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Early spectra of supernova 1993J in M81
Optical spectra covering the region 5500 to 7000 A with a resolution of2.1 A were obtained during the first visual maximum of supernova 1993J.Thirteen spectra of the supernova were recorded between March 30.2 andMarch 31.5 (UT). They show narrow emission features of hydrogen, helium,and highly ionized iron. The H alpha line appears with a P-Cygni profileand its width decreases during the observing interval. All the sharpemission lines show a rapid drop in flux which can be attributed tocircumstellar gas being overtaken by the expanding supernova envelope.Supplementing our data with spectra taken by the Isaac Newton TelescopeGroup, we find the temporal behavior of the circumstellar lines suggeststhat the mass-loss rate of the progenitor intensified about a yearbefore the supernova explosion. A similar effect, but with a highervelocity wind, was seen in SN 1984E and suggests that enhanced mass-lossmay precede some supernovae events by a few years or less. We derive amass-loss rate during the year before the supernova of more than 3 x10-4 solar mass/yr, or about ten times the rate estimatedfrom radio techniques which sample more distant gas. The observed heliumto hydrogen emission line flux ratio is larger than expected and impliesthat the helium abundance was enriched in the precursor wind, as mightbe expected from a star stripped of most of its hydrogen envelope.Broad, shallow, blue-shifted absorption features attributed to H alphaand He I 5875 A are also seen in the spectra and indicate the velocityof the expanding supernova photosphere to be approximately 14 000 km/s.The time series of individual spectra shows a rapid color evolution forthe spectral range between 5500 and 6900 A, with the bluest continuumoccurring between March 30.5 and 30.9 (UT).
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