The Physics and Chemistry of Translucent Molecular Clouds. IV. HCO + and N 2H +

Details The Physics and Chemistry of Translucent Molecular Clouds. IV. HCO + and N 2H + The Physics and Chemistry of Translucent Molecular Clouds. IV. HCO + and N 2H +

Aug 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...449..635t&link_type=abstract

Astrophysical Journal v.449, p.635

Astronomy and Astrophysics

Astronomy

36

Ism: Clouds, Ism: Molecules, Molecular Processes

Scientific paper

A survey of HCO+ and N2H+ has been conducted in the 11 cirrus cloud cores and 28 Clemens-Barvainis translucent objects whose structures and chemistry have been studied earlier in this series. HCO+ (J = 1-0) emission is seen in all objects and emulates 12CO in its distribution. By contrast, J = 3-2 emission requires exceptional excitation conditions, is seen in only a few cases, and is generally much more localized. In several objects HCO+ (J = 1-0) is also seen in emission from associated diffuse gas. HCO+ emission appears to trace a wide range of physical conditions, including diffuse, translucent, and shocked gas. Observations are modeled in terms of our previous hydrostatic equilibrium and n ˜ r-α structures together with other chemical and physical properties derived earlier. We find that the abundance ratio HC+/1 12CO is ˜10 times higher toward regions with extinctions Avo 1 mag than toward regions where AυO ≳ 1.5 mag, at which dense- cloud chemistry appears to onset. This behavior is well described by a chemistry which combines the diffuse cloud formation process C+ + OH → CO+ + H for HCO+ and CO with the dense-cloud processes H+3 + CO → HCO+ and HCO+ + ɛ → CO. The transition region between these regimes is precisely that of the translucent objects comprising the bulk of this study.
By contrast with HCO+, N2H+ (J = 1-0) is detected in only two translucent objects of 16 searched. This is expected, since there is no analogous diffuse-cloud formation process for N2H+, but only the "dense-cloud" process H3+ + N2 → N2+ + H2. Significant amounts of N2 can form only when photoprocesses are minimal, so only the regions with highest Avo exhibit detectable N2H+. Conventional N2H+ astrochemistry is consistent with all present N2H+ observations. The unusual source CB 17, exhibiting very strong lines of both HCO+ and N2H+, is explained as having an unusually large column density together with a strong external radiation field, consistent with previous analyses of the object.

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