High-resolution measurements of NH3 absorption toward Cassiopeia A

Details High-resolution measurements of NH3 absorption toward Cassiopeia A High-resolution measurements of NH3 absorption toward Cassiopeia A

Apr 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...425..127g&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 425, no. 1, p. 127-131

Astronomy and Astrophysics

Astronomy

6

Absorption Spectra, Carbon Monoxide, Cassiopeia A, Methane, Molecular Clouds, Radio Astronomy, Supernova Remnants, Brightness, Continuum Flow, Optical Thickness, Stellar Temperature, Very Large Array (Vla)

Scientific paper

We have observed NH3 absorption against the bright supernova remnant Cassiopeia A. The 3 sec resolution Very Large Array (VLA) map shows absorption against the brightest continuum peak in the NH3 (J, K) = (1, 1) inversion transition. The absorption maximum is at R.A. = 23h20m54.8s, Decl. = +50 deg 32 min 23 sec (1950.0), projected against the region of brightest continuum emission. This is near where the deepest absorption was found with the 40 sec beam of the Effelsberg 100 m telescope. The apparent optical depth of the NH3 absorption, when averaged over the strongest continuum emission, is approximately equals 0.1. The NH3 absorption region has a size of at least 60 sec. In order to characterize the NH3-containing cloud, J = 1-0 and J = 2-1 spectra of CO and CO-13 were taken for the nearest CO cloud, 20 sec from the NH3 maximum. In addition, a spectrum of the J = 2-1 transition of CO-13 was taken at the position of maximum NH3 absorption. These CO and CO-13 data show that the Vlsr of the cloud is -39 km/s the FWHP line width is 3.5 km/s, and the column density, N(H2), is 3 x 1021/sq cm. The density, n(H2), is about 1000/cu cm and kinetic temperature, Tk, is 18 K. The virial mass estimated 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 same physical region. This region is significantly warmer than a typical dark dust cloud, shows a significantly larger line width, and has about 0.1 of the cloud mass in H I. The n(H2) and N(H2) values are comparable to dark dust cloud values. The enhanced kinetic temperature may be caused by an increased cosmic-ray rate, or by cloud-cloud collisions.

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