THE PHYSICS AND CHEMISTRY OF SMALL TRANSLUCENT MOLECULAR CLOUDS. Ⅵ. ORGANO-SULFUR SPECIES

摘要

A survey of HCS~+ and CS has been conducted in most of our standard samples of 11 cirrus cores and 27 Clemens-Barvainis translucent objects whose structures and chemistry have been studied earlier in this series. CS (J = 2-1) is seen quite strongly in all cirrus cores and CB objects, and CS (J = 3-2) in seven cirrus clouds and 25 CB objects. HCS~+ (J = 2-1) is detected in four of seven cirrus cores searched, and 15 of 20 CB objects. These 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 high CS abundances, over the wide range 10~(-9) to 10~(-7.2), increasing monotonically with increasing extinction in the 1.2-2.6 mag range. The HCS~+ abundances are much lower, in the range 10~(-10) to 10~(-9.3), again increasing monotonically with increasing extinction. Thus these species display the same characteristic transition between diffuse and dense cloud chemistry as do SO, SO_2, HCO~+, and other species studied in this series. The same simple ion-molecule network as used by Turner (1995b; Paper Ⅴ) is extended to include the organo-sulfur species, and when solved analytically, predicts the observed parameters of SO, SO_2, and HCS~+ successfully. The median abundance of CS is also predicted but the slope with respect to extinction is flatter than observed. The overall success of the model strongly endorses ion-molecule chemistry for all sulfur species so far studied. Other sulfur chemistries (grain catalysis, shock processes) are entirely inconsistent with the observed abundances of any of the four sulfur species studied to date in these translucent objects.