The Type IIb Supernova 2013df and Its Cool Supergiant Progenitor
Schuyler D. Van Dyk, WeiKang Zheng, Ori D. Fox, S. Bradley Cenko, Kelsey I. Clubb, Alexei V. Filippenko, Ryan J. Foley, Adam A. Miller, Nathan Smith, Patrick L. Kelly, William H. Lee, Sagi Ben-Ami, Avishay Gal-Yam
aa r X i v : . [ a s t r o - ph . S R ] D ec To Appear in the Astronomical Journal
Preprint typeset using L A TEX style emulateapj v. 5/2/11
THE TYPE IIB SUPERNOVA 2013DF AND ITS COOL SUPERGIANT PROGENITOR
Schuyler D. Van Dyk , WeiKang Zheng , Ori D. Fox , S. Bradley Cenko , Kelsey I. Clubb , AlexeiV. Filippenko , Ryan J. Foley , Adam A. Miller , Nathan Smith , Patrick L. Kelly , William H. Lee , SagiBen-Ami , and Avishay Gal-Yam To Appear in the Astronomical Journal
ABSTRACTWe have obtained early-time photometry and spectroscopy of Supernova (SN) 2013df in NGC 4414.The SN is clearly of Type IIb, with notable similarities to SN 1993J. From its luminosity at secondarymaximum light, it appears that less Ni ( . . M ⊙ ) was synthesized in the SN 2013df explosion thanwas the case for the SNe IIb 1993J, 2008ax, and 2011dh. Based on a comparison of the light curves, theSN 2013df progenitor must have been more extended in radius prior to explosion than the progenitorof SN 1993J. The total extinction for SN 2013df is estimated to be A V = 0 .
30 mag. The metallicityat the SN location is likely to be solar. We have conducted
Hubble Space Telescope ( HST ) Target ofOpportunity observations of the SN with the Wide Field Camera 3, and from a precise comparisonof these new observations to archival
HST observations of the host galaxy obtained 14 years priorto explosion, we have identified the progenitor of SN 2013df to be a yellow supergiant, somewhathotter than a red supergiant progenitor for a normal Type II-Plateau SN. From its observed spectralenergy distribution, assuming that the light is dominated by one star, the progenitor had effectivetemperature T eff = 4250 ±
100 K and a bolometric luminosity L bol = 10 . ± . L ⊙ . This leads toan effective radius R eff = 545 ± R ⊙ . The star likely had an initial mass in the range of 13–17 M ⊙ ; however, if it was a member of an interacting binary system, detailed modeling of the systemis required to estimate this mass more accurately. The progenitor star of SN 2013df appears to havebeen relatively similar to the progenitor of SN 1993J. Subject headings: galaxies: individual (NGC 4414) — stars: evolution — supernovae: general —supernovae: individual (SN 2013df) INTRODUCTION
A core-collapse supernova (SN) is the final stage ofevolution for stars with initial mass M ini & M ⊙ (e.g.,Woosley et al. 2002). The compact remnant of this ex-plosion is thought to be either a neutron star or a blackhole. Stars which reach the terminus of their evolutionwith most of their hydrogen-rich envelope intact — sin-gle red supergiants (RSGs) — produce Type II (specif-ically Type II-Plateau) supernovae (SNe). As the enve-lope is stripped away, either through vigorous mass lossor via mass transfer in a binary system, the results arethe hydrogen-free, yet helium-rich, Type Ib SNe and thehydrogen-free and helium-poor (or helium-free) Type IcSNe; see Filippenko (1997) for a review of SN classifica- Spitzer Science Center/Caltech, Mail Code 220-6, Pasadena,CA 91125; email: [email protected]. Department of Astronomy, University of California, Berke-ley, CA 94720-3411. Astrophysics Science Division, NASA Goddard Space FlightCenter, Mail Code 661, Greenbelt, MD 20771. Harvard-Smithsonian Center for Astrophysics, 60 GardenStreet, Cambridge, MA 02138. Department of Astronomy, University of Illinois, Urbana-Champaign, IL 61801. Jet Propulsion Laboratory, MS 169-506, Pasadena, CA91109. Hubble Fellow. Steward Observatory, University of Arizona, Tucson, AZ85720. Instituto de Astronom´ıa, Universidad Nacional Aut´onomade M´exico, Apdo. Postal 70-264, Cd. Universitaria, M´exico DF04510, M´exico Benoziyo Center for Astrophysics, The Weizmann Instituteof Science, Rehovot 76100, Israel. tion. Intermediate to