Observations of the Type II-P SN 1991G in NGC 4088

Abstract

We present VRI photometry and optical spectra of SN 1991G in NGC 4088. The strong Hα P-Cygni feature observed in the early spectrum coupled with the shape of the light curve demonstrate that SN 1991G is of type II-P (plateau). SN 1991G's plateau, which lasts ~120 days, is longer than that of other well-observed SNe. We combine our photometry and spectroscopy to estimate the time of the explosion to be JD 2448280+/-3 (1991 January 23 UT), 26 days before our first photometric observation. The expansion velocity of the material at the photosphere, derived from absorption lines in our spectra, of 5590+/-340 km/s at 22 days after the explosion is slower than that observed for all the SNe II studied by Schmidt et al. (ApJ, 395,366(1992)] other than SN 1987A. We find that the bolometric luminosity of SN 1991G is lower than that of typical SNe II-P during both its plateau phase and exponential tail and find that SN 1991G ejected less ^56^Ni than most other type II-P SNe. Using the analytical models of SNe II-P by Popov [ApJ, 414,712 (1993)], we compare the explosion properties of SN 1991G to SN 1969L and SN 1992am. This comparison suggests that the explosion energy of SN 1991G was likely smaller than that of the others, and that there may be a correlation of explosion energy with the mass of ejected ^56^Ni.