Discovery of the Dust-Enshrouded Progenitor of SN 2008S with Spitzer
We report the discovery of the progenitor of the recent type IIn SN 2008S in the nearby galaxy NGC 6946. Surprisingly, it was not found in deep, pre-explosion optical images of its host galaxy taken with the Large Binocular Telescope, but only through examination of archival Spitzer mid-IR data. A source coincident with the SN 2008S position is clearly detected in the 4.5, 5.8, and 8.0 m IRAC bands, showing no evident variability in the three years prior to the explosion, yet is undetected at 3.6 and 24 m. The distinct presence of K dust, along with stringent LBT limits on the optical fluxes, suggests that the progenitor of SN 2008S was engulfed in a shroud of its own dust. The inferred luminosity of L implies a modest mass of M. We conclude that objects like SN 2008S are not exclusively associated with the deaths or outbursts of very massive Carinae-like objects. This conclusion holds based solely on the optical flux limits even if our identification of the progenitor with the mid-IR source is incorrect.
Over the last 20 years, several significant milestones have been reached in the pre-explosion detection of core-collapse supernova progenitors. These began with the “peculiar” type II-P supernova 1987A in the Large Magellanic Cloud (e.g., Menzies et al. 1987), where a cataloged blue supergiant star was identified as the progenitor (Sk 69 202; e.g., West et al. 1987). Next came the transition type IIb 1993J in M81, with a progenitor identified as a red supergiant in a binary system (e.g., Podsiadlowski et al. 1993; Maund et al. 2004). During the last decade, analyses of pre-explosion archival optical imaging of nearby galaxies obtained (mainly) with the Hubble Space Telescope have convincingly shown red supergiants with masses to be the typical progenitors of type II-P supernovae (e.g., Smartt et al. 2004; Li et al. 2007), the most common core-collapse supernovae. Curiously, the progenitors of nearby type Ib/c supernovae, thought to result from very massive ( ) stars with strong winds that end their lives as Wolf-Rayet stars, have evaded optical detection (e.g., Crockett et al. 2008).
The rarest and most diverse class of core-collapse supernovae are the type IIn (Schlegel 1990), which represent of all type II supernovae (e.g., Capellaro et al. 1997). Their optical spectra, dominated by Balmer lines in emission, and slowly declining light curves show clear signatures of interactions between the supernova ejecta and a dense, hydrogen-rich circumstellar medium (e.g., Filippenko 1997). Mainly due to their low frequencies, high mass loss rates, and the massive circumstellar envelopes generally required to explain the observations, some luminous type IIn supernovae have been associated with the deaths of the most massive stars (e.g., Gal-Yam et al. 2007; Smith 2008 and references therein). Recently, evidence for this association has increased with the report of a very luminous source in pre-explosion images of the type IIn SN 2005gl (Gal-Yam et al. 2007) and the discovery of an LBV eruption two years before the explosion of SN 2006jc (Pastorello et al. 2007). On the other hand, some low luminosity type IIn have been associated with the super-outbursts of LBVs like Carinae (e.g., Van Dyk et al. 2000; Van Dyk et al. 2006).
The appearance of the type IIn SN 2008S in the nearby galaxy NGC 6946 ( Mpc; Sahu et al. 2006) was fortuitous, since a massive stellar progenitor would be relatively easy to find. However, pre-explosion images serendipitously obtained from the Large Binocular Telescope revealed nothing at the position of SN 2008S, allowing us to put stringent limits on the optical emission. In this Letter, we report the discovery of an infrared point source coincident with the site of SN 2008S using archival Spitzer Space Telescope data. The Spitzer mid-IR detection, and deep optical non-detections, of the progenitor are the tell-tale signs of a M star obscured by dust. We describe the available data in § 2, our analysis in § 3, and our conclusions in § 4.
