[Paper Review] The SN 2008S Progenitor Star: Gone or Again Self-Obscured?

J. L. Prieto, D. Szczygiel+6 more|arXiv (Cornell University)|Jun 30, 2010

Gamma-ray bursts and supernovae2 references3 citations

TL;DR

The study investigates whether SN 2008S's progenitor star survived its 2008 outburst or remains hidden by newly formed dust. Using late-time LBT observations, it finds the source is undetected in optical bands (limit ~25 mag) but detected in near-IR (K-band ~20 mag, fading rapidly), consistent with shock-heated dust at ~900 K and luminosity ~40,000 L☉. The rapid fading at constant temperature favors transient emission over a surviving star, though Spitzer observations are needed to confirm a dust photosphere or exclude a surviving progenitor.

ABSTRACT

We obtained late-time optical and near-IR imaging of SN 2008S with the Large Binocular Telescope (LBT). We find that (1) it is again invisible at optical (UBVR) 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 detections in March and May 2010 are consistent with dust emission at a blackbody temperature of T ~ 900 K and a total luminosity of L ~ 40000 Lsun, 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 support a near-IR dust echo and the decline in luminosity is shallower than the 56Co slope. If it is reprocessed emission from a surviving progenitor, a dust photosphere must have reestablished itself closer to the star than before the transient (~40 AU rather than 150 AU), unless there is a second, cooler dust component that dominates at 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.

Motivation & Objective

• To determine whether the progenitor star of SN 2008S survived its 2008 outburst or remains hidden by self-obscuring dust.
• To distinguish between a surviving massive star and transient dust emission as the source of near-IR emission observed in 2010.
• To assess whether the system has re-established a dust photosphere closer to the star than before the transient.
• To evaluate the viability of shock-heated dust versus a re-emerging progenitor in explaining the observed luminosity and decay rate.
• To guide future observations with HST and Spitzer to resolve the nature of the transient source.

Proposed method

• Conducted late-time optical (UBVR) and near-IR (HK) imaging with the Large Binocular Telescope (LBT) to detect the transient source.
• Measured fluxes and upper limits in multiple bands across 2008–2010, focusing on the K-band light curve.
• Fitted the spectral energy distribution (SED) to a blackbody model to estimate dust temperature (~900 K) and luminosity (~40,000 L☉).
• Compared the observed near-IR decay rate (1.023 mag/100 days) to the 56Co decay slope to assess energy source.
• Evaluated the plausibility of shock heating of pre-existing dust using mass-loss rate and shock velocity models.
• Proposed that future Spitzer observations are essential to detect mid-IR emission and distinguish between a surviving star and transient dust.

Experimental results

Research questions

• RQ1 Is the near-IR emission from SN 2008S due to a surviving progenitor star or transient dust emission?
• RQ2 Has the dust photosphere re-established itself closer to the star than the pre-outburst distance (~40 AU vs. ~150 AU)?
• RQ3 Does the observed near-IR luminosity and decay rate match expectations for shock-heated dust or 56Co decay?
• RQ4 Could a cooler, mid-IR-emitting dust component dominate the bolometric luminosity and explain the discrepancy in decay rates?
• RQ5 Will future Spitzer observations resolve whether the source is a surviving star or transient dust?

Key findings

• The transient remains undetected in optical bands (UBVR) to a 3σ limit of ~25 mag, indicating no detectable star or bright emission in the optical.
• Near-IR detections in March and May 2010 show the source at ~20 mag, fading rapidly at a rate consistent with 1.023 mag per 100 days, matching 56Co decay.
• The SED is well-fit by a blackbody at ~900 K with a luminosity of ~40,000 L☉, comparable to the pre-outburst progenitor's luminosity.
• The constant temperature and rapid fading rule out a near-IR echo, which would require emission from dust at ~70,000 AU, too distant to be heated by the transient's peak luminosity.
• The observed luminosity decay is slower than expected for 56Co decay, suggesting the energy source is not purely radioactive decay.
• The data favor shock heating of pre-existing dust at ~1000–2000 AU, though the required shock velocity (~3000 km/s) and mass-loss rate (~10⁻⁴ M☉/yr) make this scenario challenging to reconcile with observed timescales.