the SNe II and SNe Ib are theType IIb, which retain a low-mass ( . M ⊙ ) hydro-gen envelope prior to explosion. SNe IIb are intrinsi-cally rare ( ∼ i absorption lines typical of SNe Ib (e.g., Filippenko et al.1993; Chornock et al. 2011), with broad H emission reap-pearing in the nebular phase (e.g., Filippenko et al. 1994;Matheson et al. 2000; Taubenberger et al. 2011; Shivverset al. 2013). Although rarely seen owing to its very shortduration early in the evolution of a SN, a rapid declineafter an initial peak has been observed among SNe IIb(e.g., Richmond et al. 1994; 1996; Roming et al. 2009; Ar-cavi et al. 2011). This is interpreted as adiabatic coolingafter the SN shock has broken out through the star’s sur-face; the duration of this cooling is governed primarily bythe radius of the progenitor (Chevalier & Fransson 2008;Nakar & Sari 2010; Rabinak & Waxman 2011; Bersten et Van Dyk et al.al. 2012). The SN light curves reach a secondary max-imum, as a result of thermalization of the γ -rays andpositrons emitted during the radioactive decay of Niand Co, followed by a smooth decline. The light-curveshapes resemble those of SNe Ib (Arcavi et al. 2012).As the number of all core-collapse SNe having directlyidentified progenitor stars is very small ( ∼
20 at the timeof this writing), we have been extraordinarily fortunatethat, up to this point, the progenitors of three SNe IIbhave been identified, including SN 1993J (Aldering et al.1994; Van Dyk et al. 2002; Maund et al. 2004; Maund& Smartt 2009), SN 2008ax in NGC 4490 (Crockett etal. 2008), and SN 2011dh in Messier 51 (Maund et al.2011; Van Dyk et al. 2011). Each of these stars exhibitsdistinctly different properties, although all show indica-tions of envelope stripping prior to explosion: the SN1993J progenitor has been characterized as a K-type su-pergiant, with initial mass ∼ M ⊙ (Van Dyk et al.2002; Maund et al. 2004); for SN 2008ax it was difficultto fit a single supergiant to the observed colors (Li et al.2008; Crockett et al. 2008), and the possible initial massrange for the progenitor is large, ∼ M ⊙ ; and forSN 2011dh we now know that the ∼ ∼ M ⊙ (Maund et al. 2011; Murphy et al. 2011; Berstenet al. 2012). In addition, Ryder et al. (2006) may havedetected at very late times the massive companion tothe SN IIb 2001ig in NGC 7424 (Silverman et al. 2009).Furthermore, Chevalier & Soderberg (2010) have dividedSN IIb progenitors into those that are compact (radius ∼ cm; e.g., SN 2008ax) and those that are extended( ∼ cm; SN 1993J), although SN 2011dh appears tobe an intermediate case (Horesh et al. 2013). Each newexample provides us with an increased understanding ofthis SN subtype and of the massive stars which give riseto these explosions.In this paper we consider SN 2013df in NGC 4414,shown in Figure 1. It was discovered by Ciabattari etal. (2013) on June 7.87 and 8.83 (UT dates are usedthroughout). Cenko et al. (2013) provided spectroscopicconfirmation on June 10.8 that it is a Type II SN. The re-semblance to SN 1993J at early times suggested to Cenkoet al. that SN 2013df would evolve to be a SN IIb. Herewe present early-time photometric and spectroscopic ob-servations of SN 2013df, which demonstrate that it isindeed a SN IIb. In a telegram, we presented a prelimi-nary identification of three progenitor candidates for theSN (Van Dyk et al. 2013a). We provide here a much bet-ter identification, through high-resolution imaging of theSN, and show that none of the three sources turned outactually to be at the SN position. We will characterizethe nature of the probable progenitor. The host galaxy,NGC 4414, is a nearby, isolated, flocculent spiral galaxy,with an inclination of 55 ◦ (Vallejo et al. 2002). Thornley& Mundy (1997) measured a global star formation rateof 1.3 M ⊙ yr − , comparable to that of other Sc galaxies.SN 2013df is at a nuclear offset of 32 ′′ E, 14 ′′ N, along anouter spiral arm. The galaxy was also host to the likelyType Ia SN 1974G (e.g., Schaefer 1998). We adopt theCepheid-based distance modulus µ = 31 . ± .
