Astron. Astrophys. 355, 333-346 (2000)

1. Introduction

Following the discovery of Galactic 2.6mm CO absorption toward BL Lac (B2200+420) by Marscher et al. (1991), we subsequently found that a wide variety of molecules could be studied in absorption in the nearby diffuse/translucent gas seen toward extragalactic compact mm-wave continuum sources (Lucas and Liszt 1993, 1994). Indeed, a survey of HCO⁺ absorption (Lucas and Liszt 1996) found that it is much more frequent than CO emission, and remarkably common, even compared to HI, at Galactic latitudes below 15^o-20^o. The abundances of polyatomics like C₃H₂ (Cox et al. 1988), HCO⁺, HCN, C₂H are much higher than can be explained by even the most exhaustive conventional chemical models (Van Dishoeck and Black 1988) unless the gas is taken to be denser (Viala et al. 1988) than we believe to be the case (Liszt and Lucas 1998).

The need to promote more rapid chemical formation in diffuse gas threaded by a strong UV radiation field has given rise to consideration of shock models (Flower and Pineau Des For^ets 1998) and somewhat more exotic schemes involving local dissipation of turbulent energy (Joulain et al. 1998). The shock models in particular imprint distinct kinematic signatures on resultant line profiles, and the existence of any kinematic peculiarities can of course be tested very directly given the high spectroscopic resolving power of modern instruments.

In an attempt to understand better the nature of the absorption profiles we have been studying, and especially to see if in these spectra there are signs of the underlying molecular formation mechanisms, we have taken spectra of OH and HCO⁺ which have some combination of higher-resolution, broader-bandwidth, and higher sensitivity. These spectra can be compared with each other and with their predecessors, to study their structure and stability (similarity across time and species). It seems to be the case that absorption profiles from the interstellar neutral gas vary on yearly time scales and on AU-sized spatial scales across a wide range of experiments, for instance, in atomic hydrogen absorption mapped on VLB scales (Diamond et al. 1989; Faison et al. 1998) or in H₂CO absorption measured toward point-sources at various epochs (Moore and Marscher 1995). The physical conditions inferred from a naive analysis of these variations, with densities hypothesized to range up to , should surely have some discernible consequences.

Sect. 2 describes the observational material used here. Sect. 3 compares HCO⁺ and OH profiles across time and species. Sect. 4 presents equilibrium models of the H- and C⁺-CO transitions in diffuse gas. Using these models, it discusses the distributed, Galactic scale contribution to molecular absorption profiles, and shows that the observed abundances of CO are the natural result of a high abundance of HCO⁺ in weakly molecular gas. Sect. 5 questions the viability of dense inclusions in neutral gas clouds, which have often been inferred from the yearly time scales or milli-arcsecond spatial scales on which absorption profiles (like ours) appear to vary. The weakness of HCO⁺ emission is a very strong constraint on the internal structure of diffuse clouds.

© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000
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