Diffuse molecular gas in the Polaris flare
H. Meyerdierks, A. Heithausen;
AaA, 1996, 313, 929
ABSTRACT:We present 21cm line observations of two galactic cirrus clouds, the Polaris Flare and LVC127, and compare the HI emission with that of CO and with
emission in the IRAS bands at 100{mu}m and 60{mu}m.
The 100{mu}m emission per
hydrogen nucleus is constant outside, but lower inside the CO emitting
volume.
We also find an infrared excess emission over and above what can be related
to HI or CO emission.
We conclude that apart from the diffuse atomic
hydrogen and the denser molecular hydrogen traced by CO emission, there is a
third gas component of diffuse molecular hydrogen.
In places the diffuse
H_2_ has a column density rivalling that of HI.
In general it does not
contain a denser CO emitting interior.
The fact that the diffuse H_2_ is not
traced by CO emission is due to the lower gas density, either because CO is not
collisionally excited, or because the H_2_ is not effective enough in shielding CO
molecules from photo-dissociation.
We find a conversion factor X between CO
line integral and H_2_ column density similar to other values from cirrus
clouds, but lower than values for galactic plane clouds derived from gamma ray
studies.
If we account for the lower 100{mu}m emission per hydrogen in the CO
emitting volume, the difference is reduced, and our X may be consistent with the
lower values found in a recent gamma ray study.
KEYWORDS: dust, extinction, ism: individual objects: polaris flare, ism: molecules, radio lines: ism
CODE: meyerdierks96