05 mag(distance 16 . ± . OBSERVATIONS
Early-Time Photometry
We have observed the SN using the 0.76 m KatzmanAutomatic Imaging Telescope (KAIT; Filippenko et al.2001) at Lick Observatory in
BV RI between 2013 June13.7 and July 18.7. Point-spread function (PSF) photom-etry was applied using the DAOPHOT (Stetson 1987)package from the IDL Astronomy User’s Library . Theinstrumental magnitudes and colors of the SN were trans-formed to standard Johnson-Cousins BV RI using twostars in the SN field which are in the Sloan Digital SkySurvey (SDSS) catalog, and by following this prescrip-tion to convert from the SDSS to the Johnson-Cousinssystem.Data in rizJH were also obtained with the multi-channel Reionization And Transients InfraRed camera(RATIR; Butler et al. 2012) mounted on the 1.5-mHarold L. Johnson telescope at the Mexican Observa-torio Astron´omico Nacional on Sierra San Pedro M´artirin Baja California, M´exico (Watson et al. 2012). Typi-cal observations include a series of 60-s exposures, withdithering between exposures. RATIR’s fixed infrared(IR) filters cover half of their respective detectors, au-tomatically providing off-target IR sky exposures whilethe target is observed in the neighboring filter. Master IRsky frames were created from a median stack of off-targetimages in each IR filter. No off-target sky frames wereobtained with the optical CCDs, but the small galaxy sizeand sufficient dithering allowed for a sky frame to be cre-ated from a median stack of all the images in each filter.Flat-field frames consist of evening sky exposures. Giventhe lack of a cold shutter in RATIR’s design, IR “dark”frames are not available. Laboratory testing, however,confirms that dark current is negligible in both IR detec-tors (Fox et al. 2012). The data were reduced, coadded,and analyzed using standard CCD and IR processingtechniques in IDL and Python, utilizing online astrome-try programs SExtractor (Bertin & Arnouts 1996) and
SWarp . Calibration was performed using a single com-parison star in the SN field that also has reported fluxesin both 2MASS (Skrutskie et al. 2006) and the SDSSData Release 9 Catalogue (Ahn et al. 2012).We did not yet possess template images of the hostgalaxy (i.e., prior to the SN discovery or when the SNhas faded to invisibility) to subtract from the imageswith the SN present in each band. Consequently, thephotometry presented here should be considered prelim-inary; however, the SN is far from the main light fromthe host galaxy, so results including template subtractionprior to photometry might not substantially differ fromwhat we present here. Early-Time Spectroscopy
We have obtained a number of spectra of the SNat early times, using the Kast spectrograph (Miller & http://idlastro.gsfc.nasa.gov/contents.html. N 2013df and Its Progenitor 3Stone 1993) on the Lick Observatory 3-m Shane tele-scope and the DEep Imaging Multi-Object Spectrograph(DEIMOS; Faber et al. 2003) on the Keck-II 10-m tele-scope. Ultraviolet (UV) spectroscopy has also been ob-tained using the
Hubble Space Telescope ( HST ) SpaceTelescope Imaging Spectrograph (STIS) as part of pro-gram GO-13030 (PI: A. V. Filippenko). The results ofthese
HST observations will be presented in a future pa-per, together with the bulk of the ground-based opticalspectra. However, here we present and analyze a rep-resentative spectrum obtained on 2013 July 11.2 withDEIMOS using the 600 l mm − grating. HST
Imaging
The region of the host galaxy containing the SN sitewas observed with the
HST
Wide Field Planetary Cam-era 2 on 1999 April 29 by program GO-8400 (PI: K. Noll),as part of the Hubble Heritage Project. The bands usedwere F439W (two individual images with 40 s exposuretimes and two with 1000 s), F555W (four 400-s expo-sures), F606W (two 60-sec exposures), and F814W (two40-s and four 400-s exposures). The F555W and F814Wdata were combined with images obtained by programsGO-5397 and GO-5972 at an earlier time for the rest ofthe galaxy, and drizzled into mosaics in each band at thescale 0 . ′′
05 pix − by Holwerda et al. (2005).We have also observed the SN on 2013 July 15 with HST using the Wide Field Camera 3 (WFC3) UVISchannel in F555W, as part of our Target of Opportunityprogram GO-12888 (PI: S. Van Dyk). The observationsconsisted of 28 5-s exposures; the short exposure timewas intended to avoid saturation in each frame by thebright SN. ANALYSIS
Light Curves
