A VLA Study of the Expansion of Tycho's Supernova Remnant

We have measured the expansion of Tycho's supernova remnant (SNR) over a 10 yr interval by comparing new VLA observations at 1375 MHz made in 1994 and 1995 with previous observations made in 1983 and 1984 using the same array configurations, bandwidths, calibrators, and integration times. To compute the expansion of the outer rim, we estimate the expansion rate for radial sectors of 4 degrees. The overall mean expansion obtained is 3." 05 but varies between 1 " and 5 " around the shell. The weighted average fractional expansion rate is 0.1126% yr(-1), corresponding to a power-law index (expansion parameter) v = dln R/dlnt = 0.471, with an error of similar to 6%. We also compute the expansion of interior features, and find a comparable global expansion parameter. The value of v obtained is in excess of the pure Sedov value of 0.4, indicating that Tycho's SNR is still in transition from an earlier phase to the Sedov adiabatic phase of evolution, and supporting expansion into a medium without a strong radial density gradient, as expected for a Type Ia supernova. In addition, the significant local variations that we observe suggest that a unique average expansion rate is an oversimplified description for Tycho. Both radio flux and polarization were found to remain almost constant during the intervening 10 years.