arXiv:1106.2897v2 [astro-ph.CO] 5 Oct 2011 eecp XT n yKsia fk(01 with (2011) on X-ray Ofek (UVOT) & the telescope Kasliwal with ultraviolet-optical by detected (2011) the and was num- Soderberg (XRT) SN & a the telescope Margutti at instance, by studied For al. intensely early et wavelengths. being (Arcavi of is IIb ber 2011dh Type SN as classified 2011). further been has SN rgeshsbe aeoe h attodcdsi the SNe. in core-collapse decades this of two In last progenitor the the over of astrophysics. identification made modern been of has areas Progress compelling most the of one u n eldvlpdPCgipolsi h Balmer H the ∼ contin- the in blue with profiles relatively P-Cygni series, by a well-developed spectrum showed and (2011) confirmation uum al. The et Silverman therein. through- references used are and dates (UT within 1 collaboration out), June and (PTF) 31 Factory May Transient between and Palomar astronomers the amateur discov- by several was by 2011dh SN independently ered M51. in 2011dh/PTF11eon SN of culrD a Dyk Van D. Schuyler nttt fSine 60 eoo,Israel. Rehovot, 76100 Science, of Institute 96822. HI Honolulu, Dr., lawn 91125. CA Pasadena, 94720-3411. CA ley, [email protected]. email: 91125; CA eemnn h tla rgn fsproa Se is (SNe) supernovae of origins stellar the Determining rpittpstuigL using Letters typeset ApJ Preprint in Appear To 17 dmL Kraus L. Adam 7 6 5 4 3 2 1 , eateto atcePyisadAtohsc,Weizmann Astrophysics, and Physics Particle of Department Wood- Fellow. 2680 Hubble Hawaii, of University Astronomy, for Institute Fellow. Einstein Technology of Institute California Department, Berke- Astronomy California, of University Astronomy, of Pasadena Department 220-6, Mailcode Center/Caltech, Science Spitzer 0 ms km 600 Letter ujc headings: Subject progenitor. Wolf-Rayet nitrogen-rich K), trcudesl oca,ee nteutailt h rsneo s a of presence the ultraviolet, the in even conceal, easily could star naHrzpugRseldarmi ossetwt niiilms of mass initial an with consistent positio is system the diagram binary near Hertzsprung-Russell mass-transfer very a a star in in unrelated companion an progenitor’s either the likely, is star detected the oerudcnrbto,adta h Nevrneti frough of prog is its magnitude environment and absolute SN 2011dh SN the has to that object extinction and the contribution, that infer foreground We 1993J. IIb SN ∼ yeIb ihamr opc rgntr(radius progenitor a compact is more SN a the Ke with that a indicate IIb, using photometry Type precision, and astrometric spectra available early-time available highest the to confirmed, Nacia,mlibn mgso h ery eryfc-ngalaxy face-on nearly nearby, the of images the multi-band archival, SN ehv dnie uiossa ttepsto fsproa(N 2 (SN) of position the at star luminous a identified have We 10 ubeSaeTelescope Space Hubble ∼ ecnie nta eut nteprogenitor the on results initial consider we , 13 a fepoin e raie l (2011) al. et Arcavi see explosion; of day 1 − m smr xeddta hthsbe nerdfrteS proge SN the for inferred been has what than extended more is cm, 1 niaigteS a fTp I The II. Type of was SN the indicating , α H RGNTRO UENV 01HPF1O NMSIR51 MESSIER IN 2011DH/PTF11EON SUPERNOVA OF PROGENITOR THE 1. 