The Rise Time of the Normal Type Ia SN 1990N

The earliest spectrum available of a well-observed, local type Ia SN, the UV-optical spectrum of SN 1990N obtained 14 days before B maximum, was modelled with a Monte Carlo code in order to determine its epoch and consequently the rise time from explosion to B max, which is important for fitting template light curves. If a standard density distribution and composition for the SN ejecta are used, and the Cepheid distance modulus to the host galaxy, NGC4639 in the Virgo cluster, μ = 32.0, is adopted, the spectrum is best reproduced for an epoch t = 5.5 ± 1 days, implying for the SN a rise time of 19.5 ± 1 days. For a shorter μ, t is smaller, but for a T-F distance μ = 31.4 the solution is not consistent.