Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...443..169m&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 443, no. 1, p. 169-180
Astronomy and Astrophysics
Astrophysics
43
Carbon Monoxide, Density (Number/Volume), Hydrogen, Interstellar Matter, Mass Distribution, Molecular Clouds, Data Conversion Routines, Galactic Mass, Molecular Excitation, Translucence
Scientific paper
In this paper we describe a new technique for obtaining the conversion factor between the molecular hydrogen column density and the CO(J = 1-0) integrated antenna temperature. This factor, typically known as XCO is often to be of order a few times 1020/sq cm/K km/s) for the molecular clouds in the Galaxy and is one of the primary means of determining the molecular cloud mass from CO observations. However, for the low-extinction interstellar clouds known as the translucent molecular clouds, estimates of XCO vary by up to a factor of 60 depending on the object and techniques employed to calibrate XCO. Since the cloud mass is directly proportional to XCO uncertainties in mass estimates of translucent clouds can be more than an order of magnitude. We calibrate the H2 content in translucent clouds by using the linear relationship between the CH and H2 column densities. The CH column density is readily determined from observations of the CH ground-state hyperfine main-line transition at 3335 MHz. Using CH as a surrogate tracer for H2 and CO(J = 1-0) observations of a sample of translucent and dark molecular clouds, we find a wide variation in values for XCO. For translucent clouds, XCO ranges from 0.3 to 6.8 x 1020 and for dark clouds the values range from 0.8 to 8.6. Although the average values for both types of cloud are similar to the canonical value determined for the Galactic molecular cloud ensemble (2-4 x 1020), the scatter in individual XCO values may indicate that XCO for a given translucent cloud cannot be determined a priori and must be obtained for each cloud so that a reliable mass determination may be made.
Magnani Loris
Onello Joseph S.
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