5,6 .Quimby M. bopinmnmmbusitdby blueshifted minimum absorption A efe .Silverman M. Jeffrey , INTRODUCTION T E 1 tl mltajv 11/10/09 v. emulateapj style X edn Li Weidong , uenve niiul(N2011dh) (SN individual supernovae: aais niiul(G 14 tr:eouin—sproa:ge supernovae: — evolution stars: — 5194) (NGC individual : 3 hiia .Kulkarni R. Shrinivas , 2 ihteAvne aeafrSres hsietfiainhsbeen has identification This Surveys. for Camera Advanced the with .BalyCenko Bradley S. , M V 0 − ≈ 2 arArcavi Iair , Swift oApa nAJLetters ApJ in Appear To 7 . n ffcietemperature effective and 7 by ; ABSTRACT , 3 frYaron Ofer , 2 ∼ as .Kasliwal M. Mansi , 7 lxiV Filippenko V. Alexei , acrmtcS bevtos h Npsto tcm at early position present SN is also The wavelengths (2011) observations. al. Ar- SN et Large panchromatic Very Soderberg Expanded the (EVLA). with Re- ray (2011b) for al. Array by et and Combined Horesh (CARMA); the Astronomy Millimeter-wave with in search (2011a) al. et Horesh NC5194; (NGC e 126 set +47 asv 0tp uegat ihblmti magni- bolometric was with progenitor al. supergiant, 1993J et tude K0-type very IIb Aldering massive SN two the a with compact. that stars and concluded progenitor (1994) extended from radii, arise different can arm. IIb spiral SNe prominent a along 1994), Ho & Turner tended, NIb20a Psoel ta.20;Conc tal. et Chornock 2008; al. et (Pastorello the 2008ax was resembles IIb closely progenitor more SN 2011dh radius 2011dh SN with SN spectroscopically compact, al. the et more Woosley (e.g., that likely 1993; 1993J al. curve indicates et SN SN light Swartz for 1994), 1993; the early observed al. from et was the How- Filippenko inferred than in temperature spectra, 1994). lower decline 2011dh al. much rapid et the (Richmond was more and 1993J which the SN 2011dh, ever, for the SN of seen for detection the also tail on report cooling (2011) al. been shock-breakout al. et et have (Maund Arcavi to later 2004). decade further- appears a 1994); companion spectroscopically al. al. recovered progenitor et inter- et Woosley Podsiadlowski the an 1993; more, (e.g., al. by et model explained Nomoto 1993; system well binary be acting could properties 1993J 10 hvle oebr 21)hv rudthat argued have (2010) Soderberg & Chevalier 11 ◦ 10 m hnwstecs o h well-studied the for case the was than cm) M ′′ ′ . bol 10 ∼ 5Ead91 and E 35 7 ′′ . n ai Polishook David and , 4 2( 92 − ≈ fteata rgntr r more or, progenitor, actual the of n × h oiino h eetdstar detected the of position The . rpe,cmat eyht( hot very compact, tripped, α yslrmtliiy h detected The metallicity. solar ly kI dpieotc mg.The image. adaptive-optics ck-II 10 ntraie rmalwGalactic low a from arises enitor α ± tipdevlp,core-collapse stripped-envelope, 13 = 7 ese 1(5)otie by obtained (M51) 51 Messier J00 13 = (J2000) 0 13 . ∼ ′′ . .Terdu ftesa a ex- was star the of radius The 8. 71 M 17–19 3 1i ahcodnt) ti off- is It coordinate). each in 01 sa Horesh Assaf , m(ose ta.19) h SN The 1994). al. et (Woosley cm 00K h trsradius, star’s The K. 6000 ′′ . 1d/T1en npre- in 011dh/PTF11eon, h 2 0Sfo h ulu fM51a of nucleus the from S 70 io.W pclt that speculate We nitor. vsa Gal-Yam Avishay , 29 m 52 ⊙ h ih fthis of light The . s . 711, h 30 7 m < δ 3 05 rnO Ofek O. Eran , 10 s . +47 = 104, ea — neral 13 ∼ 7 m nfact, In cm. Robert , δ 10 J00 = (J2000) ◦ 5 11 ′ 42 ′′ . 3,4 62; , 2 Van Dyk et al.