In Figure 2 we display the early-time KAIT andRATIR light curves in all bands for SN 2013df. Wealso include the very early V measurement from June11.202 by Stan Howerton . We compare these curveswith those at BV RIJH for SN 1993J (Richmond et al.1994; 1996; Matthews et al. 2002) and for SN 2011dh(Arcavi et al. 2011; Van Dyk et al. 2013b; Ergon et al.2013). We also compare the z -band light curve of SN2008ax (Pastorello et al. 2008). The curves for the com-parison SNe were adjusted in time and relative brightnessto match the curves of SN 2013df, particularly at the sec-ondary maximum in each band. Clearly, from its overallphotometric similarity with the other SNe, SN 2013dfappears to be a SN IIb; see Arcavi et al. (2012) for ageneral description of SN IIb light-curve shapes. Whatis most notable from the comparison is that the post-shock-breakout cooling of SN 2013df occurred at a laterepoch in all bands, relative to that of SN 1993J. Thepost-breakout decline of SN 2011dh occurred at an evenearlier relative epoch (e.g., Arcavi et al. 2011; Bersten etal. 2012).We show in Figure 3 the absolute V -band light curveof SN 2013df, relative to those of SNe 1993J, 2008ax, and2011dh. (We do not make this comparison based on bolo-metric or pseudo-bolometric luminosity, since, as noted, we consider the present photometry of SN 2013df to bepreliminary.) The light curve of SN 2008ax is from Pa-storello et al. (2008), rather than from Taubenberger etal. (2011), since the former has somewhat more completecoverage through the secondary maximum. The epochsof the secondary maxima in V for SNe 1993J, 2008ax,and 2011dh are from Richmond et al. (1996), Pastorelloet al. (2008), and Ergon et al. (2013), respectively. Thisepoch for SN 2013df was determined by comparing itslight curve to that of each of the three comparison SNe.The light curves have all been adjusted for extinction andfor the distances to their host galaxies (distance to SN2013df, above; 3.6 Mpc for SN 1993J, Freedman et al.2001; 8.4 Mpc for SN 2011dh, Van Dyk et al. 2013b; and9.6 Mpc for SN 2008ax, Pastorello et al. 2008). We dis-cuss below our estimate of the extinction for SN 2013df.Ergon et al. (2013) have estimated the total extinction forSN 1993J and SN 2011dh to be A V ≈ .
53 and A V ≈ . A V = 0 .
93 mag for SN 2008ax (all assuming a Cardelli,Clayton, & Mathis 1989 reddening law and R V = 3 . Ni mass (e.g.,Perets et al. 2010). For comparison, the Ni producedin the comparison SNe was ∼ . . M ⊙ , ∼ . . M ⊙ , and ∼ . M ⊙ in SN 2008ax (Pastorello et al.2008; Taubenberger et al. 2011), SN 1993J (Young et al.1995; Richardson et al. 2006), and SN 2011dh (Berstenet al. 2012), respectively.From the overall comparison of the SN 2013df lightcurves in Figures 2 and 3 with the light curves of SNe1993J and 2011dh, we can make an estimate of the ex-plosion date based, in particular, on when SN 2013dfappears to have reached secondary maximum in BV RI .We find that all of the light curves indicate that this dateis JD 2,456,447.8 ± .
5, or about June 4.3 (indicated inFig. 2). This is certainly consistent with the earliest dis-covery epoch of June 7.87 (JD 2,456,451.37). We areunable to constrain the explosion date based on KAITSN search monitoring, since the observation of the hostgalaxy with KAIT prior to discovery was on May 25 (i.e.,about 10 days before discovery), at a limiting (unfiltered)magnitude of 18.5.
Spectrum
In Figure 4 we show the Keck/DEIMOS spectrum ofSN 2013df and a comparison with spectra of the SNe IIb1993J (Filippenko et al. 1993), 2008ax (Pastorello et al.2008; obtained from SUSPECT ; the data are also avail-able at WISeREP ), and 2011dh (available from WIS-eREP), all at nearly the same age. The SN 1993J ex-plosion date was 1993 March 27.5 (Lewis et al. 1994), sothe spectrum of SN 1993J from April 30 shown in thefigure is at age ∼
34 d. The age of the spectrum of SN2008ax is ∼
30 d (Pastorello et al. 2008). The age ofthe SN 2011dh spectrum is ∼
28 d (assuming an explo-sion date of 2011 May 31.275; Arcavi et al. 2011). Fromour estimate of the explosion date, above, the SN 2013dfspectrum shown in Figure 4 was obtained on day ∼ http://suspect.nhn.ou.edu/ ∼ suspect/. Van Dyk et al.This spectrum bears a greater similarity with that of SN1993J than SN 2008ax. Several of the He i lines, notablythe λ α emission profile, ap-pear to have been stronger in the SN 1993J spectrum(even at a somewhat earlier age) than for SN 2013df.This could imply that the H layer for SN 2013df mayhave been more substantial (larger mass, larger radius)at explosion, as suggested as well by the post-breakoutlight-curve behavior of SN 2013df, relative to SN 1993J.Visible in the spectrum of SN 2013df are absorptionfeatures due to Na i D. In Figure 5 we show these featuresafter the overall continuum in the spectrum has beennormalized. One pair of lines appears to be weakly vis-ible at effectively zero redshift, which would correspondto the Galactic foreground extinction contribution; theseare indicated as “MW” in the figure. Another pair isfar more distinct, at a redshift of 0.002874, indicated as“Host” in the figure. This is consistent with the red-shift for NGC 4414, 0.002388, given by NED . We havemeasured the equivalent width (EW) of each of the Na i features, D and D , for each system to be EW( D ) =0 . ± .