2. Searching for the Progenitor
NGC 6946 is quite a remarkable galaxy, giving birth to (at least) nine SNe in the last century. The latest event discovered in NGC 6946 is SN 2008S, found on February 1.79 UT at mag (Arbour & Boles 2008) and located 52 West and 196 South of the nucleus of NGC 6946. It was spectroscopically classified as a likely young type IIn supernova from the presence of narrow Balmer lines in emission, highly reddened by internal extinction with a measured Na D absorption equivalent width of 5 Å (Stanishev et al. 2008). Steele et al. (2008) later reported that it had a peculiar spectrum due to the presence of narrow emission lines from the [Ca II] 730 nm doublet, Ca II infrared triplet, and many weak Fe II features. The spectral properties and low peak luminosity led Steele et al. (2008) propose that SN 2008S was a supernova impostor such as SN 1997bs (Van Dyk et al. 2000).
Accurate coordinates are needed in order to search for the progenitor in pre-explosion images. Fortunately, Swift started monitoring SN 2008S with UVOT and XRT shortly after the discovery. We retrieved the UVOT optical images obtained on Feb. 4.8, 6.0, and 10.5 (UT) from the Swift archive. We used WCSTools v3.6.7 (Mink 1999) and the USNO-B astrometric catalog (Monet et al. 2003) to obtain astrometric solutions for the images. The mean coordinates of SN 2008S are , (J2000.0), with rms uncertainties of and .
The Large Binocular Telescope (Hill et al. 2006) obtained deep optical images of NGC 6946 on 1921 May 2007, 225 days before discovery, during Science Demonstration Time using the LBC/Blue camera (Ragazzoni et al. 2006; Giallongo et al. 2008). We combined the sec images obtained using the filter (seeing ), and the sec images obtained using the and filters (seeing ). After finding an astrometric solution for the combined images using the USNO-B catalog (), we do not detect a source at the position of SN 2008S (see Fig. 1). After calibrating the images using ancillary optical data obtained by the Spitzer Infrared Nearby Galaxies Survey (SINGS; Kennicutt et al. 2003) and Swift, we obtain 3 upper limits on the progenitor magnitudes of , and , which correspond to absolute magnitudes , , and , correcting for mag of Galactic extinction (Schlegel et al. 1998). The upper limits are calculated using aperture photometry from the standard deviation of the sky at the SN position using a 10 pixel () diameter aperture. We correct these values with aperture corrections estimated using bright stars. The mag uncertainties in the 3 upper limits are due to the uncertainties in the aperture corrections and the standard deviation of the sky (which is estimated from the rms variations in the standard deviation measured in equal-sized apertures placed in the background around the SN position). Welch et al. (2008) reported 3 upper limits from pre-explosion Gemini/GMOS observations of , and . These correspond to absolute magnitudes , , and , correcting for Galactic extinction.
Such a deep non-detection led us to investigate IRAC ( m; Fazio et al. 2004) and MIPS ( m; Rieke et al. 2004) images obtained by the SINGS Legacy Survey in 2004. We astrometrically calibrated the images in the same way as the optical images from Swift and LBT. We detect a point source at , in the 4.5, 5.8, and 8.0 m IRAC bands (see Fig. 1), with rms uncertainties , . This is consistent with the position of SN 2008S given the estimated uncertainties, and thus likely to be the progenitor. The source is not detected at 3.6, 24, or 70 m. We estimate a probability of random coincidence given the uncertainty in the SN position () of 0.8% (0.02%) from the density of 4.5 micron sources (with [3.6][4.5] 1.5 mag) detected within a radius of the SN position.
We searched the Spitzer archive for all the programs that have observed NGC 6946. Observations by the SINGS survey (PID: 159), and two programs (PIs: Meikle, Sugerman, Barlow) monitoring the type II-P SNe 2002hh and 2004et (PID: 230, 20256, 30292, 30494) provide a 2.5-year baseline (June 2004 January 2007) of IRAC and MIPS observations prior to the discovery of SN 2008S. We used aperture photometry (a 2 pixel extraction radius with aperture corrections) in the flux-calibrated images provided by the Spitzer Science Center to derive light curves for the progenitor. Fig. 2 shows the flux density as a function of time in the 4.5, 5.8, and 8.0 m bands starting from June 2004. There is no sign of variability at the level. The non-detection at 3.6, 24, and 70 m in single and stacked images allows us to place useful upper limits on these fluxes.
