Bright Infrared Progenitor of Luminous Transient in NGC300 implies Explosion of a Massive Star

ATel #1550; Jose L. Prieto (Ohio State University)
on 29 May 2008; 15:57 UT
Credential Certification: Jose Prieto (prieto@astronomy.ohio-state.edu)

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We report analysis of pre-explosion, archival Spitzer images of NGC300 obtained in Dec. 28, 2007 (PI: R. Kennicutt). We detect an infrared source at RA=00:54:34.53, DEC=-37:38:31.71 (J2000), which is within 1 sigma (0.25") of the position of the luminous transient discovered by Monard (IAUC # 8946 ), and reported by Berger & Soderberg (ATEL #1544). The source is detected in all four IRAC bands at 3.6, 4.5, 5.8 and 8 micron, and in the MIPS 24 micron band. The flux densities of the source are (in micro Jy, errors approx. 10%): 6.7 (3.6 micron), 76.6 (4.5 micron), 324.8 (5.8 micron), 877.2 (8.0 micron) and 2523.0 (24 micron). The spectral energy distribution of the Spitzer source suggests radiation from warm dust (see "http://www.astronomy.ohio-state.edu/~prieto/ngc300_sed.gif"). A black-body fit to the SED gives an integrated luminosity of L=9x10^4 Lsun (using a distance to NGC300 of 1.9 Mpc) and dust temperature of 300 K. The luminosity of the source implies a massive star with 15-20 Msun. The properties of this SED are remarkably similar to the properties of the progenitor of SN2008S in NGC6946 detected in Spitzer images and undetected in deep optical imaging (Prieto et al. 2008, ApJL accepted, http://arxiv.org/abs/0803.0324). Furthermore, the luminosity of the optical transient (relatively faint compared to typical core-collapse supernovae, too bright for novae) and spectrum reported in IAUC # 8946 also have striking similarities. We propose that the luminous transient in NGC300 and SN2008S share the same origin: they are most likely the energetic explosion of a massive star enshrouded in its own dust. It is still not clear if they are the result of core-collapse or something like the super-outburst of an LBV.