011 ˚A and EW( D ) = 0 . ± .
001 ˚A from theGalactic system, for a total EW(Na i ) = 0.146 ˚A. Forthe host-galaxy system, EW( D ) = 0 . ± .
002 ˚A andEW( D ) = 0 . ± .
001 ˚A, for a total EW(Na i ) = 0.691˚A. The ratio EW( D )/EW( D ) is 1.98 for the Galacticcomponent and 1.67 for the host-galaxy component. Theratio of the oscillator strengths of these two lines is 2.0;this intrinsic value is approached only at the lowest opti-cal depths (Munari & Zwitter 1997). We can see that theEW(Na i ) [host] is ∼ i ) [Galac-tic]. If we assume the Galactic foreground extinctionestimated by Schlafly & Finkbeiner (2011; A V = 0 . A V = 0 .
25 mag.Poznanski et al. (2011) have stressed that one can-not accurately infer interstellar visual extinction fromEW(Na i ) in low-resolution spectra. The resolution ofthe Keck DEIMOS spectrum may be just at the limit,however, where we can use the more accurate relationsfrom Poznanski et al. (2012). If we apply their Equa-tion 7 for EW( D ), we obtain E ( B − V ) = 0 . ± . D ), we obtain E ( B − V ) = 0 . ± .
04 mag; and from their Equation9 for the total EW( D + D ), E ( B − V ) = 0 . ± . E ( B − V ) = 0 . ± .
01 mag. If weassume the Cardelli, Clayton, & Mathis (1989) redden-ing law, with R V = 3 .
1, then the host-galaxy extinctionis A V = 0 . ± .
04 mag. (We note that Poznanski et al.2012 assume the older, somewhat higher values of Galac-tic extinction from Schlegel et al. 1998.) This estimateof the host-galaxy extinction is quite consistent with theone we made above, which we adopt. We therefore as-sume hereafter that the total (Galactic foreground plushost galaxy) extinction for SN 2013df is A V = 0 .
30 mag.We adopt an uncertainty in the extinction of ± .
05 mag.
Progenitor We display in Figures 6a and 6b the subregion of theWFPC2 mosaic around the SN position at F555W andF814W, respectively. We show the combination of allthe WFC3 exposures in Figure 6c. From a precise com-parison between the pre-explosion WFPC2 images andpost-explosion WFC3 images, we can identify the SNprogenitor. We had previously attempted an identifi-cation of the star (Van Dyk et al. 2013a), but this wasemploying much lower-resolution, ground-based imagesof the SN. None of the three objects that we had pre-viously nominated as candidates, numbered in Figure 6,turned out to be the likely SN progenitor. Using 11 starsin common between the WFPC2 and WFC3 images, weastrometrically registered the images to an accuracy of0.11 drizzled WFPC2 pixel, or 5.5 milliarcsec, and found,instead, that the SN corresponds to the position of thestar indicated by the tick marks in Figures 6a and 6b.We therefore identify this star as the likely progenitor ofSN 2013df.We extracted photometry for the source from the pre-explosion WFPC2 images using Dolphot v2.0 (Dolphin2000). (We disregarded the short F439W, F606W, andF814W exposures, since the signal-to-noise ratio is quitelow in these.) The progenitor location is in the WFPC2chip 3. The Dolphot output indicates that the progenitoris likely stellar, with an “object type” of 1; the “sharp-ness” parameter, at − .
37, is slightly outside the accept-able range ( − . − .
25. We therefore consider it most likelythat this is a well-resolved stellar object. The star isnot detected by Dolphot in the F439W images (nor is itdetectable upon visual inspection of the images). How-ever, we estimate a 3 σ upper limit to its detection. TheDolphot output automatically includes the transforma-tion from HST flight-system magnitudes to the corre-sponding Johnson-Cousins magnitudes (Holtzman et al.1995). We present the Dolphot results for the progenitorin Table 1. We also show the resulting spectral energydistribution (SED) of the star in Figure 7.To our knowledge, no measurement exists of the metal-licity in the vicinity of the SN position (such as fromspectroscopy of nearby H ii regions). So, we are un-able to quantify accurately the metallicity at the SN sitewith available data. However, we can provide at leastan approximate estimate, based on the assumption thatthe abundance gradient in the host spiral galaxy followsthe same behavior as for the sample of spirals analyzedby Zaritsky et al. (1994). After deprojecting NGC 4414from its inclination and position angle (56 ◦ and 160 ◦ , re-spectively; Jarrett et al. 2000), we find that SN 2013dfoccurred ∼ ′′ (or ∼ . / H] ≈ .
