It is clear that the evolutionary stage of the progenitor star
determines the kind of SN that occurs. However, it is only rarely that
we have comprehensive data on the progenitor. Typically,
classification is made from the observation of the supernova event and
its aftermath, and a great diversity is seen in these catastrophic
explosions. Brian Schmidt (ANU) gave a comprehensive review of
SN classification emphasising this diversity and the fact that many
events do not fit the existing sub-type classifications, based on
studies of light curves and optical spectra (Filippenko 1997).
Because there are so many SNe which are atypical, it may be that the
broader groupings of ``thermonuclear explosions'' (involving
predominantly white dwarfs) and ``core collapse of massive stars''
might lead to better predictions of the progenitor star type. It is
clear that variations in age, mass and metallicity can all affect the
SN light curve and spectrum. In the core collapse scenario, there are
five phases which produce different spectral characteristics. These
phases relate to shock break-out, adiabatic cooling, transfer of
energy and subsequent radioactive heating in the core, and the
eventual transition to the nebula phase. However, it is unclear what
are the primary causes of differences in observed events. Present
models involving variation in the energy of the initial explosion,
mass loss rates and the condition of the CSM do not seem to explain
the observed diversity. In a further twist, it may be that some types
of SNe (for example Type Ib/c) may be linked to the phenomenon of
Gamma-Ray-Bursters (GRBs).
The first specific example selected to demonstrate CSM interaction was
SN 1987A, in the Large Magellanic Cloud. Ray Stathakis (AAO)
presented results from optical and infrared monitoring with several
instruments mounted on the Anglo Australian Telescope (AAT). Hubble
Space Telescope (HST) images show evidence for the SN ejecta
interacting with the edge of the CSM, from H
and Ly-
observations. At the AAT the source is not well resolved. However,
monitoring of optical CSM lines establishes valuable baseline levels
against which future changes due to increasing interaction may be
measured. Several spectral lines, both in the optical (eg. H,
OI) and infrared (eg. FeII, Br
)
regimes are becoming strong
enough to image. It is anticipated this program will continue.
Radio observations of SN 1987A, made with the Australia Telescope
Compact Array (ATCA), were presented by Lister Staveley-Smith
(ATNF). Evolution of both angular size and flux density is seen. Radio
frequency observations have been an effective way of monitoring the
expanding shock front (Gaensler et al. 1997). Finding a consistent
model to explain the results is more problematic. The data show that
the overall radio luminosity is increasing linearly and that the EW
asymmetry in the brightness of the observed circular ring is also
becoming more pronounced. From the change in image size over several
years, it appears that the expansion velocity has slowed
significantly. The morphology of the images suggests a thin spherical
shell with an EW asymmetry, expanding and now very close to the ring
of CSM. Evidence for the onset of interaction is seen in the HST
H
and Ly-
observations. It is speculated that the
emission is coming from the reverse shock, consistent with the low
value for the expansion velocity. Two possible models which might
explain the observed results both have some unsatisfactory
features. The minimum energy solution is inconsistent with a low shock
velocity and the model invoking a dense HII torus to account for the
slow shock velocity would not predict the symmetric ring observed, nor
the inferred spherical shock. Overall, it seems that SN 1987A was an
atypical Type II SN. It is expected that the shock will heavily impact
the CSM ring in about 2004. High resolution observations at 20 GHz are
planned with the ATCA for the anticipated impressive display.
The second object selected to illustrate early interaction with the
CSM is SN 1993J. Michael Rupen (NRAO) showed results from VLBI
observations of this young SNR (Bartel et al. 1994; Rupen et
al. 1998), which was the brightest optical SN seen in the northern
hemisphere since 1937. Early observations classified this event as a
core collapse SN (Type IIb) of a massive progenitor star, probably
about 15 M
.
This object has been closely monitored since 30
days after the explosion over several wavelengths in the range 1-20
cm. The SN occurred in M81, a galaxy 3.63 Mpc away. Distance estimates
from the SN observations agree well with the independent estimate from
Cepheid measurements. The object is now seen as a nearly-circular
expanding shell with an asymmetric brightness distribution. There is
some indication that the core may be located off-centre. However,
there is clear evidence of source evolution and the shell is
noticeably decelerating, even if the most extreme opacity effects are
included.
From the review by Schmidt and the specific data on SN 1987A and SN
1993J, it is clear that even for very young remnants, the
peculiarities of the individual SN explosion and the pre-existing CSM
are far stronger influences than any underlying generic
characteristics. This makes it hard to develop global theories and
emphasises the need for continuing searches and subsequent long-term
monitoring of SNe.
© Copyright Astronomical Society of Australia 1997