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Date: Thu, 15 Apr 93 17:08:56 EDT From: Kurt w. Weiler <kweiler@shimmer.nrl.navy.mil> Subject: SN1993J Radio Information Update SN 1993J in NGC 3031 N. Panagia, Space Telescope Science Institute and University of Catania; S. D. Van Dyk and K. W. Weiler, Naval Research Laboratory report that the available radio observations of SN 1993J are already sufficient for initial modelling of the progenitor star and projection of the likely evolution of the radio emission. Applying the Chevalier mini-shell model which has been shown appropriate for other radio supernovae (Weiler, et al. 1986, ApJ 301, 790) with the usual assumptions for the stellar wind from the red supergiant progenitor star and assigning a normal spectral index of -0.7 and an index for the emission decay rate of -0.7, the model yields a presupernova mass loss rate of about 2 x 10 ^-6 Msun/yr. The stellar evolution models of Maeder & Meynet (1988, A&AS, 76, 411) then imply a ZAMS mass for the progenitor of SN1993J of <15 Msun. Comparison with other known radio supernovae, however, indicate that SN1993J is quite similar in radio properties to SN1980K and SN1981K, and may represent the lower end of the mass range of stars which can explode as Type II supernovae. This relative consistency of properties may indicate a relatively firm lower limit to the radio properties of Type II supernovae, and therefore a possible minimum distance indicator. Our modelling also results in predictions of the radio flux density to be expected from SN1993J at various frequencies to assist planning observations at other telescopes. Taking 26 March 1993 as Day = 1, the expected peak flux density at different radio frequencies is ~20 mJy on Day 20 (14 April) at 23 GHz, ~20 mJy on Day 25 (19 April) at 15 GHz, ~25 mJy on Day 40 (4 May) at 8.4 GHz, ~30 mJy on Day 55 (19 May) at 5 GHz, and ~35 mJy on Day 130 (2 August) at 1.4 GHz. After peak, the flux density at each frequency should decline approximately as t^-0.7.
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