest, detected at 3 . σ with 24.60 mag in F814W only, is 3 . ′′
21 (190 pc) to thesoutheast. The closest source detected in both bands(with 24.06 mag, 11 . σ at F450W; 23.47 mag, 9 . σ atF814W) is 3 . ′′
59 (210 pc) to the northeast. The 3 σ limiting magnitude corresponds to m F450W > . m F814W > . ANALYSIS
SN 2014dt is spectroscopically a SN Iax. The spectrumpresented in Figure 1 is compared to one of SN 2002cx,the prototypical member of the class (Filippenko 2003;Li et al. 2003), at 17 days after maximum brightness.They are nearly identical except SN 2014dt appears tohave a slightly lower ejecta velocity. Importantly, thereis no indication of hydrogen in the spectrum.At D = 12 . M ≈ − . M ≈ −
18 mag, also consis-tent with other SNe Iax.Although it is difficult to measure host-galaxy redden-rogenitor of SN 2014dt 3
Fig. 2.—
HST /WFC3 F438W image of SN 2014dt (b) and its surrounding environment. Aligned pre-explosion
HST /WFPC2 F450W(a) and F814W (c) PC1-chip images are also shown to the same scale. The position of SN 2014dt in the pre-explosion images is marked.No star is detected at the SN location in either pre-explosion image. ing from SN Iax colors (F13), SN 2014dt does not appearto have any host-galaxy reddening. The SN has a rea-sonably blue continuum and there is no Na D absorptionin our high signal-to-noise ratio low-resolution spectrum.This is consistent with the expected reddening from arelatively clean part of a face-on spiral galaxy.No sources were detected at the position of SN 2014dtin pre-explosion
HST images. With our given distancemodulus ( µ = 30 .
45 mag), and our Milky Way and host-galaxy reddening estimates ( E ( B − V ) = 0 .
02 and 0 mag,respectively), the 3 σ limits correspond to M F450W > − . M F814 > − . M F435W = − . ± .
15 and M F814W = − . ± .
16 mag). Formally, the SN 2014dt limits are0.40 mag deeper than the SN 2012Z progenitor systemdetection in F435W/F450W. However, this is only 1.4 σ different when including uncertainties in the SN 2012Zprogenitor system photometry and the distance to M61.We therefore cannot rule out a progenitor system forSN 2014dt that is similar to that of SN 2012Z.Our limits are not quite deep enough to rule outa progenitor system similar to that of the source co-incident with SN 2008ha at late times ( M F814W = − . ± .
22 mag). Moreover, a system similar to thatof V445 Pup (the only known He nova; Kato & Hachisu2003), thought to be a C/O WD with a He-star com-panion (Kato et al. 2008; Woudt et al. 2009), would bebarely undetected in the pre-explosion images. There-fore, SN 2014dt could have a progenitor system similarto any of these comparison objects.Nonetheless, a large number of potential progenitorsystems have been excluded. Using a projected off-set of 2.3 kpc, and a metallicity map of M61 (Pilyu-gin et al. 2014), we find a metallicity at the SN po-sition of 12 + log(O / H) = 8 .
68, which on the scaleadopted by Pilyugin et al. (2014) is roughly 1.5 times so-lar. From single-star evolutionary models at this metal-licity (Bertelli et al. 2009), the pre-explosion limits areinconsistent with systems containing a red giant (RG)or horizontal branch (HB) star with M init & ⊙ ormain-sequence stars with M init &
16 M ⊙ . Of course, these kinds of stars are unlikely to have been the pro-genitor because of the lack of hydrogen in the SN spec-trum; nonetheless, these limits apply to companion starsas well.The data do not exclude very hot Wolf-Rayet stars,which can be relatively faint at optical wavelengths(Shara et al. 2013). However, Wolf-Rayet starswould likely be physically close to other massive stars.SN 2014dt exploded in a region that is ∼
100 pc fromany detected sources and &
200 pc from any particu-larly bright sources, making the Wolf-Rayet scenario lesslikely. DISCUSSION AND CONCLUSIONS
Using
HST images obtained 13 years before SN 2014dt,a clear SN Iax, we place limits on its progenitor system.This is the third SN Iax with pre-explosion
HST imagesand the second deepest (after SN 2012Z).With these data, we are able to exclude many massivestars as progenitors for SN 2014dt. Specifically, main-sequence stars with M init &
16 M ⊙ and RG/HB starswith M init & ⊙ are excluded. Some Wolf-Rayet starsare still allowed, but the lack of bright nearby stars makea Wolf-Rayet progenitor unlikely.If the progenitor system of SN 2012Z were in M61,we would perhaps marginally detect it. Since we didnot detect any sources at the position of SN 2014dt, itsprogenitor system was likely fainter (in B ) than that ofSN 2012Z.Many of the He stars in the He-star–C/O-WD progen-itor models of Liu et al. (2010) have roughly the sameluminosity but varying temperature. Since the He-starspectral energy distribution peaks in the ultraviolet, theeffective temperature dictates the brightness in the HST bands. A large region of the He-star parameter space isstill allowed by the current data. However, future data,particularly at shorter UV wavelengths, should be able toplace strong constraints on the existence of such a star.The pre-explosion data also do not exclude a red giantstar similar to what may be the companion of SN 2008ha,although the data are within ∼
