Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...185.8406m&link_type=abstract
American Astronomical Society, 185th AAS Meeting, #84.06; Bulletin of the American Astronomical Society, Vol. 26, p.1457
Astronomy and Astrophysics
Astronomy
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 taken to be of order a few times 10(20) cm(-2) [K km s(-1) ] 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 molecular 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 H_2 content in translucent clouds by using the linear relationship between the CH and H_2 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 H_2 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 times 10(20) and for dark clouds the values range from 0.8 to 8.6. Although the average values for both types of cloud are typical of the canonical value determined for the Galactic molecular cloud ensemble (2 - 4 times 10(20) ), 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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