THE PHYSICS AND CHEMISTRY OF TRANSLUCENT MOLECULAR CLOUDS .4. HCO+ AND N2H+

A survey of HCO+ and N2H+ has been conducted in the 11 cirrus cloud co res and 28 Clemens-Barvainis translucent objects whose structures and chemistry have been studied earlier in this series. HCO+ (J = 1-0) emi ssion is seen in all objects and emulates (CO)-C-12 in its distributio n. By contrast, J = 3-2 emission requires exceptional excitation condi tions, is seen in only a few cases, and is generally much more localiz ed. In several objects HCO+ (J = 1-0) is also seen in emission from as sociated diffuse gas. HCO+ emission appears to trace a wide range of p hysical conditions, including diffuse, translucent, and shocked gas. O bservations are modeled in terms of our previous hydrostatic equlibriu m and n similar to gamma(-alpha) structures together with other chemic al and physical properties derived earlier. We find that the abundance ratio HCO+/(CO)-C-12 is similar to 10 times higher toward regions wit h extinctions A(v0) less than or similar to 1 mag than toward regions where A(v0) greater than or similar to 1.5 mag, at which dense-cloud c hemistry appears to onset. This behavior is well described by a chemis try which combines the diffuse cloud formation process C+ + OH --> CO + H for HCO+ and CO with the dense-cloud processes H-3(+) + CO --> HC O+ and HCO+ + e --> 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 n o analogous diffuse-cloud formation process for N2H+, but only the ''d ense-cloud'' process H-3(+) + N-2 --> N2H+ + H-2. Significant amounts of N-2 can form only when photoprocesses are minimal, so only the regi ons with highest A(v0) exhibit detectable N2H+. Conventional N2H+ astr ochemistry is consistent with all present N2H+ observations. The unusu al source CB 17, exhibiting very strong lines of both HCO+ and N2H+, i s explained as having an unusually large column density together with a strong external radiation field, consistent with previous analyses o f the object.