The CS J = 2-1 emission line at 98 GHz has been searched for in 10 diffuse molecular clouds. CS column densities are derived by performing statistical equilibrium calculations for the rotational... Show moreThe CS J = 2-1 emission line at 98 GHz has been searched for in 10 diffuse molecular clouds. CS column densities are derived by performing statistical equilibrium calculations for the rotational population distribution which includes collisional excitation by electrons as well as by neutral species. Because CS has a large dipole moment, the electron collisions completely dominate the CS excitation in diffuse clouds, where the electron fraction is high. This result suggests that, in general, molecules with large dipole moments may be relatively more detectable by millimeter emission in diffuse clouds than in dense clouds. A detailed model of the gas-phase sulfur chemistry in diffuse clouds is developed to interpret the observations. Show less
Recent high-resolution observations of interstellar absorption lines of CH and CN toward Zeta Oph obtained by Crane et al. (1986), and Palazzi et al. (1988), exhibit line widths that suggest... Show moreRecent high-resolution observations of interstellar absorption lines of CH and CN toward Zeta Oph obtained by Crane et al. (1986), and Palazzi et al. (1988), exhibit line widths that suggest thermal line broadening at high temperature, T about 1200 K. Observations of CO line emission at 2.6 mm toward Zeta Oph (Langer et al.,1987) indicate that the molecular gas resides in four distinct velocity components that span less than 3 km/s in Doppler velocity. Simulated CH and CN absorption line profiles are compared for high-temperature (T = 1200 K) thermal broadening and for a combination of low-temperature (T = 50 K) thermal plus turbulent broadening. It is shown that the two broadening models reproduce existing observations comparably well and are virtually indistinguishable at a lambda/Delta-lambda ratio of about 100,000. The observed differences in the CH and CN line widths may reflect slightly different distributions of those molecules along the line of sight. The simulations use very recent, improved laboratory spectroscopic data on CH (Bernath). Some related consequences of such unresolved velocity structure on the ultraviolet absorption lines of CO are examined. Indirect diagnostics of temperature in the Zeta Oph cloud favor low-temperature thermal plus turbulent broadening, and the implied rate of dissipation of turbulence is in harmony with estimates of the global input of mechanical energy into to interstellar medium. Show less