Type II Supernova Photospheres and the Distance to Supernova 1987A

Among the many historic opportunities provided by the recent supernova in the LMC is that to improve our understanding of the physical conditions in the neighborhood of supernova photospheres, even though 1987A was initially characterized by radial and time scales smaller (by a factor 5–10) than “standard” more luminous SNII. Two consequences of this understanding, which we shall focus on in this contribution, are (a) an estimate of the (frequency-dependent) location and thickness of the photosphere and (b) the only direct determination of the distance of the supernova (via the generalized Baade method). We find that the photosphere is sharp enough to allow the use of plane-parallel geometry in the calculation of the emergent continuum spectral flux, if we confine our attention to those epochs (temperature T ~ 5000−6000 K) at which hydrogen is recombining at the photosphere. We also find that the distance to this supernova is 43 ± 4 kpc. The reliability of this determination should improve when accurate spectrophotometric data for dates other than March 1 become available to us.