The SN 2008S Progenitor Star: Gone or Again Self-Obscured?
J. L. Prieto, D. M. Szczygiel, C. S. Kochanek, K. Z. Stanek, T. A. Thompson, J. F. Beacom, P. M. Garnavich, C. E. Woodward
aa r X i v : . [ a s t r o - ph . S R ] J un D RAFT VERSION S EPTEMBER
21, 2018
Preprint typeset using L A TEX style emulateapj v. 04/20/08
THE SN 2008S PROGENITOR STAR: GONE OR AGAIN SELF-OBSCURED?
J. L. P
RIETO , , D. M. S ZCZYGIEŁ , , C. S. K OCHANEK , , K. Z. S TANEK , , T. A. T HOMPSON , , J. F. B EACOM , , ,P. M. G ARNAVICH , C. E. W OODWARD Draft version September 21, 2018
ABSTRACTWe obtained late-time optical and near-IR imaging of SN 2008S with the Large Binocular Telescope (LBT). Wefind that (1) it is again invisible at optical (
UBV R ) wavelengths to magnitude limits of approximately 25 mag,and (2) while detected in the near-IR ( HK ) at approximately 20 mag, it is fading rapidly. The near-IR detectionsin March and May 2010 are consistent with dust emission at a blackbody temperature of T ≃
900 K and a totalluminosity of L ≃ L ⊙ , comparable to the luminosity of the obscured progenitor star. If it is a supernova,the near-IR emission is likely due to shock heated dust since the elapsed time from peak is too long to supporta near-IR dust echo and the decline in luminosity is shallower than the Co slope. If it is reprocessed emissionfrom a surviving progenitor, a dust photosphere must have reestablished itself closer to the star than beforethe transient ( ∼
40 AU rather than 150 AU), unless there is a second, cooler dust component that dominatesat mid-IR wavelengths. The continued rapid fading at roughly constant temperature favors transient emission,but the SED peaks in the mid-IR and future Spitzer observations will be needed to close the case.
Subject headings: stars: evolution – stars: supergiants – supernovae:individual (SN 2008S) INTRODUCTIONSN 2008S is one of the most mysterious optical transientscreated by a massive star in the last decade. It was discoveredin February 2008 by Arbour & Boles (2008) in the prolificsupernova factory NGC 6946. It was initially classified asa likely “supernova impostor" due to its faint absolute peakmagnitude ( M V ∼ -
13 mag) and optical spectra dominated bynarrow Balmer and [Ca II] lines in emission (Stanishev et al.2008; Steele et al. 2008). NGC 6946 had been observed bythe Large Binocular Telescope (LBT) the previous year, andthe key piece of evidence from these observations was thatthere was no optical progenitor (Prieto et al. 2008), which wassurprising since the “supernova impostors” are believed tobe eruptions from very massive ( > M ⊙ ), evolved stars(e.g., Smith et al. 2010 and references therein) that shouldhave been easily visible in the LBT observations.The only means of having an optical eruption from a mas-sive star and an invisible progenitor is for the star to be self-obscured by dust that is largely destroyed by the transient.This possibility was confirmed when Prieto et al. (2008) foundthe progenitor star as a log L / L ⊙ ≃ . T ≃
440 K black-body in archival Spitzer data. This luminosity is comparableto that of an evolved ∼ M ⊙ star, and is well below that cor-responding to the more massive stars thought to be requiredfor non-supernova eruptions. Subsequent analyses of the pro-genitor by Botticella et al. (2009) and Wesson et al. (2010)were consistent with those by Prieto et al. (2008).More remarkably, an almost identical event then occurred Carnegie Observatories, 813 Santa Barbara St., Pasadena, CA, 91101 Department of Astronomy, The Ohio State University, 140 W. 18th Ave.,Columbus OH 43210 Department of Physics, The Ohio State University, 191 W. WoodruffAve., Columbus OH 43210 Center for Cosmology and AstroParticle Physics, The Ohio State Uni-versity, 191 W. Woodruff Ave., Columbus OH 43210 University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame,IN 46556 Department of Astronomy, School of Physics and Astronomy, 116Church Street, S. E., University of Minnesota, Minneapolis, MN 55455 Hubble and Carnegie-Princeton Fellow in NGC 300 (Monard 2008). The progenitor was invisible inthe optical to even tighter limits (Berger & Soderberg 2008;Bond et al. 2009; Berger et al. 2009), but we again found theprogenitor as a self-obscured star of similar luminosity and(dust photosphere) temperature in Spitzer mid-IR data (Prieto2008; Thompson et al. 2009). A subsequent analysis of theprogenitor by Berger et al. (2009) agreed with our estimates,and an investigation of the progenitor based on its neighbor-ing stars by Gogarten