Finally, we searched the Chandra archive to determine if the progenitor was an X-ray source. All five ACIS-S observations of NGC 6946 include the location of SN 2008S. These observations include a 60 ks exposure in 2001, a 30 ks exposure in 2002, and ks exposures in 2004. No source is detected at the supernova position in any of these images. We set a 3 upper limit on the flux of the progenitor of erg cm s ( erg s) in the broad X-ray band (0.58 keV), which rules out a bright X-ray binary as the progenitor. This flux limit corresponds to 20 counts in the longest exposure. Table 1 summarizes the detections and 3 upper limits on the progenitor fluxes.
|0.64 m||Welch et al. (2008)|
|0.80 m||Welch et al. (2008)|
3. Beneath the Shroud
The measured fluxes and upper limits in the mid-IR bands are shown in Fig. 3. The shape of the spectral energy distribution (SED) suggests thermally-radiating dust as the source of the emission. We derive a best-fit single-temperature blackbody of K, with a luminosity of L ( Mpc; Sahu et al. 2006), which implies a blackbody radius111These values would change to L and AU if we assume an extreme distance to NGC 6946 of 8.5 Mpc, which is the 3 upper limit of the distance used by Li et al. (2005; 5.51.0 Mpc). AU. This luminosity points to a M star at the end of its life (e.g., Meynet & Maeder 2003). The 3 upper limit at 70 m further limits the total luminosity of the dust-enshrouded source and the geometry of obscuring dust distribution.
As shown in Fig. 3, a blackbody yields a relatively poor fit to the data ( per d.o.f.). The inability of a single-temperature blackbody to accommodate the data follows primarily from the rapid change in the SED implied by the 3.6 m upper limit and the 4.5 m detection. Radiation transport calculations using DUSTY (Ivezic & Elitzur 1997) were performed as a sanity check. Using a central incident blackbody with K we calculated the emergent spectrum from a spherical dusty shell extended over approximately one decade in radius. As expected, the best correspondence with the data is obtained for a total optical depth at 8.0 m of order unity, although the precise value depends on the assumed radial gradient of the density, the radial extent of the obscuring medium, and the mixture of grain types. Although a detailed investigation of the dust properties is beyond the scope of this Letter, we note that the strong evolution in the SED between 3.6 and 4.5 m may signal the need for relatively large grains (e.g., Ivezic & Elitzur 1996).
We can estimate the mass of obscuring gas and dust by assuming that the medium is marginally optically thick at 8.0 m. Setting , and assuming , we find that M, where cm g is a typical value for the Rosseland-mean dust opacity for gas at K (e.g., Semenov et al. 2003). This suggests a gas density on the scale of . We also estimate a minimum mass loss rate from the progenitor of M, where km s is the gas sound speed in the medium on the scale of .
The lack of variability in the mid-IR fluxes (see Fig. 2) limits the expansion velocity of the photosphere. Given our estimated temperature and luminosity, keeping the mid-IR fluxes constant to within % over the days covered by the observations means that the dust photosphere cannot be expanding by more than km s, which is below the escape velocity of 13 km s for a M star at the estimated photospheric radius of AU. This is further evidence that the dust is part of a relatively steady, massive wind rather than an explosively-expelled dust shell.
