Molecular and atomic clouds associated with infrared cirrus in Ursa Major
H. W. De Vries, P. Thaddeus, A. Heithausen;
ApJ, 1987, 319, 723
ABSTRACT:Observations of CO and H I revealed that in Ursa Major the high-latitude far-infrared 'cirrus' emission discovered by the Infrared Astronomical Satellite
comes from molecular and atomic clouds.
These clouds differ sufficiently
from the large clouds in the Galactic plane so that the ratio of H2 column
density to velocity-integrated CO radiation temperature, N(H2)/W(CO),
derived from Galactic plane surveys, may not apply to them.
On the assumption
of a constant gas-to-dust ratio, it is argued that the cirrus emission in
Ursa Major is a good mass tracer, since both the atomic and the molecular gas
are probably optically thin at visual wavelengths, and the grains are
heated not by local sources but by the background field of Galactic
starlight.
The N(H2)/W(CO) ratio thus derived for those diffuse clouds, is
significantly lower than the ratio applicable to Galactic plane surveys.
KEYWORDS: astronomical spectroscopy, atomic spectra, eclipsing binary stars, hydrogen clouds, infrared spectra, molecular clouds, carbon monoxide, infrared astronomy satellite, milky way galaxy, nebulae, optical thickness
CODE: de_vries87