et al. (2009) was consistent with theprogenitor being a massive star of order 10-20 M ⊙ , where theanalysis favored the upper portions of this range but, strictlyspeaking, the method only provides an upper mass bound.In Thompson et al. (2009) we surveyed the galaxy M33 formid-IR sources with similar properties to these progenitorsand found that they were astonishingly rare, with only a fewsuch sources in the entire galaxy. In the mid-IR, these sourceshave the properties of super-AGB stars, with properties dis-tinct from other classes of massive stars such as LBVs andred supergiants. The rarity of these sources compared to allmassive stars, confirmed in our survey of additional galaxies(Khan et al. 2010), means that the progenitors of the transientsare a very short lived ( ∼ years) phase in the evolution ofthese massive stars and that there is a causal connection be-tween obscuration and explosion.Thompson et al. (2009) concluded that there are a num-ber of possible mechanisms to explain the nature of thesetransients and their progenitors: (1) massive white-dwarfbirth; (2) electron-capture supernova; (3) intrinsically low-luminosity iron core-collapse supernova; and (4) massive staroutbursts. Debates about these possible origins have been rag-ing ever since then, based both on theoretical and observa-tional arguments. They are basically divided into the (somekind of) supernova camp (Prieto et al. 2008; Botticella et al.2009; Pumo et al. 2009) and the (some kind of) massive staroutburst camp (Berger et al. 2009; Smith et al. 2009; Bondet al. 2009; Kashi et al. 2010). The outburst camp generallyargues that the progenitor was not a ∼ M ⊙ super-AGB starbut a more massive 15 - M ⊙ star (supported by Gogartenet al. 2009), despite their position at the red, high luminos- TABLE 1LBT
MAGNITUDES OF
SN 2008SDate MJD
Us B V R H K (UT) (mag) (mag) (mag) (mag) (mag) (mag)2008-05-03 54589.4 21 . ± .
07 20 . ± .
03 19 . ± .
04 18 . ± . · · · · · · . ± .
08 20 . ± . · · · . ± . · · · · · · . ± .
07 22 . ± .
03 21 . ± .
04 20 . ± . · · · · · · · · · . ± .
05 22 . ± . · · · · · · · · · · · · . ± .
06 22 . ± . · · · · · · · · · · · · . ± .
05 22 . ± . · · · · · · · · · · · · . ± .
06 22 . ± . · · · · · · · · · < . < . < . . ± . · · · · · · < . < . < . < . · · · · · · < . < . < . < . · · · · · · · · · · · · · · · · · · . ± . · · · · · · · · · · · · · · · < . . ± . < . < . < . < . · · · · · · · · · · · · · · · · · · · · · . ± . σ . The estimated start date of thetransient is MJD 54485 . ± B , V and R are Besselfilters, Us is a high throughput U -band interference filter. ity end of the AGB sequence in mid-IR color-magnitude dia-grams (Thompson et al. 2009; Khan et al. 2010) and the lowmass compared to typical stars with LBV outbursts (see Smithet al. 2010). The massive-star outburst interpretation is seri-ously called into question by our Spitzer IRS spectrum of theNGC 300 event (Prieto et al. 2009). The mid-IR spectrum re-sembles that of carbon-rich proto-planetary nebulae and lacksthe silicate-dominated dust features typical of massive staroutbursts (e.g., Humphreys et al. 2006). Wesson et al. (2010),analyzing post-event Spitzer observations of SN 2008S, alsofound that the silicate dust characteristics of high mass starswere inconsistent with the observations. Prieto et al. (2009)also note that proto-planetary nebulae (initial masses < ∼ M ⊙ )have most of the optical spectral features that led Smith et al.(2009), Bond et al. (2009) and Berger et al. (2009) to argue foran outburst from a more massive ( ∼ M ⊙ ) star. Since “TypeIIn” optical spectroscopic properties are seen in some proto-planetary nebulae, massive supergiants, supernova impostors,and the genuine, but very diverse, Type IIn supernovae, theyappear only to be a diagnostic for the presence of strong in-teractions between ejecta and a dense circumstellar mediumrather than a diagnostic for the source of the ejecta.In the end, however, the question is easy to answer – eitherthe stars survived, or they did not. We have been following theSN 2008S event with the LBT in both the optical and near-IR,and here we report that the source is again too faint to detectin the optical, and while detected in the near-IR, it presentlyis only as luminous as the progenitor and fading rapidly. Wedescribe our observations and results in §2 and discuss theirimplications in §3. OBSERVATIONS AND RESULTSThe optical observations were done with the Large Binoc-ular Cameras (LBC, Giallongo et al. 2008), using theLBC/Blue camera for U , B and V and the LBC/Red camerafor R . The pixel scale of the LBC cameras is 0 . ′′