35 20 10 5 3 2Temperature (10 K)−3.0−3.5−4.0−4.5−5.0−5.5−6.0−6.5 M F W ( m a g ) H e s t a r SN 2014dtProgenitorExcluded
V445Pup 12Z 08ha O • O • O •
10 M O • Fig. 3.—
Hertzsprung-Russell diagram for SN Iax progenitorsystems. SN 2014dt is not detected, and the region of the H-R diagram excluded is shaded dark gray. The allowed region iswhite. The black curve represents the single-band 3 σ limits forblack body sources. The gray gradient band indicates the addi-tional 1 σ distance-modulus uncertainty. The white region is al-lowed for the progenitor system. The SN 2012Z progenitor system(black cross; M14a) and the V445 Pup progenitor system (browncross; F814W magnitude estimated from temperature and V -bandmagnitude; Woudt et al. 2009) are shown. The source coincidentwith SN 2008ha in post-explosion images is also marked with ablack cross. This source may be the companion star or the boundremnant star (Foley et al. 2014). Also plotted are stellar evolutiontracks for stars with initial masses of 4, 6, 8, and 10 M ⊙ (Bertelliet al. 2009) and the region predicted for one set of He-star progen-itor models from Liu et al. (2010, blue region). be somewhat ambiguous.There are now three SNe Iax with reasonably deeppre-explosion images. A probable luminous blue progen-itor system was detected for SN 2012Z (M14a), the SN with the deepest pre-explosion data, while no progenitorwas detected for either SN 2008ge (Foley et al. 2010b)or 2014dt. Foley et al. (2014) detected a luminous redsource coincident with SN 2008ha 4 years after the ex-plosion that could be a companion star or the boundremnant of the progenitor WD. These four SNe inde-pendently and jointly rule out massive stars as SN Iaxprogenitors.SN 2014dt, being at D = 12 . ∼
20 Mpc (SNe 2008ge, 2008ha, 2010ae, 2010el,and 2014dt), SN 2014dt is the best SN for such observa-tions; SNe 2010ae and 2010el have significant reddening,SN 2008ge was close to a bright galactic nucleus, andSN 2008ha was the faintest and most distant. SNe Iaxfade by ∼
12 mag in the first two years (McCully et al.2014b). SN 2014dt peaked at M ≈ −
18 mag, and willbe roughly as bright as the pre-explosion image limits intwo years. Around that time, deep images may be ableto detect emission from either a companion star or theWD remnant.
Facility:
Hubble Space Telescope (WFPC2, WFC3),Lick Shane (Kast)Based on observations made with the NASA/ESA
Hubble Space Telescope , obtained at the Space TelescopeScience Institute (STScI), which is operated by the As-sociation of Universities for Research in Astronomy, Inc.,under NASA contract NAS 5–26555. These observa-tions are associated with and funded through programGO-13683. Data were obtained through the HubbleLegacy Archive, which is a collaboration between theSpace Telescope Science Institute (STScI/NASA), theSpace Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre(CADC/NRC/CSA).SN Iax research at the University of Illinois issupported in part through NASA/
HST grant GO–12999.01. This research at Rutgers University was sup-ported through NASA/
HST grant GO–12913.01 and Na-tional Science Foundation (NSF) CAREER award AST–0847157 to S.W.J. A.A.M. acknowledges support for thiswork by NASA from Hubble Fellowship grant HST–HF–51325.01, awarded by STScI. A.V.F.’s group at U.C.Berkeley is supported by the Christopher R. RedlichFund, the TABASGO Foundation, and NSF grant AST–1211916.
REFERENCESBertelli, G., Nasi, E., Girardi, L., & Marigo, P. 2009, A&A, 508,355 []Bose, S., & Kumar, B. 2014, ApJ, 782, 98 []Branch, D., Baron, E., Hall, N., Melakayil, M., & Parrent, J.2005, PASP, 117, 545 []Chornock, R., Filippenko, A. V., Branch, D., Foley, R. J., Jha, S.,& Li, W. 2006, PASP, 118, 722 []Dolphin, A. E. 2000, PASP, 112, 1383 []Filippenko, A. V. 2003, in From Twilight to Highlight: ThePhysics of Supernovae, ed. W. Hillebrandt & B. Leibundgut,171 []Fink, M., et al. 2014, MNRAS, 438, 1762 []Foley, R. J. 2008, Central Bureau Electronic Telegrams, 1576, 2 [] Foley, R. J., Brown, P. J., Rest, A., Challis, P. J., Kirshner, R. P.,& Wood-Vasey, W. M. 2010a, ApJ, 708, L61 []Foley, R. J., et al. 2013, ApJ, 767, 57 []——. 2009, AJ, 138, 376 []Foley, R. J., McCully, C., Jha, S. W., Bildsten, L., Fong, W.-f.,Narayan, G., Rest, A., & Stritzinger, M. D. 2014, ApJ, 792, 29[]Foley, R. J., et al. 2003, PASP, 115, 1220 []——. 2010b, AJ, 140, 1321 []Horne, K. 1986, PASP, 98, 609 []Jha, S., Branch, D., Chornock, R., Foley, R. J., Li, W., Swift,B. J., Casebeer, D., & Filippenko, A. V. 2006, AJ, 132, 189 [] rogenitor of SN 2014dt 5rogenitor of SN 2014dt 5