Dusty Massive Stars: the Origin of the Luminous Optical Transient in M85

J.L. Prieto, M.D. Kistler, K.Z. Stanek, T.A. Thompson, C.S. Kochanek, J.F. Beacom

We report analysis of pre-discovery archival Spitzer data of the luminous transient in the Virgo galaxy M85 (Kulkarni et al. 2007, Nature, 447, 458; Pastorello et al. 2007, Nature, 449, 1) obtained on UT Dec. 28.72, 2005 (PI: Andreas Zezas), 8.8 days before the optical discovery of the transient reported by the KAIT supernova search (Jan 6.6, 2006). We detect an infrared source at the position of the transient in all four IRAC bands (3.6-8.0 micron). The flux densities of the source are (in micro Jy, errors approx. 10%): 176 (3.6 micron), 206 (4.5 micron), 239 (5.8 micron), and 185 (8.0 micron). This bright infrared source is most likely associated with the transient, and not the progenitor, since it was not detected in archival Spitzer data of M85 obtained in 2004 (Rau et al. 2007, ApJ, 659, 1536). The transient is also detected in infrared images obtained with Spitzer 7 months later (Rau et al. 2007), although the infrared fluxes have decreased by a factor of ~5 from the earlier epoch. If we assume that the optical fluxes of the transient were roughly constant between the Spitzer pre-discovery detection and the initial post-discovery optical+NIR photometry obtained on Jan. 2006 by Kulkarni et al., the spectral energy distribution of the transient is well-fit by the sum of two black-bodies with T1~3900 K and T2~800 K (see Fig. 1). The colder, infrared-bright component is likely due to reprocessing of UV/optical light by circumstellar dust. This bright, early infrared detection of the transient resembles the early spectral energy distribution of SN 2008S (Prieto et al. 2008, ApJ, 681, L9; Wesson et al. 2008, CBET #1381; see Fig. 2). Furthermore, the optical luminosity at discovery and the spectral properties of the M85 transient are similar to SN 2008S and the luminous transient in NGC 300 (Monard 2008, IAUC # 8946 ; Berger & Soderberg 2008, ATeL #1543; Prieto 2008, ATeL #1550). We propose that the transient in M85, SN 2008S, and the luminous transient in NGC 300 form a new class of explosions of massive stars embedded in their own optically-thick dust.

Fig. 1

Fig. 2