22. Sincethese observations are part of a program whose overall goalis to use difference imaging to characterize variable sources,the sub-images obtained for each epoch were not ditheredand SN 2008S was always located at approximately the samepoint on Chip 2 of the cameras. Image exposure times were300 sec, generally with two exposures for U , B and V and 6exposures for R . The near-IR observations were made withLUCIFER (Seifert et al. 2003; Mandel et al. 2008; Ageorges et al. 2010) in the H and K bands using the F3.75 camera witha pixel scale of 0 . ′′
12. At each dither position we obtained 3exposures of 33 (10) sec for H ( K ) band. We obtained 10 on-source and 6 off-source dither positions in a 2-5-2-5-2 off-on-off-on-off pattern, where the off-source position was shifted8 arcmin away from the galaxy.The optical and near-IR data were reduced using stan-dard methods in IRAF. The photometry was obtained usingDAOPHOT and ALLSTAR (Stetson 1987; Stetson 1992).The optical data was calibrated using 4 -
24 local standardsfrom Welch et al. (2007) for the V and R bands and from Bot-ticella et al. (2009) for the U and B bands. The near-IR datawere calibrated using 3 - σ upper limit on the magnitude using the standard deviation ofthe sky in a region around the source.Figure 1 shows the H , K and R -band light curves fromBotticella et al. (2009) and our LBT observations, and Fig-ure 2 shows the current SED. The left panel of Fig. 2 showsthe constraints on the progenitor’s spectral energy distribu-tion (SED) as compared to typical massive stars. To makethe comparison we used a Galactic plus intrinsic extinctionof A V = 2 .
13 mag (Botticella et al. 2009) and the distance of D = 5 . L = 4 π D ν F ν . For compar-ison we show the extincted SEDs of 10 M ⊙ and 20 M ⊙ redsupergiants (RSG) using luminosities and effective temper-atures from Marigo et al. (2008), a 20 M ⊙ blue supergiant(BSG) modeled on SN1987A, and the blackbody that best fitthe SN 2008S progenitor data.In the optical ( UBV R ), the source is again too faint to corre-spond to a massive ( > M ⊙ ) evolved star, with limits on itsbrightness similar to those for the progenitor (see right panelin Fig. 2). The extinction would have to be increased from the A V ≃ . A V ∼ . - . ± K band that is significantly steeper than the F IG . 1.— The R , H and K -band light curves of SN 2008S from Botticella et al. (2009, open black points) and the Large Binocular Telescope (filled red points).The last R and H -band points are upper limits. The dashed line shows the Co decay slope. This should properly be compared with the bolometric light curve,but this will require Spitzer observations. Botticella et al. (2009) found that the bolometric light curve observed after day 120 was slightly shallower than the Co decay slope. mean slope of 2 . ± . H - K > . K -band light curve, we estimate H ≃ . H - K ≃ . H - K ≃ . K -band flux and either the H -band magnitude limit(20 . . T ≃
900 K and 750 K and luminosities of L ≃ L ⊙ and 130000 L ⊙ , respectively. With a λ - emis-sivity law, the estimated temperatures and luminosities arelower, with T ≃
800 K and 700 K and L ≃ L ⊙ and95000 L ⊙ . With the further fading between March and May2010, the source luminosity is now comparable to the esti-mated luminosity L ≃ L ⊙ of the progenitor star (Prietoet al. 2008; Botticella et al. 2009; Wesson et al. 2010). DISCUSSIONThompson et al. (2009) proposed that SN 2008S and theNGC 300 transient were the archetypes of a new class of tran-sients potentially including the M85 OT-1 transient (Kulka-rni et al. 2007; Pastorello et al. 2007), SN 1999bw (Li et al.2002 and references therein), and now PTF10fqs (Kasliwalet al. 2010). The initial defining characteristics were (1)a dust-enshrouded progenitor without optical counterpart andmid-IR magnitudes that places them at the tip of the AGB se-quence in a mid-IR CMD, and (2) a low-luminosity transient( - & M V & -
15) with narrow lines in emission in the spec-tra ( v . F IG . 2.— The pre-explosion, progenitor SED (left) and the current SED (right) of SN 2008S. The measured magnitudes are converted to fluxes, and these areconverted to a luminosity as L = 4 π D ν F ν where D = 5 . A V = 2 .
13 mag of total extinction. The 10 M ⊙ and20 M ⊙ red supergiant models (RSG, dashed curves) are from Marigo et al. (2008) and have T ≃ L / L ⊙ = 4 .
68 and 5 .
29, respectively.The blue supergiant model (dotted curve) is based on SN1987A and has T ≃ L / L ⊙ = 5 .