8, which is comparable tothe solar abundance (8 . ± .
05; Asplund et al. 2009).Therefore, it is reasonable to assume that the SN site isof roughly solar metallicity.We compared the observed SED of the progenitor withsynthetic SEDs derived from MARCS (Gustafsson etN 2013df and Its Progenitor 5al. 2008) supergiant model stellar atmospheres . Themodel atmospheres are spherical, with standard compo-sition at solar metallicity, surface gravity log g = 1 .
0, andmicroturbulence of 5 km s − . The spherical-geometrymodel atmospheres were computed, generally, for starswith masses 0.5, 1.0, 2.0, 5.0, and 15 M ⊙ ; we chosemodels for this last mass as most appropriate for thelikely massive progenitor, given what has been inferredfor the progenitor initial masses for other SNe IIb. Wefound that the synthetic V − I colors, in particular, de-rived from models at a given temperature are essentiallymass-independent (they differ by ∼ .
03 mag). The mod-els were reddened based on the assumed total extinctionfor SN 2013df, above, assuming the Cardelli, Clayton,& Mathis (1989) reddening law, and were normalized at V . The model that provided the best comparison withthe photometry has T eff = 4250 K, which we show inFigure 7. Warmer and cooler models did not comparewell with the observations. We therefore adopt this ef-fective temperature for the progenitor, with a generousuncertainty of ±
100 K.The absolute V magnitude of the progenitor, correctedfor the assumed distance modulus and extinction to theSN, is M V = − . ± .
10. The uncertainty arises fromthe uncertainties in the photometry, the extinction, andthe distance modulus, all added in quadrature. Theuncertainty in the transformation from flight-system toJohnson-Cousins magnitudes is ≪ .
01 mag (Holtzmanet al. 1995). The bolometric correction obtained fromthe MARCS model at T eff = 4250 K is BC V = − . BC V , resulting from the un-certainty in T eff , is ∼ .
05 mag. Including this uncer-tainty, the bolometric magnitude is M bol = − . ± . L bol = 10 . ± . L ⊙ .We show the locus for the progenitor in a Hertzsprung-Russell (HR) diagram in Figure 8. DISCUSSION AND CONCLUSIONS
The SN 2013df progenitor’s position in the HR diagramis significantly blueward of the RSG terminus of modelmassive-star evolutionary tracks with rotation, such asthe 15 M ⊙ track (Ekstr¨om et al. 2012) in Figure 8. Thisindicates that the star likely has had its envelope some-what stripped by some mechanism. The expectation sofar for the progenitor scenario for SNe IIb is an inter-acting massive star binary system, such as for SN 1993J(e.g., Podsiadlowski et al. 1993; Maund et al. 2004) andSN 2011dh (Bersten et al. 2012; Benvenuto et al. 2013).(However, see Crockett et al. 2008 for other possibili-ties regarding the progenitor of SN 2008ax.) A binarychannel is also strongly favored for SNe IIb, in general,based on statistical arguments accounting for their ob-served relative rate among core-collapse SNe (Smith et al.2011), as well as on the small ejecta masses and H/Heline ratios compared to detailed models (Dessart et al.2011; Hachinger et al. 2012). A blue binary companionis expected to survive the explosion for SNe IIb (Maundet al. 2004; Benvenuto et al. 2013). The locus of theSN 2013df progenitor in the HR diagram is quite similarto, albeit possibly somewhat less luminous than (thoughwithin the uncertainties), that of the supergiant progen- http://marcs.astro.uu.se/. itor of SN 1993J (Maund et al. 2004; see also Fig. 8).We can attempt to assign an initial mass to the pro-genitor, which, following Van Dyk et al. (2011) for SN2011dh, would have been ∼ M ⊙ , based on com-paring the locus in T eff and L bol to the correspondingvalues of the 15 and 20 M ⊙ tracks in the figure. Onthe other hand, adopting the approach of Maund et al.(2011), of basing the likely initial mass on the luminos-ity at the terminus of the available tracks, we estimatethe mass to be ∼ M ⊙ . As Maund et al. (and ref-erences therein) pointed out, the final luminosity at thetop of the RSG branch is more relevant to the conditionof the yellow supergiant. The timescale for the transitionredward for massive stars across the HR diagram is com-paratively fast, and the stars will subsequently sit at theRSG locus for the majority of their post-main-sequenceevolution. In a binary model, mass transfer will trun-cate the outer radius of the donor star, resulting in ahotter photosphere; however, the core nuclear-burningluminosity will be relatively unaffected. Note that, if theprogenitor experienced mass transfer in a binary, sub-stantial mass stripping can lower the core temperatureand, hence, lower the surface luminosity. So, the progen-itor could have been somewhat more massive initially,perhaps ∼ M ⊙ .The effective radius of the SN 2013df progenitor 14 yrbefore explosion, based on our estimates of T eff and L bol ,was R eff = 545 ± R ⊙ . Unfortunately, we cannot pro-duce an independent estimate from the light curves us-ing the relations between SN progenitor radius and pho-tospheric temperature by Chevalier & Fransson (2008),Nakar & Sari (2010), and Rabinak & Waxman (2011),since these relations tend to break down only a few daysafter explosion, and the earliest set of photometric datapoints for SN 2013df is at age ∼
