SN 2008S: A cool super-eddington wind in a supernova impostor

We present visual-wavelength photometry and spectroscopy of supernova (SN) 2008S. Based on the low peak luminosity for a SN of M_R = -13.9 mag, photometric and spectral evolution unlike that of low-luminosity SNe, a late-time decline rate slower than ⁵⁶Co decay, and slow outflow speeds of 600-1000 km s^-1, we conclude that SN 2008S is not a true core-collapse SN and is probably not an electron-capture SN. Instead, we show that SN 2008S more closely resembles an "SN impostor" event like SN 1997bs, analogous to the giant eruptions of luminous blue variables (LBVs). Its total radiated energy was 10^47.8 erg, and it may have ejected 0.05-0.2 M in the event. We discover an uncanny similarity between the spectrum of SN 2008S and that of the Galactic hypergiant IRC+10420, which is dominated by narrow Hα, [Ca II], and Ca II emission lines formed in an opaque wind. We propose a scenario where the vastly super-Eddington (Γ 40) wind of SN 2008S partly fails because of reduced opacity due to recombination, as suggested for IRC+10420. The range of initial masses susceptible to eruptive LBV-like mass loss was known to extend down to 20-25 M, but estimates for the progenitor of SN 2008S (and the similar NGC 300 transient) may extend this range to ≲15 M. As such, SN 2008S may have implications for the progenitor of SN 1987A.