0. The best fit blackbody model (solid curve) for theprogenitor has T = 440 K and log L / L ⊙ = 4 .
54 (Prieto et al. 2008). what stronger than those for the progenitor, and the near-IRdetections already rule out RSGs more massive than 10 M ⊙ .The total luminosity is now comparable to that of the progeni-tor and emerges mainly in the mid-IR, but it is also continuingto rapidly fade. Recently, Ohsawa et al. (2010) presented late-time AKARI mid-IR observations of the NGC 300 transientthat show the transient is again self-enshrouded with an SEDthat peaks at ∼ - µ m ( T ∼
600 K) and total bolometricluminosity ∼ R BB ≃
40 AU , although we can’trule out the presence of a cooler dust component that dom-inates the bolometric luminosity and peaks in the mid-IR. Ifthe optical depth is due to a constant velocity wind, this inturn requires decreasing the mass loss rate ˙ M (or opacity perunit mass) or increasing the wind velocity v w by a factor of 4relative to the progenitor. Producing the near-IR time variabil-ity is difficult in this scenario because the characteristic timescale R BB / v w is ∼ v w ≃
100 km/s while the K bandflux changed by over a factor of 2 in only 60 days. Thus, itseems unlikely that the system has returned to its pre-transientstate.The rapid fading strongly suggests that the present emis-sion is a continuation of the transient. Since we are now There are differences in the sizes quoted for the dust around SN 2008S.Prieto et al. (2008) assume an infinite wind and estimate a photospheric radiusof 150 AU, while Botticella et al. (2009) and Wesson et al. (2010) generallydiscuss the geometric boundaries of the dust distribution.
800 days post explosion, the emission can no longer be ex-plained as an infrared echo. At this point, echos from thetransient peak are produced from a minimum distance of ct / ≃ L ⊙ (Botticella et al. 2009) to produce significant near-IR emis-sion. While the near-IR emission has roughly decayed atthe rate expected from Co decay (1.023 mag/100 days, seeFig. 1) the drop in the estimated bolometric luminosity is sig-nificantly slower, and it is unclear how the positron heatingcould be efficiently converted to near-IR emission. The lastpossibility is shock heating of pre-existing dust. Botticellaet al. (2009) and Wesson et al. (2010) estimate that dust sur-vived outside of 1000 - - ˙ M ∼ - M ⊙ /yearwind believed to have surrounded the progenitor, the shockluminosity of (1 / ˙ Mv s / v w ∼ L ⊙ for v w ∼
50 km/s and v s ∼ ∼
10% efficiency of emission byshocked dust (Draine 1981). However, this seems a stretchgiven the time and velocity scales, and it would be simplerto use dust forming in the shocked material as advocated byBotticella et al. (2009).At its present rate of fading in the near-IR, SN 2008S willeffectively be invisible to ground based observatories when itnext rises, and finally closing the case will need a combina-tion of HST and, more importantly, Spitzer observations thatwill be obtained over the next year. The HST observationscan detect or rule out the presence of a star in the near-IRto significantly deeper limits than possible from the grounddue to both its sensitivity and resolution. With two epochs ofdata showing some variability, the source can be unambigu-ously identified even if very faint. The Spitzer observationswill accurately determine the temperature and luminosity ofthe source. If it continues to decay as rapidly as we observedin the near-IR, it should be significantly fainter than the pro-genitor star in 2011.These late time observations will be crucial to understand-ing this new class of transient sources, particularly since it isalso possible for the survivor to be fainter than the progeni-tor in several of the possible scenarios. It could be sublumi-nous as a result of the outburst and then will slowly return tothermal equilibrium (Smith et al. 2009). Or, as suggested byThompson et al. (2009) and discussed more fully in Prieto etal. (2009), if SN 2008S was the explosive birth of a massivewhite dwarf, we would expect the bolometric luminosity toapproach nearly Eddington for a ∼ M ⊙ object, ∼ × L ⊙ . We thank G. Cresci, J. Hill, R. Humphreys, and A. Quir-renbach for suggestions and comments. Based in part on ob-servations made with the Large Binocular Telescope. TheLBT is an international collaboration among institutions in theUnited States, Italy and Germany. The LBT Corporation part-ners are: the University of Arizona on behalf of the Arizonauniversity system; the Istituto Nazionale di Astrofisica, Italy;the LBT Beteiligungsgesellschaft, Germany, representing theMax Planck Society, the Astrophysical Institute Potsdam, andHeidelberg University; the Ohio State University; and theResearch Corporation, on behalf of the University of NotreDame, University of Minnesota and University of Virginia.JLP acknowledges support from NASA through Hubble Fel-lowship grant HF-51261.01-A awarded by STScI, which isoperated by AURA, Inc. for NASA, under contract NAS 5-2655. CSK, JLP, KZS, DS and TAT are supported in part byNSF grant AST-0908816. JFB is supported by NSF CAREERGrant PHY-0547102. Facilities: