Observations of the H I self-absorption in Heiles' Cloud 2

Details Observations of the H I self-absorption in Heiles' Cloud 2 Observations of the H I self-absorption in Heiles' Cloud 2

Publishing date

Feb 1977

URL

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977a%26a....54..933w&link_type=abstract

Journals

Astronomy and Astrophysics, vol. 54, no. 3, Feb. 1977, p. 933-941.

Science

Astronomy and Astrophysics

Field

Astrophysics

Additional info 2

65

Additional info 3

Astronomical Maps, Cosmic Rays, H Lines, Meteoroid Dust Clouds, Optical Thickness, Correlation, Electromagnetic Absorption, Gas Density, Gas Temperature, Radiative Transfer, Stellar Evolution

Type

Scientific paper

Abstract

The dust cloud referred to as Cloud 2 by Heiles (1968) was mapped in the 1.42 GHz self-absorption line of H I, and comparison of these results with molecular lines and optical absorption yields a detailed description of the cloud structure. An upper limit of 30 K was calculated for the kinetic temperature of the cloud, and assuming a value of 10 K, the average H I column density is 1.2 times 10 to the 19th power per sq cm. There are areas where two velocity components are present, although the weaker component accounts for less than 5% of the equivalent width of the deepest line. Comparison of H I and H2CO results shows that H2CO density increases toward the center of the cloud. H I does not increase, hydrogen probably being converted into H2 molecules. Toward the boundary, H2CO and H I densities are linearly proportional. The average H2CO/H2 column density ratio is 6 times 10 to the negative 9th power. Total cosmic ray ionization rate for H I is calculated at about 4 times 10 to the negative 17th power per sec. Assuming the cloud to be a single entity with a cylindrical shape of length 4 pc, radius 0.7 pc, and average H2 density of 3000 per cu cm, the mass is about 1,000 solar masses, and the free-fall time for such an unstable cloud is 700,000 years.

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