2011; Taubenberger et al. 2011), the progenitor of which Figure 1a. Comparison of the expected ACS pixel posi- Chevalier & Soderberg (2010) estimated to have had a tion for the SN site, derived from the transformation, to radius ∼ 1011 cm. Soderberg et al. (2011) suggest a ra- the actual measured position of the source results in an dius ∼ 1011 cm for the SN 2011dh progenitor as well. We uncertainty of 0.049 ACS pixel = 2.5 mas, or ∼ 19,000 find this evidence for a compact SN 2011dh progenitor AU = 0.09 pc (at the distance of M51). Accordingly, to be plausible and have assumed it to be the case in this we have very high confidence in the probability that this Letter. object, the same as the one identified by Li & Filippenko Li & Filippenko (2011) first identified a progenitor (2011), is spatially coincident with SN 2011dh. candidate for SN 2011dh in archival Hubble Space Tele- We analyzed the ACS “flt” images in all bands using 10 scope (HST) Advanced Camera for Surveys (ACS) im- Dolphot (Dolphin 2000), which is especially designed ages. Li, Filippenko, & Van Dyk (2011) performed fur- for ACS. We measured the relative offsets between the ther preliminary studies, measuring initial properties of dithered exposures in each band, with respect to one fidu- the candidate. Here we undertake a more extensive anal- cial image, before running the package. The output from ysis and attempt to constrain the properties of the SN Dolphot automatically includes the transformation from progenitor system. flight-system F435W, F555W, and F814W to the corre- The nearly face-on M51, the “Whirlpool ,” also sponding Johnson-Cousins (Bessell 1990) magnitudes in hosted the Type I SN 1945A, Type Ic SN 1994I, and BV I, following Sirianni et al. (2005). Since color cor- Type II-Plateau (II-P) SN 2005cs. We adopt a distance rections are required in the Sirianni et al. (2005) rela- modulus to M51 of µ = 29.42±0.27 mag (distance = 7.66 tions to transform the flight-system broad-band colors Mpc), from surface brightness fluctuation measurements into the standard magnitude system, and no such color (Tonry et al. 2001).8 corrections exist for F658N, the flight-system magnitude in this narrow bandpass could not be transformed to a 2. OBSERVATIONS AND ANALYSIS standard system. Dolphot indicates with a flag, as well 2 M51 was observed on 2005 January 20–21 by the as with measurements of χ and the parameter “sharp- bona Hubble Heritage Team (GO/DD 10452; PI: Beckwith) ness,” whether a detected source is most likely a fide with the ACS Wide Field Channel (WFC). A four-band star. All of these indicators point to the detected (F435W, F555W, F658N, and F814W) image mosaic of object being stellar. M51a and NGC 5195 (M51b) was obtained in six ACS The SN site was also imaged in a pair of 1300 s expo- pointings, with four dithered exposures at each point- sures with WFPC2 in F336W on 2005 November 13 (GO ing. These data had been previously analyzed by Li et al. 10501; PI: Chandar). We performed photometry of these (2006) to determine the properties of the SN 2005cs pro- images with HSTphot. A source is detected at 4.1σ at genitor. The “drizzled” mosaics in each band were taken the SN position in one of the two exposures, but not in from the Hubble Legacy Archive (HLA). Individual “flt” the other one. The results of all of the photometry for exposures were also acquired from the HST Archive. The the detected star are in Table 1. approximate location of the SN site was first established 3. THE NATURE OF THE DETECTED STAR comparing the HLA mosaics to early-time SN images, ob- To estimate the properties of this star, we need the tained with the Katzman Automatic Imaging Telescope M51 distance (§ 1) and the SN extinction. To infer (KAIT; Filippenko et al. 2001) at Lick Observatory. the latter, we plot the star’s photometry in a color- We subsequently (2011 June 6) took high-resolution, color diagram in Figure 2. We also show the colors adaptive-optics (AO; Wizinowich et al. 2006) K -band p from Dolphot of stars in a ∼ 50 × 50 pixel (∼ 91 × 91 (central wavelength 2.124 µm; bandwidth 0.351 µm) 2 images of the SN, using the Near Infrared Camera pc ) region centered around the SN site. Further- 2 (NIRC2) instrument on the Keck-II 10-m telescope, more, we show the locus for normal supergiants (syn- to precisely pinpoint the SN location in the archival thetic Johnson-Cousins colors extracted using the pack- HST data. The geometric distortion corrections from age STSDAS/Synphot within IRAF from model super- Yelda et al. (2010) were first applied to each of the in- giants; Castelli & Kurucz 2003), and the reddening