10 d. This star was evi-dently more extended than the progenitor of SN 2011dh— the yellow supergiant progenitor of SN 2011dh mod-eled by Bersten et al. (2012) has radius R ≈ R ⊙ .The later post-shock breakout cooling for SN 2013df im-plies that its progenitor may have had a larger radiusthan that of the SN 1993J progenitor. However, bothMaund et al. (2004) and Van Dyk et al. (2013b) calcu-lated that the progenitor of SN 1993J had a radius of ∼ R ⊙ , which is somewhat greater than, althoughwithin the uncertainties of, what we have estimated forthe SN 2013df progenitor. In order for the SN 1993Jprogenitor to have had a smaller effective radius, it musthave been hotter ( > < . L ⊙ ) than currently understood (e.g., Maund etal. 2004). To have a larger radius, the SN 2013df pro-genitor would have had to be cooler and also potentiallymore luminous. However, to be cooler would require theextinction to be even lower than the small amount thatwe have estimated. Additionally, the somewhat largerbolometric correction with the lower temperature wouldbe offset by the smaller extinction correction, leading,ultimately, to very little change in the estimated lumi-nosity. At the least, we can say that the SN 2013df andSN 1993J progenitor stars appear to have been relativelycomparable in nature.In summary, we have shown that SN 2013df in NGC4414 is a SN IIb, based on early-time photometry andspectroscopy, and that its properties are most similar Van Dyk et al.to those of SN 1993J. However, the mass of Ni syn-thesized in the explosion likely differed from that inSNe 1993J, 2008ax, and 2011dh. Furthermore, the lightcurves of SN 2013df — specifically, the late cooling frompost-shock-breakout maximum — indicate that its pro-genitor must have been more extended in radius thanthat of SN 1993J. Using archival pre-SN, high spatialresolution
HST images, together with
HST images ofSN 2013df, we have identified a star that we consider tobe the likely SN progenitor; it had T eff ≈ L bol ≈ . L ⊙ . Ultimately, verification of this identi-fication of the SN 2013df progenitor should occur whenthe SN itself has greatly faded, as has been done forthe SNe IIb 1993J (Maund & Smartt 2009) and 2011dh(Ergon et al. 2013; Van Dyk et al. 2013b). This canbe accomplished with high-quality images obtained with HST . Deep, very late-time
HST imaging in the blue orultraviolet could reveal a putative blue binary compan-ion star at the SN location, although the fact that SN2013df is at least a factor of two more distant than bothSNe 2011dh and 1993J could make such a detection amore challenging prospect.We thank the referee for their comments. This work isbased in part on observations made with the NASA/ESA
Hubble Space Telescope , obtained from the Data Archiveat the Space Telescope Science Institute (STScI), whichis operated by the Association of Universities for Re-search in Astronomy (AURA), Inc., under NASA con-tract NAS5-26555. Some of the data presented hereinwere obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the Califor-nia Institute of Technology, the University of Californiaand NASA; the observatory was made possible by the generous financial support of the W. M. Keck Founda-tion. KAIT and its ongoing research were made possibleby donations from Sun Microsystems, Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Obser-vatory, the NSF, the University of California, the Sylvia& Jim Katzman Foundation, and the TABASGO Foun-dation. We thank the RATIR instrument team and thestaff of the Observatorio Astron´omico Nacional on SierraSan Pedro M´artir. RATIR is a collaboration between theUniversity of California, the Universidad Nacional Au-ton´oma de M´exico, NASA Goddard Space Flight Center,and Arizona State University, benefiting from the loanof an H2RG detector from Teledyne Scientific and Imag-ing. RATIR, the automation of the Harold L. JohnsonTelescope of the Observatorio Astron´omico Nacional onSierra San Pedro M´artir, and the operation of both arefunded by the partner institutions and through NASAgrants NNX09AH71G, NNX09AT02G, NNX10AI27G,and NNX12AE66G, CONACyT grants INFR-2009-01-122785, UNAM PAPIIT grant IN113810, and a UCMEXUS-CONACyT grant. Support for this research wasprovided by NASA through grants GO-12888 and GO-13030 from STScI. A.V.F. and his group at UC Berkeleyalso wish to acknowledge generous support from Garyand Cynthia Bengier, the Richard and Rhoda GoldmanFund, the Christopher R. Redlich Fund, the TABASGOFoundation, and NSF grant AST-1211916. Researchby A.G. is supported by the EU/FP7 via ERC grantn 307260, “The Quantum Universe” I-Core program bythe Israeli Committee for planning and budgeting, theISF, GIF, and Minerva grants, and the Kimmel award.S.B. is supported by the Ilan Ramon Fellowship fromISA.
