Physical State of Molecular Gas in High Galactic Latitude Translucent
Clouds
release_5pnsnci36zfupafziwahkrkpfq
by
James G. Ingalls, T. M. Bania, Adair P. Lane, Matthias Rumitz, Antony
A. Stark
1999
Abstract
The rotational transitions of carbon monoxide (CO) are the primary means of
investigating the density and velocity structure of the molecular interstellar
medium. Here we study the lowest four rotational transitions of CO towards
high-latitude translucent molecular clouds (HLCs). We report new observations
of the J = (4-3), (2-1), and (1-0) transitions of CO towards eight
high-latitude clouds. The new observations are combined with data from the
literature to show that the emission from all observed CO transitions is
linearly correlated. This implies that the excitation conditions which lead to
emission in these transitions are uniform throughout the clouds. Observed
13CO/12CO (1-0) integrated intensity ratios are generally much greater than the
expected abundance ratio of the two species, indicating that the regions which
emit 12CO (1-0) radiation are optically thick. We develop a statistical method
to compare the observed line ratios with models of CO excitation and radiative
transfer. This enables us to determine the most likely portion of the physical
parameter space which is compatible with the observations. The model enables us
to rule out CO gas temperatures greater than 30K since the most likely
high-temperature configurations are 1 pc-sized structures aligned along the
line of sight. The most probable solution is a high density and low temperature
(HDLT) solution. The CO cell size is approximately 0.01 pc (2000 AU). These
cells are thus tiny fragments within the 100 times larger CO-emitting extent of
a typical high-latitude cloud. We discuss the physical implications of HDLT
cells, and we suggest ways to test for their existence.
In text/plain
format
Archived Files and Locations
application/pdf 622.9 kB
file_pge74m7765h25jgbuezxmrpdkq
|
arxiv.org (repository) core.ac.uk (web) web.archive.org (webarchive) archive.org (archive) |
astro-ph/9912079v1
access all versions, variants, and formats of this works (eg, pre-prints)