vec- dividual NIRC2 frames before combination of all frames tor from the Cardelli, Clayton, & Mathis (1989) redden- into a single image mosaic. Measuring the positions using ing law. The detected star, as well as other stars in imexamine in IRAF9 of 18 stars seen in common between the immediate environment, appear to be subject to the Keck AO and ACS images, we were able to obtain relatively low reddening. This agrees with the relative an astrometric transformation with formal uncertainty lack of dust emission at the SN site, as seen in pre- (∆X, ∆Y ) = (0.102, 0.109) pixel, for a total root-mean SN (2004 May 18 and 22), archival Spitzer Space Tele- square uncertainty of 0.149 ACS pixel, or 7.45 milliarcsec scope images of M51 at 8 µm (program ID 159; PI: (mas), in the relative astrometry. The SN position, seen Kennicutt). This is also consistent with weak or unde- in Figure 1b, coincides with the point source visible in tectable Na i D absorption in a SN spectrum obtained on June 3 at the Keck 10-m telescope using LRIS (however, 8 This distance modulus is consistent with 29.41 ± 0.22 mag, see Poznanski et al. 2011 regarding the limited utility of determined by Poznanski et al. (2009) using the SN II-P standard- this absorption feature in low-resolution spectra to de- candle method applied to SN 2005cs, assuming a Hubble constant −1 −1 10 H0 = 70 km s Mpc . The ACS module of Dolphot is an adaptation of the pho- 9 IRAF (Image Reduction and Analysis Facility) is distributed tometry package HSTphot (a package specifically designed for by the National Optical Astronomy Observatories, which are oper- use with HST Wide-Field Planetary Camera 2 [WFPC2] images; ated by the Association of Universities for Research in Astronomy, Dolphin 2000). We used v1.1, updated 2010 January 6, from Inc., under cooperative agreement with the National Science Foun- http://purcell.as.arizona.edu/dolphot/, with, e.g., updated ACS dation. zeropoints. The SN 2011dh/PTF11eon Progenitor 3 termine SN extinction), and via a comparison of early via a wind or mass exchange with its companion (similar SN photometry, obtained using KAIT, with the dered- to SN 1993J; e.g., Podsiadlowski et al. 1993). dened colors of SN 2008ax (e.g., Pastorello et al. 2008). The compact primary star would therefore be very hot, The Galactic contribution to the extinction is also rel- probably a star in a Wolf-Rayet (WR) phase. From Fig- atively low, AV = 0.12 mag, and E(B − V ) =0.04 ure 3 one can see that the presumed secondary star alone and E(V −I)=0.05 mag (Schlegel, Finkbeiner, & Davis can account for much of what was marginally detected in 1998). We adopt this foreground value as the extinction the F336W bandpass. (Additionally, nothing is detected toward SN 2011dh. at the SN position in archival Galaxy Evolution Explorer We also require the metallicity of the SN environ- [GALEX] ultraviolet NUV and FUV band data for M51.) ment, which can be inferred from spectroscopy of H ii A star with characteristics similar to a weak-lined, early- 5 regions nearest the SN site. The regions Bresolin et al. type, N-rich WR, with Teff ≈ 10 K, log(Lbol/L⊙) ≈ 5.3, 0 (2004) labelled “53” (nearest region), “54,” and “55” and MV ≈−2.1 mag (adopting the corresponding WNE have oxygen abundances 12 + log(O/H) = 8.66 ± 0.09, model from Hamann & Gr¨afener 2004), would be & 2.2 8.49 ± 0.08, and 8.60 ± 0.08, respectively. We assume mag fainter in the F336W bandpass and would also be these are representative of the SN environment. Given concealed by its brighter companion in all redder bands. that the solar value is 12 + log(O/H) = 8.66 ± 0.05 This WR star, therefore, would have little effect on the (Asplund et al. 2004) and the average abundance for H ii total light of the system, although any WR more lumi- regions in the Large Magellanic Cloud is 12 + log(O/H) nous than this would have been detected at F336W. =8.37 ± 0.22 (Russell & Dopita 1990), we conclude that In Figure 4 we show the loci of the detected secondary the SN 2011dh site is most likely of roughly solar metal- star and the hypothetical primary in a Hertzsprung- licity. Russell diagram. (The main uncertainty in the lumi- We can estimate the effective temperature, Teff , of the nosity of the detected star arises from the assumed detected star by modeling its spectral energy distribution distance modulus uncertainty, ±0.27 mag.) We also (SED) across all observed bands. We produced tem- show model stellar evolutionary tracks for massive −1 plate SEDs via synthetic photometry, extracted using stars with equatorial rotation (vrot = 300 km s ; Synphot, from Castelli & Kurucz (2003) model