HIGH-RESOLUTION MEASUREMENTS OF NH3 ABSORPTION TOWARD CASSIOPEIA-A

We have observed NH3 absorption against the bright supernova remnant C assiopeia A. The 3'' resolution VLA map shows absorption against the b rightest continuum peak in the NH3 (J, K) = (1,1) inversion transition . The absorption maximum is at R.A. = 23h20m54s.8, Decl. = +50-degrees -32'23'' (1950.0), projected against the region of brightest continuum emission. This is near where the deepest absorption was found with th e 40'' beam of the Effelsberg 100 m telescope. The apparent optical de pth of the NH3 absorption, when averaged over the strongest continuum emission, is almost-equal-to 0.1. The NH3 absorption region has a size of at least 60''. In order to characterize the NH3-containing cloud, J = 1-0 and J = 2-1 spectra of CO and (CO)-C-13 were taken for the nea rest CO cloud, 20'' from the NH3 maximum. In addition, a spectrum of t he J = 2-1 transition of (CO)-C-13 was taken at the position of maximu m NH3 absorption. These CO and (CO)-C-13 data show that the V(lsr) of the cloud is -39 km s-1, the FWHP line width is 3.5 km s-1, and the co lumn density, N(H2), is 3 x 10(21) cm-2. The density, n(H2), is about 1000 cm-3 and kinetic temperature, T(k), is 18 K. The virial mass esti mated for this cloud is about 60 times the mass obtained from CO data. It is likely that the NH3 absorption and CO emission arise in the sam e physical region. This region is significantly warmer than a typical dark dust cloud, shows a significantly larger line width, and has abou t 0.1 of the cloud mass in H I. The n(H2) and N(H2) values are compara ble to dark dust cloud values. The enhanced kinetic temperature may be caused by an increased cosmic-ray rate, or by cloud-cloud collisions.