A study of SN 1992H in NGC 5377

A photometric and spectroscopic analysis is presented of SN 1992H in NGC 5377 which was discovered by W. Wren at McDonald Observatory on UT 1992 February 11.5. SN 1992B displayed a distinct but truncated plateau suggesting that it was in some sense intermediate between a type II-linear and a fully developed type II-plateau event. Explosion is estimated to have occurred about 3 days before discovery. BV(RI)c light curves plus bolometric corrections based on the behavior of SN 1987A are used to compute a UVOIR bolometric light curve. This light curve shows a broad decline from early epochs that merges smoothly into the ''plateau'' at about 50 days after explosion. About 85 days after the explosion the light curve begins a precipitate decline of about 2 mag in 45 days. At that point, 130 days after the explosion, the light curve begins a linear decline with the timescale appropriate to that of Co-56 that lasts to the end of the observation at about 450 days. The early spectra show a continuum with H alpha in emission and H beta and He I lambda 5876 with broad P Cygni profiles. Velocities at this phase are in excess of 10 000 km s(-1). By 50 days after the explosion, H alpha shows a well-developed P Cygni profile. Lines of Ca H&K, Fe II, Sc II lambda 5526 and 5658, Na D, [Ca II] lambda lambda 7291, 7324, and the Ca II IR triplet are also observed on the plateau. On the tail, lines of [O I] lambda 5577 and [O I] lambda lambda 6300, 6364, and blends of [Fe II] are also observed. The latest spectra at about 450 days show a blue continuum. The B-V color evolution, the equivalent width of interstellar Na D, and the epoch of disappearance of He I lambda 5876 are used to estimate a color excess of E(B-V)=0.09+/-0.06. For this extinction, the expanding photosphere method is used to estimate a distance of 20.2+/-3.0 Mpc (mu=31.53+/-0.74). Models of the light curve show that the total mass ejected is about 11 M. with 0.075 M. of Ni-56. The latter is necessary to match both the distance-dependent amplitude of the exponential decline, but also the contrast between the plateau and the tail, which is independent of distance. The envelope of the progenitor must have extended to about 1000 R. with a density profile that dropped more slowly than exponential in order to match the relatively slow decline from the initial peak to the plateau. The decline from the plateau to the exponential tail was too precipitate in models with a distinct core-envelope discontinuity. It is unclear whether the smoother density contrast required is a constraint on the progenitor structure or may be induced by mixing in the explosion. Synthetic spectra in the late photospheric phase (50 and 119 days) are used to identify lines and estimate abundances. The results are consistent with solar abundances in the outer envelope. The evolution of the Ha line strength is consistent with an ejecta mass of 10-14 M., as derived from the light curve calculation. This is somewhat less that the lower limit to the ejecta mass derived by the constraint that sufficient gamma ray opacity be required to maintain the light curve near the Co-56 decay line for 450 days, which is greater than or similar to 15 M.. The evolution of the [Ca II] doublet and Ca II IR triplet were used to estimate the electron temperature and density and the mass of calcium. The electron temperature of the matter within 2500 km s(-1) is found to be similar to 5500 K, the electron density to be 2.5x10(9) cm(-3). The mass of Ca is estimated to be 3.5x10(-4) M.. If this matter has solar abundances, the total mass of hydrogen containing this Ca would be 4.4 M.. (C) 1996 American Astronomical Society.