stars at Hirschi, Meynet, & Maeder 2004). These single-star solar metallicity with log g = 1.0 (see below), reddened tracks are meant to be merely suggestive of possible by our assumed value. We made the comparison in flight- masses for the binary components; in particular, these system magnitudes, since the F658N measurement from tracks do not even adequately account for the position of Dolphot and the F336W upper limit from HSTphot could the hypothetical primary in the diagram. Clearly, what not be transformed to a standard system. The results are is required is a full modeling of the components of this shown in Figure 3. The model with Teff = 6000 K com- possible interacting binary system and its evolution up pares well with the observations (particularly those in until the primary’s explosion. the ACS bands). We therefore adopt this temperature 4. for the observed star, with a conservative uncertainty of DISCUSSION AND CONCLUSIONS ±100 K. We have detected in archival HST images a star at From the adopted distance modulus, extinction, and the precise location of SN 2011dh/PTF11eon. The star reddening, we find that the object had an absolute in- has colors consistent with mid-F-type, although its lumi- 0 0 trinsic magnitude of MV = −7.73, and intrinsic colors nosity is higher (MV ≈ −7.7 mag) and its radius more (B − V )0 =0.61 and (V − I)0 =0.55 mag. We estimate extended (∼ 290 R⊙) than is the case for a normal su- that the probability is ∼ 4 × 10−6 that a star more lu- pergiant. The early properties of SN 2011dh, however, minous than this in M51a could be found at this exact point to its having a compact progenitor (Arcavi et al. ∼ 11 location. From the Teff = 6000 K model, above, we esti- 2011; radius 10 cm, Soderberg et al. 2011), indicat- mate that the bolometric correction for the detected star ing that the detected star is likely not the star that ex- is BCV = 0.00 mag, and therefore Mbol = −7.73 mag. ploded. (Maund et al. 2011 favor the interpretation that The bolometric luminosity with respect to the Sun (as- the star is the yellow supergiant progenitor of the SN.) suming Mbol⊙ =4.74 mag) is log(Lbol/L⊙)=4.99. The It is possible that the detected star is just a very close star has a radius of ∼ 290 R⊙, and we estimate its sur- neighbor of, yet generally unrelated to, the actual pro- face gravity to be log g ≈ 0.77 (assuming the star’s mass genitor. We consider it more likely, though, that the is ∼ 18 M⊙; see below), so our choice for the model log g star is the companion to the progenitor in a binary sys- is warranted. We note that the star is significantly more tem. The extended radius for the detected star and the extended than a normal supergiant at this temperature compact radius for the progenitor implies that the two (e.g., Drilling & Landolt 2000). stars may have been interacting. We note, however, that It is unlikely that the detected star is the one that ac- no direct observational indication yet exists that the SN tually exploded. As already noted, the early SN data 2011dh progenitor was a member of a binary system. indicate that the progenitor was far more compact (ra- The compact SN 2011dh progenitor and the extended dius ∼ 1011 cm). Either the detected star is unrelated SN 1993J progenitor are within similar ranges of initial to the progenitor and is merely a very close (. 0.1 pc) mass (13–22 M⊙ for the latter; Van Dyk et al. 2002). neighbor, or, more likely, it is the companion to the pro- (Note that Crockett et al. 2008 concluded that, if the genitor in a binary system. The fact that He i lines were compact progenitor of SN 2008ax were also in a binary seen in the SN spectra within the first few weeks of explo- system, its initial mass range would have been signifi- sion (Marion et al. 2011) implies that the progenitor had cantly lower, 10–14 M⊙.) We note that many of the been substantially stripped of its H envelope, presumably model SN Ib progenitors experiencing Case A/Case B mass transfer, which Yoon, Woosley, & Langer (2010) 4 Van Dyk et al. have recently considered, are also in a mass range similar servatory, which is operated as a scientific partnership to that suggested for the SN 2011dh progenitor. As both among the California Institute of Technology, the Uni- Chevalier & Soderberg (2010) and Dessart et al. (2011) versity of California, and NASA, with generous finan- point out, the difference between SN IIb and Ib progen- cial support from the W. M. Keck Foundation; and the itors could be razor thin, depending on the H mass re- Lick Observatory, operated by the University of Califor- maining in the progenitor star’s envelope. nia. KAIT