Facilities:
HST(WFPC2), HST(WFC3), Keck, KAIT,RATIR.
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Fig. 1.—
A greyscale composite of rizJH images obtained with RATIR on 2013 June 24, showing SN 2013df in NGC 4414. The positionof the SN is indicated by the tick marks. North is up, east is to the left.
N 2013df and Its Progenitor 9
Fig. 2.—
BV RIzJH light curves of SN 2013df from KAIT (solid points) and RATIR (open points; with r and i converted to R and I BV RIJH light curves of SN 1993J (Richmond et al. 1994; 1996; Matthewset al. 2002; dashed lines) and of SN 2011dh (Arcavi et al. 2011; Van Dyk et al. 2013b; Ergon et al. 2013; dot-dashed lines), and the z -bandlight curve of SN 2008ax (Pastorello et al. 2008; solid line), all adjusted in time and relative brightness. The estimated time t = 0 of theexplosion is indicated. Fig. 3.—
Absolute V light curve for SN 2013df (solid points), compared to those at V for SNe 1993J (dashed line; colored red in theonline version), 2008ax (solid line; colored green in the online version), and 2011dh (dot-dashed line; colored blue in the online version)shown in Figure 2. All of the curves are displayed relative to the day of V maximum; see text. N 2013df and Its Progenitor 11
Fig. 4.—
Optical spectrum of SN 2013df obtained on 2013 July 11.2 with DEIMOS on the Keck II 10-m telescope. Also shown forcomparison are the spectra, at approximately the same age, of SN 1993J from 1993 April 30 (Filippenko et al. 1993), SN 2008ax from2008 April 2 (Pastorello et al. 2008), and SN 2011dh from 2011 June 29 (unpublished; from WISeREP, Yaron & Gal-Yam 2012). Severalspectral features are indicated.
Fig. 5.—
Portion of the spectrum of SN 2013df shown in Figure 4, after normalization of the continuum, focusing on the region includingthe interstellar Na i D λλ N 2013df and Its Progenitor 13
F555W (a) 1999 Apr 29
SN 2013df
Fig. 6.—
A portion of the archival
HST
WFPC2 images of NGC 4414 from 1999 in (a) F555W and (b) F814W. The likely progenitorof SN 2013df is indicated by tick marks. (c) A portion of the
HST
WFC3 F555W image of SN 2013df, to the same scale and orientation.The SN is indicated by tick marks. We have astrometrically registered the WFPC2 and WFC3 images to an accuracy of 5.5 milliarcsec.The three progenitor candidates initially identified by Van Dyk et al. (2013a) are indicated (“1,” “2,” “3”) in panel (a). North is up, eastis to the left.
F814W (b) 1999 Apr 29
SN 2013df
Fig. 6.— (Continued.)
N 2013df and Its Progenitor 15
F555W (c) 2013 July 15
SN 2013df
Fig. 6.— (Continued.)
Fig. 7.—
Spectral energy distribution of the probable progenitor star of SN 2013df, based on the photometry presented in Table 1. Shownfor comparison is synthetic photometry derived from a MARCS (Gustafsson et al. 2008) supergiant model atmosphere at solar metallicitywith effective temperature 4250 K, reddened assuming A V = 0 .
30 mag and the Cardelli, Clayton, & Mathis (1989) reddening law. Themodel has been normalized at V . The hashed region in the diagram represents the range in brightness for the model within the totaluncertainty in the observed V magnitude. N 2013df and Its Progenitor 17
Fig. 8.—
Hertzsprung-Russell diagram, showing the locus of the probable progenitor of SN 2013df (solid symbol; colored red in the onlineversion). For comparison we also illustrate the massive-star evolutionary tracks with rotation from Ekstr¨om et al. (2012) at initial masses12, 15, and 20 M ⊙⊙