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The rise time of the normal Type Ia SN 1990N
Paolo A. Mazzali 1 and Brian P. Schmidt 2
1 Osservatorio Astronomico, Trieste, Italy 2 Mt. Stromlo, Australia
The earliest spectrum available of a well-observed, local SN Ia,
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 tting template light curves.
If a standard density distribution ( / r 7 ) and composition for
the SN ejecta are used, and the Cepheid distance modulus to the
host galaxy, NGC4639 in the Virgo cluster,  = 32:00, is adopted,
the 14d spectrum of SN 1990N is best reproduced if the epoch
is t = 5  1 days, implying for the SN a rise time of 19  1 days.
For smaller epochs the temperature of the ejecta is too high, or
the photosphere must be at velocities so large that there is not
enough material above it to form strong absorption lines. For
larger epochs the opposite problem arises, the temperature is too
low or a low velocity must be used, leading to exceedingly strong
lines. Better ts in these cases can only be obtained for non-
standard Si abundances. Alternatively, rise times ranging around
dierent values can be obtained if dierent distances to the SN are
used. If  = 31:70, t = 41 days, but the best solution is obtained
if the ejecta mass is reduced below the commonly accepted value of
1.4M . The opposite trend holds for longer distances. Therefore
the Cepheid distance and a rise time of 19  1 days appear to be
the best set of parameters if SN 1990N is a standard SN Ia.
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