Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a...101...72s&link_type=abstract
Astronomy and Astrophysics, Vol.101, P. 72, 1981
Astronomy and Astrophysics
Astrophysics
32
Scientific paper
The visual extinction, Aυ,, has been determined over Heiles Cloud 2 (1 = 175°, b = -14°) for a scale size which matches the resolution of H2CO and H I data. A minimum visual extinction is required before H I self absorption and H2CO are detected. The size of the cloud in the visual is larger than that of the H I self-absorption region, which in turn is larger than the size of the H2CO region. The following relations between the column densities, N, and A hold for these data
N(J=1, ortho)=1.5 1013 (Aυ, -2.1) for Aυ, ≧ 2.1
=0 for A≧ < 2.1 and
N(H I, self-absorption) = 7.8 1018 (Aυ, -0.5)
for Aυ, ≧ 0.5
=0 for Aυ, < 0.5.
The H2CO column densities could be uncertain by a factor of 2, because of a dependence on the excitation temperature. In spite of the uncertainty, we believe that our result is significantly different from that obtained by Kutner (1973), for a cloud which is only 3° away. In the dark part of the cloud, the ratio of ortho-H2CO to H2 is 2 10-8. Also the molecular-to-atomic hydrogen ratio, 116, is less than half that found in other molecular clouds by McCutcheon et al. (1978). That is, Heiles Cloud 2 is relatively rich in cold H I. We also investigate regions in Heiles Cloud 2 which are completely void of stars and discuss these in terms of fragmentation sizes and masses.
Sherwood W. A.
Wilson Thomas L.
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