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Session 40 - Supernova Remnants & SN 1987A.
Display session, Thursday, January 08
Exhibit Hall,

[40.10] Radio Evidence for Rapid Presupernova Stellar Evolution

K. W. Weiler (NRL), S. D. Van Dyk (UCLA), M. J. Montes (NRL), R. A. Sramek (NRAO), N. Panagia (STScI)

SN 1979C prior to 1990 (age < 4000 days) follows a standard, albeit sinusoidally modulated declining radio emission. However, after \sim 4000 days a slow increase in the radio light curve occurs at all wavelengths. By day \sim 6500, this change in evolution implies an increase in flux density by a factor of \sim 1.8 with respect to the standard model or a density enhancement by a factor of \sim 1.35. This may possibly be understood as a change of the CSM density profile from the r^-2 law, which was applicable until day \sim 4000, to an appreciably flatter behavior (\sim r^-1.2).

SN 1980K prior to day \sim 4000 is also well behaved. However, more recent measurements show a steep decline in flux density at all wavelengths by a factor of \sim 2 occurring between day \sim 4000 and day \sim 4500. Such a sharp decline in flux density implies a decrease in \rho_CSM by a factor of \sim 1.6.

SN 1988Z, similarly to SN 1980K, shows a sharp drop in its flux density with respect to its expected radio evolution. Although the parameters of the change are yet to be quantified, it appears to also have evolved rapidly in the last few thousand years before explosion.

Thus, for at least SN 1979C, SN 1980K, and SN 1988Z (and well known for SN 1987A), we have significant changes in radio flux density occurring a few thousand days (\sim 10 years) after the explosion. Since the SN shock is moving \sim 1000 times faster than the wind material of the RSG progenitor (\sim 10,000 km s^-1 vs. \sim 10 km s^-1), such a time interval implies a significant change in the presupernova stellar wind properties \sim 10,000 years before the explosion. Although short compared to the lifetimes of typical RSN progenitors, such an interval is comparable to the evolutionary time since the last ``blue loop'' episode for stars with masses up to \sim 12 -- 14 M_ødot for solar metallicity (and higher for lower metallicities) so that changes in the radio emission may indicate a significant transition in the evolution of the presupernova stars.

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Program listing for Thursday