Proc. Neutrino Astrophysics - MPP Theory Group

Phenomenology of Supernova Explosions
Wolfgang Hillebrandt
Max-Planck-Institut für Astrophysik, D-85748 Garching, Germany
Abstract
An attempt is made to match observed properties of supernovae, such as spectra, lightcurves
etc. with theoretical ideas and predictions. It is demonstrated that neither do observational
data constrain the models in a unique way nor does theory allow for an unambiguous interpretation
of the data. Possible improvements on the theoretical side are briefly discussed.
Some Observational Facts
In general, supernovae are classified according to their maximum light spectra. Those showing
Balmer lines of H are called Type II’s, and all the others are of Type I. Those Type I’s which
show a strong Si absorption feature at maximum light are named Type Ia, and the others are
Ib’s or Ic’s, depending on whether or not they have also He I features in their spectra [1]. At
later times, several months after the explosion, when the supernova ejecta become optically
thin, Type II spectra are dominated by emission lines of H, O, and Ca, whereas Type Ia’s
have no O, but Fe and Co. Type Ib,c’s, on the other hand side, show emission lines of O and
Ca, just like the Type II’s [1]. However, the spectral classification is not always as clear. For
example, SN 1987K started out as a Type II with H lines, but changed into a Type Ib,c like
spectrum after 6 months [2].
As far as the light curves are concerned, Type II’s seem to be more complicated than
Type I’s. Type II-L are characterized by a peak lasting for about 100 days, followed by a
“linear” decay in the blue magnitude vs. time diagram. In contrast, Type II-P’s have a
somewhat wider peak, followed by a “plateau” phase and an occasionally rather complicated
tail decaying not like a single exponential. Typically, Type II-L’s are brighter than Type II-
P’s in the blue. In addition, there are objects like SN 1987A which possess a very complicated
light curve, with an early narrow peak, a first minimum, followed by a broad hump after a
few months, and a final nearly exponential decay with indications of some flattening at late
times. SN 1987A-like objects are much dimmer than all other Type II’s [1].
In contrast, all Type Ia light curves are quite similar, making them good candidates for
standard candles to measure the cosmic distance scale. Moreover, since they are the brightest
among all supernovae, they can be observed even at high redshifts and, in fact, a Type Ia
supernova at a redshift of about 1 has recently been observed [3]. Although their absolute
peak luminosity may vary by about one magnitude, an observed correlation between the
luminosity and light curve shape (the brighter ones have broader light curves) allows one
to correct for the differences. So recently observations of Type Ia’s have become a powerful
tool in attempts to determine cosmological parameters [3]. Of course, one has to make sure
that the supernovae one is observing are indeed of Type Ia, and not of Type Ib,c which are
intrinsically fainter, but this can be achieved if a spectrum near maximum light is available.
Other observational information on supernovae is, in general, less solid. With the exception
of SN 1987A, we do not have direct information on the properties of the progenitors,
the energetics, or the masses of the ejecta and of the (compact) remnants, if there are any.
47