Vikram Dwarkadas , Lewis Ball , James Caswell , Anne Green , Simon Johnston , Brian Schmidt , Mark Wardle, PASA, 17 (1), 83.
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Title/Abstract Page: SUPERNOVA REMNANTS, PULSARS AND
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Supernova remnants and the Surrounding Medium I
The first session in this workshop dealt with supernovae (SNe) and their interaction with surrounding circumstellar material (CSM). In particular, papers were presented on the diversity of SNe in general, and on some detailed observations of two very young objects (SN 1987A and SN 1993J) which now show evidence for interaction between the expanding ejecta and the surrounding material. 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.
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