Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...313..929m&link_type=abstract
Astronomy and Astrophysics, v.313, p.929-937
Astronomy and Astrophysics
Astrophysics
27
Dust, Extinction, Ism: Individual Objects: Polaris Flare, Ism: Molecules, Radio Lines: Ism
Scientific paper
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 100μm and 60μm. The 100μm 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 100μm 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.
Heithausen Andreas
Meyerdierks H.
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