and its ongoing research were made possible We will, of course, also develop a clearer picture once by donations from Sun Microsystems, Inc., the Hewlett- the SN has significantly faded, most likely several years Packard Company, AutoScope Corporation, Lick Obser- in the future. At that time, imaging of the SN site can vatory, the NSF, the University of California, the Sylvia be undertaken, presumably with HST, and we can deter- & Jim Katzman Foundation, and the TABASGO Foun- mine if the possible secondary star is still there. For ex- dation. We thank the staffs of the Lick and Keck Obser- ample, Ryder, Murrowood, & Stathakis (2006) detected vatories for their assistance with the observations. what they concluded to be the late-B- to late-F-type su- We thank Peter Nugent for useful comments, and pergiant companion to the compact SN IIb 2001ig in the referee for helpful suggestions which improved this ground-based Gemini images obtained ∼ 1000 d after manuscript. Support for this research was provided explosion. If the star has vanished or significantly faded, by NASA through grants GO-11575, AR-11248, and we can investigate if any fainter stars were present, con- AR-12126 from the Space Telescope Science Institute, tributing to the observed point-spread function of the which is operated by AURA, Inc., under NASA contract star detected in the pre-SN HST images. Further work NAS 5-26555. A.V.F. and his group at UC Berkeley is clearly required to understand more fully the nature also wish to acknowledge generous support from Gary of this interesting, and potentially important, SN. and Cynthia Bengier, the Richard and Rhoda Goldman Fund, NASA/Swift grant NNX10AI21G, NASA/Fermi grant NNX1OA057G, NSF grant AST–0908886, and This work was based in part on observations made with the TABASGO Foundation. A.G. is supported by the the NASA/ESA Hubble Space Telescope, obtained from ISF. E.O.O. is supported by an Einstein Fellowship and the Data Archive at the Space Telescope Science Insti- NASA grants. J.M.S. thanks Marc J. Staley for a grad- tute, which is operated by the Association of Univer- uate fellowship. sities for Research in Astronomy (AURA), Inc., under Facilities: Keck:NIRC2, Keck:LRIS, HST(ACS), NASA contract NAS 05-26555; the W. M. Keck Ob- Spitzer(IRAC), GALEX, Lick:KAIT.

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TABLE 1 Photometry of the Star Detected at the SN 2011dh Positiona

F336W F435W B F555W V F658N F814W I (mag) (mag) (mag) (mag) (mag) (mag) (mag) (mag) 23.434(339) 22.451(005) 22.460 21.864(006) 21.808 21.392(021) 21.216(005) 21.208

a Uncertainties (1σ) are given in parentheses as millimagnitudes.

SN 2011dh SN 2011dh (a) F814W (b) Kp

N N 1" 1" E E

Fig. 1.— (a) A portion of the archival HST F814W image of M51 from 2005; the star detected at the precise location of SN 2011dh/PTF11eon is indicated by tickmarks. (b) A portion of the Kp-band AO image obtained using NIRC2 on the Keck-II telescope on 2011 June 6; the SN is indicated by tickmarks. 6 Van Dyk et al.

Fig. 2.— Color-color diagram showing the star detected at the precise location of SN 2011dh/PTF11eon (filled square), along with other stars (open squares) within an area ∼ 91×91 pc2 around this location. Also shown for comparison are the locus of (unreddened) supergiants (solid line), derived from Castelli & Kurucz (2003) models, and the reddening vector (dashed line), assuming a Cardelli, Clayton, & Mathis (1989) reddening law. The SN 2011dh/PTF11eon Progenitor 7

Fig. 3.— Spectral energy distribution (SED) of the detected star (points) in HST ACS/WFC and WFPC2 flight-system magnitudes. Also shown are the SEDs derived from the Castelli & Kurucz (2003) models for supergiants with effective temperatures 5750 K (short-dashed line), 6000 K (solid line), and 6250 K (long-dashed line), after application of the assumed reddening toward the SN (see text). The models are normalized to the F555W brightness of the star. 8 Van Dyk et al.

Fig. 4.— Hertzsprung-Russell diagram showing the locus of the star detected at the precise location of SN 2011dh/PTF11eon (filled circle). We speculate that this star is the companion to the progenitor in an interacting binary system. We also indicate the locus of a hypothetical compact, hot, Wolf-Rayet primary (five-pointed star), which we further speculate is the actual SN progenitor. Also shown are −1 model single-star evolutionary tracks at solar metallicity with equatorial rotation (vrot = 300 km s ) for initial masses 15, 20, and 25 M⊙ (Hirschi, Meynet, & Maeder 2004), which are intended merely to be suggestive of the possible mass ranges for the two stars.