The Variation of the CO to H 2 Conversion Factor in Two Translucent Clouds

Details The Variation of the CO to H 2 Conversion Factor in Two Translucent Clouds The Variation of the CO to H 2 Conversion Factor in Two Translucent Clouds

Sep 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...504..290m&link_type=abstract

Astrophysical Journal v.504, p.290

Astronomy and Astrophysics

Astronomy

23

Ism: Clouds, Ism: Molecules, Radio Lines: Ism

Scientific paper

The conversion factor, XCO, between the velocity-integrated CO(1-0) antenna temperature, W(CO), and the H2 column density, N(H2), is determined for 32 positions in two translucent high-latitude molecular clouds, MBM 40 and MBM 16. XCO is calculated using CH observations of the 2 Pi 1/2 F = 1-1 hyperfine transition to infer N(H2). The latter quantity, divided by W(CO) yields XCO for various positions across the clouds. We observed 24 positions in MBM 16, and XCO values were derived in the range (1.6-17.3) x 1020 cm-2 (K km s-1)-1, with a mean value of 7.6 x 1020. Eleven lines of sight were sampled in MBM 40 yielding values of XCO in the range (0.7-9.7) x 1020, with a mean value of 2.6 x 1020. An inverse relationship between XCO and W(CO) may exist, suggesting that the variation in XCO for these two translucent clouds arises from varying CO abundances. This paper also reports the existence of a broad component in the CH spectra observed throughout much of MBM 16. This component possibly originates in the disturbed outer regions of the cloud where the gas is not gravitationally bound to the core of the cloud. It is unclear how sensitive the CO rotational transitions are to this component, but it is likely that the CO/H2 ratio in this broad-line gas is less than 10-5. However, if the CH/H2 ratio is the same for the gas in the extended wings as it is for the typical cloud gas, then up to 40% of the cloud mass could be contained in this difficult to trace molecular component.

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