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Session 74 - Supernova Remnants.
Display session, Thursday, June 13
Between 1983 and 1984, Tycho's supernova remnant was observed with the VLA using all array configurations at several wavelengths within the 20 and 6 cm bands (Dickel, van Breugel, and Strom, 1991 AJ 101, 2151). By matching these observations in the current epoch (1994 to 1995) we have successfully measured Tycho's expansion over a 12-year period of time, using the same array configurations, frequencies (1375 and 1635 MHz), bandwidths, calibrators, and integration times. We obtain an expansion rate for each 4^\circ radial sector by fitting the cross-correlation of the derivative of the averaged profile at each epoch. The overall mean expansion rate during the mean separation of 10.9 years is 3.21^'' \pm 0.08^'' but varies between 1^'' and 5^'' around the shell. This average rate, with a mean radius for Tycho of 241^'' \pm 3^'', corresponds to a power-law index (expansion parameter) m \equiv d \ln R/d \ln t = 0.517 \pm 0.043, where the uncertainty is dominated by the spread in observation times at each epoch (of order 1.5 years to get all VLA configurations). This value is substantially in excess of the pure-Sedov value of 0.4, indicating that Tycho's remnant is still in transition from an earlier phase to the Sedov adiabatic phase of evolution. Our value is also somewhat larger than the earlier determinations of 0.47 \pm 0.05 (Strom, Goss, and Shaver, 1982 MNRAS 200, 473) and 0.462 \pm 0.024 (Tan and Gull, 1985 MNRAS 216, 949). However, the observed rates could be consistent within the quoted errors. In addition, the significant local variations we observe may make an overall expansion rate an inappropriate description for Tycho. We discuss the significance of these observations for the evolutionary stage of Tycho and for possible interactions with inhomogeneous ambient material.
\bigskip ^*The VLA is a facility of the National Radio Astronomy Observatory, operated by Associated Universities, Inc., under a cooperative agreement with the National Science Foundation.
Program listing for Thursday