Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a...103..269m&link_type=abstract
Astronomy and Astrophysics, Vol.103, P. 269, 1981
Astronomy and Astrophysics
Astrophysics
11
Scientific paper
The 21-cm H I self-absorption line has been mapped over an area of 3° × 2° in the q Ophiuchi dark cloud with a 8'.7 resolution and a 10' spacing. The distribution of visual extinction in the same region is also obtained by the star counting method. The comparisons of the angular distribution of the cold H I with those of extinction and molecules show that they do not coincide. The primary H I density peak is located about 33' west of the extinction peak and many molecular source positions. This corresponds to a linear distance of about 2 pc. A secondary H I density peak appears at the halfway position from the extinction peak to the boundary.
The cold H I is over a much larger area than molecular distributions. The H I column density at the H I peak is N(H I) = 10.0 1019 cm-2 and the volume density is N(H I)/Aυ = 5 cm-3. The ratio of H I to extinction is N(H I)/Aυ = 2.5 1019 cm-2 mag -1 and the column density ratio of molecular and atomic hydrogens is N(H2)/N(H I) = 56. The cosmic ionization rate for H I for the inner part of the cloud is 2 10-17 s-1. The threshold value of extinction for the detection of H I self- absorption line is Aυ ≧ 2 mag. The relation between the H I column density and extinction shows a large scatter. However, it is apparent that at extinction higher than 2 mag the H I column density does not increase with extinction. The column density ratios between various molecules and H I range from the order of 10-7 for CN and H2CO to 10-3 for CO. The H I cloud in the region has a velocity gradient of ˜0.5 km s-1 from the north-west to south-east directions. The velocity difference between H I and most of molecular species and radio recombination lines are less than 1 km s-1.
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