Physical State of Molecular Gas in High Galactic Latitude Translucent Clouds
J. G. Ingalls, T. M. Bania, A. P. Lane, M. Rumitz, A. A. Stark;
ApJ, 2000, 535, 211
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
toward high-latitude translucent molecular clouds (HLCs).
We report new
observations of the J=(4-3), (2-1), and (1-0) transitions of CO toward 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 that 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 that
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 that is compatible with the
observations.
The model enables us to rule out CO gas temperatures greater than ~30 K,
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, with volume density,
n=104.5+/-0.5 cm-3, kinetic temperature, Tk~8 K, and CO
column density per velocity interval
NCO/ΔV=1016.6+/-0.3 cm-2/( km s-1).
The CO cell size is L~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.
KEYWORDS: ism: clouds, ism: molecules, submillimeter
CODE: ingalls2000