SN 1994I in M51 (NGC 5194) was one of the nearest supernovae in the past few decades. Despite some confusion during the first few days after outburst, it was eventually classified ``Type Ic" based on the absence of obvious hydrogen and helium lines in its early-time optical spectra. We present a series of near-ultraviolet through near-infrared spectra of SN 1994I obtained with the 3-m Shane reflector at Lick Observatory; the data span the range from a week prior to maximum brightness through several months past maximum. In general, the details of the spectra resemble those of the well-observed Type Ic SN 1987M. For example, the transition to the nebular phase, dominated by strong emission lines of intermediate-mass elements, began to occur about two months after the explosion. Our most important discovery, however, is that SN 1994I showed strong He I lambda 10830 absorption between one and four weeks past maximum brightness. (The earliest spectra, as well as spectra obtained 1--2 months past maximum, do not extend sufficiently far to the infrared. The line was probably still present, but weak, two months past maximum.) Thus, if SN 1994I was a typical Type Ic supernova, the atmospheres of these objects cannot be completely devoid of helium. To test this hypothesis, more Type Ic supernovae must be observed at near-infrared wavelengths; in the optical region the He I lines are weaker, and they are also contaminated by lines of other species (e.g., He I lambda 5876 is blended with Na I lambda 5892). The presence of He I lambda 10830 in the spectrum of SN 1994I suggests that instead of falling into two distinct subtypes (Ib = helium, Ic = no helium), Type I supernovae lacking silicon absorption span a continuous range in their atmospheric helium abundance (Type Ibc). Their progenitors are probably massive stars that have lost different fractions of their outer layers of hydrogen and helium, through either winds or mass transfer onto a bound companion.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1994