4. Discussion and Conclusions
Our pre-explosion detection of the progenitor of the type IIn SN 2008S is, to the best of our knowledge, the first in the mid-IR. The Spitzer observations suggest an enshrouded star with a mass of M, buried in M of gas and dust. If SN 2008S was a real supernova explosion, this is direct evidence that relatively low-mass stars can end their lives as type IIn SNe when they have a sufficiently dense CSM from a massive wind, as proposed by Chugai (1997). If this event was the luminous outburst of an LBV, it presents evidence for low-luminosity, low-mass LBVs that have not been observed before222The lowest-mass LBVs known have initial masses of M and luminosities L (e.g., Smith et al. 2004; Smith 2007).. These conclusions about the relatively low mass hold even if the identification of the progenitor with the Spitzer source is incorrect. In this case, we know the total extinction from the colours of the SN (see below). As shown in Fig. 3, our optical limits with this extinction correspond to mass limits of M for red supergiants and M for blue supergiants333We obtain an upper limit in the absolute optical magnitude of the progenitor of if we assume an upper limit on the extinction estimate from the SN color ( mag; bluest black-body possible) and an extreme distance to NGC 6946 of 8.5 Mpc..
Interestingly, we see luminous dust-enshrouded stars in the Milky Way and the LMC whose physical properties match well the observed properties of the progenitor of SN 2008S. van Loon et al. (2005, and references therein) studied the properties (, , , ) of dust-enshrouded AGB stars and red supergiants in the LMC using mid-IR observations. These are M-type stars with effective temperatures K, which have strong winds with high (gas + dust) mass loss rates ( M yr), and warm dust emission from their dusty envelopes (200 K 1300 K). Due to these similarities, we conclude that the progenitor of SN 2008S was likely a dust-enshrouded AGB (core-collapse produced from electron capture in the O-Ne-Mg core, e.g., Eldridge et al. 2007; Poelarends et al. 2008) or red supergiant like the ones observed in the LMC.
Although the detection and physical properties of the progenitor are the main results of this study, we can also try to understand something about the progenitor and explosion mechanism from the supernova itself. The classification spectrum of SN 2008S is similar to the published spectrum of SN 1997bs (Van Dyk et al. 2000), which showed narrow Balmer lines in emission and many weaker Fe II lines (V. Stanishev, priv. comm.; Steele et al. 2008). SN 2003gm had photometric and spectroscopic characteristics similar to SN 1997bs (Maund et al. 2006). Since both of these were faint ( mag) compared with the typical absolute magnitudes at maximum of type II SNe ( to mag), it is still debated whether they were intrinsically faint explosions or super-outbursts of LBVs.
The early optical photometry obtained with Swift also indicates that SN 2008S was a low-luminosity object, with mag after correcting for the total extinction along the line of sight. We estimate the total extinction for to be mag from the observed color mag and assuming a typical intrinsic temperature of K at this early phase of the evolution. This value is roughly consistent with the estimated reddening obtained from the reported equivalent width of the Na D absorption feature (; based on Turatto et al. 2002). This implies the presence of significant internal extinction with mag after correcting for mag. Although the light from the supernova likely destroyed the dust that obscured the progenitor to significantly beyond the blackbody scale of AU, the existence of internal extinction in the supernova light curve implies a more tenuous dusty obscuring medium on larger scales. In fact, the rare detection of the [Ca II] 730 nm doublet in emission by Steele et al. (2008) may provide direct, and independent, evidence for a significant amount of dust in the CSM that was destroyed by the UV-optical flash (e.g., Shields et al. 1999). The future spectra and light curves of SN 2008S, optical as well as radio and X-ray, should further probe the environment as they show signs of interactions with the progenitors’s wind.
The field of supernova forensics has advanced rapidly in recent years, with SN progenitors now known (e.g., Smartt et al. 2004; Li et al. 2007). Moving forward, several groups are obtaining the data required to more fully characterize the progenitors of future nearby SNe (e.g., Kochanek et al. 2008). We note that the discovery of the progenitor of SN 2008S itself would not have been possible only a few years ago without Spitzer. Future multi-wavelength surveys of the local universe are thus encouraged in order to catch other unexpected stellar phenomena, potentially even before they occur.
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