Atomic and molecular gas in interstellar cirrus clouds
W. T. Reach, B. C. Koo, C. Heiles;
ApJ, 1994, 429, 672
ABSTRACT:The dust, atomic gas, and molecular gas content of a sample of 26 of isolated, degree-sized infrared clouds are compared.
Half of the clouds have an
infrared excess indicating the presence of H2, and 14 contain compact
CO-emitting regions.
Complete, high angular resolution H I and CO maps of one
cloud, G236+39, resolve the transition between atomic and molecular H, as
well as the location of CO formation.
Assuming the infrared emission
traces the total column density, H2 is inferred to be much more widely
distributed than the CO.
The CO rotational levels are subthermally excited, and
the (2 - 1)/(1 - 0) line ratios suggest a density n(H2) approximately
200/cm(exp 3) where CO was detected.
A model of H2 formation on grain surfaces
balanced by self-shielded photodissociation fits the variation of infrared
brightness with H I column density.
Assuming a temperature of 80 K, typical of
diffuse H2 (Savage et al.
1977), the H2 chemistry requires an average density
n(H + 2H2) approximately 50/cm(exp 3).
For G236+39, if the distance is 100
pc, the H I and H2 masses are estimated to be 90 and 70 solar mass,
respectively.
High-resolution H I and infrared maps of a smaller cloud, G249.0+73.7, reveal no
evidence for molecular gas, which is likely due to the low total column density
through this cloud.
These results suggest the H2 and H I content are comparable
for some interstellar cirrus clouds with column densities N(H I) greater
than 4 x 10(exp 20)/cm(exp 2), even where CO was not detected.
KEYWORDS: carbon monoxide, cirrus clouds, hydrogen, infrared radiation, interstellar gas, mapping, molecular gases, monatomic gases, radio spectra, angular resolution, brightness, data integration, high resolution, radio telescopes, root-mean-square errors, signal to noise ratios, variations
PERSOKEY:h_i, h2, co, 21 cm, dust, fir, iras, draco, cirrus, ,
CODE: reach94