The peculiar type II supernova 1993J in M81: Transition to the nebular phase

We present optical spectra of the bright, peculiar Type II supernova 1993J in M81 spanning the first 14 months of its existence, revealing its transition to the nebular phase. Unlike the case in normal Type II supernovae, during the first 2-10 months the Hα emission line gradually becomes less prominent relative to other features such as [O I] λλ6300, 6364 and [Ca II] λλ7291, 7324, as we had predicted based on early-time (τ <~ 2 months) spectra. The nebular spectrum resembles those of the Type Ib/Ic supernovae 1985F and 1987M, although weak Hα emission is easily visible even at late times in SN 1993J. At τ = 8 months a close similarity is found with the spectrum of SN 1987K, the only other Type II supernova known to have undergone such a metamorphosis. The emission lines are considerably broader than those of normal Type II supernovae at comparable phases, consistent with the progenitor having lost a majority of its hydrogen envelope prior to exploding. Consequently, there is now little doubt that Type Ib, and probably Type Ic, supernovae result from core collapse in stripped, massive stars; models of the chemical evolution of galaxies in which these subtypes are ascribed to exploding white dwarfs must be appropriately modified. Although all of the emission lines in spectra of SN 1993J fade roughly exponentially for a considerable time, the fading of Hα begins to slow down at τ = 8 months, and in the interval τ = 10-14 months its flux is constant, or even slightly rising in the wings of the line. This behavior, together with the box-like shape and great breadth (full width at half maximum [FWHM] ~ 17000 km s^-1^) of the line profile, suggests that the Hα emission is being produced by the high-velocity outer layer of hydrogen ejecta interacting with circumstellar gas released by the progenitor prior to its explosion. A similar phenomenon has previously been seen at later phases in several Type II supernovae, most notably SN 1980K. Bumps (FWHM ~ 1000 km s^-1^, amplitude ~20%) in the Hα profile are probably indicative of Rayleigh-Taylor instabilities in the cool gas behind the reverse shock. A very narrow component (unresolved, FWHM <~ 200 km s^-1^) of Hα at the systemic velocity of SN 1993J may instead be produced by a superposed H II region, or perhaps by recombination in a large circumstellar shell or ring that was ionized during the first few hours after outburst. In the near future the spectrum of SN 1993J should become increasingly dominated by broad Hα emission.