Astron. Astrophys. 358, 1069-1076 (2000)

1. Introduction

The simple hydrocarbons CH (Swings & Rosenfeld, 1937) and CH⁺ (Douglas & Herzberg, 1941) were among the first molecules identified in the interstellar medium, in what have come to be known as diffuse clouds. Although Swings and Rosenfeld noted that CH, OH, NH, CN, and C₂ were obvious candidates for identification, the program of finding all of these simple species was not completed for 55 years (Meyer & Roth, 1991). Indeed, the notion of a separately identifiable molecular component of the interstellar medium was slow to develop and we are actually still discovering, and being surprised by, the ubiquity of molecules in the diffuse ISM.

One of these surprises was the widespread presence (Matthews & Irvine, 1985; Cox et al., 1988) of the cyclic ring species C₃H₂ (Thaddeus et al., 1985b; Vrtilek et al., 1987); having 5 atoms, and forming (almost certainly) as the recombination or dissociation product of a even larger species (at the very least, from C₃H + e C₃H₂ + H) we are reminded once again that the actual complexity of diffuse cloud chemistry is viewed only very dimly in those species which we have yet surveyed.

In our work, we have shown that many species viewed through the technique of mm-wave absorption pioneered by (Marscher et al., 1991) are present with abundances remarkably like those which occur in dense dark clouds (Lucas & Liszt, 1993, 1994, 1996): examples are HCO⁺, HCN, C₂H and species familiar from earlier work like H₂CO (Liszt & Lucas, 1995) and CN. Although the high abundances of such species in diffuse gas are not understood, they imply that some of classical problems of interstellar chemistry must be recast. For instance, if the observed, relatively constant abundance of HCO⁺ is inserted into very standard models of diffuse gas undergoing the HI- transition, it follows with no other assumptions that the observed variation of N(CO) and N() can be explained over the entire range N(CO) simply by the reaction HCO⁺ + e CO + H (Liszt & Lucas, 2000). In like fashion, with high demonstrated abundances of HCN, C₂H or C₃H₂ need one really worry very much about making CH, CN, C₂, or C₃?

Here we report the results of a survey of mm-wave absorption from various hydrocarbons - C₂H, ortho- and para-cyclic-C₃H₂ C₃H, and C₄H - which occur in the clouds occulting our usual sample of compact extragalactic mm-wave continuum sources. Most of the work consists of a very large survey of the 87.3 GHz C₂H N=0-1 lines which fully complements our earlier, analogous report on HCO⁺ (Lucas & Liszt, 1996). Although it is C₃H₂ which is understood to be so ubiquitous, and although it was only several years after its discovery that C₂H was found in any molecular clouds except the densest GMC's (Wootten et al., 1980), C₂H emission is widespread over the entire inner galactic plane (Liszt, 1995). C₂H is more abundant than C₃H₂ by roughly a factor thirty, and is more favorably observed despite its lower dipole moment (0.8 vs. 3.27 Debye). The abundances of C₂H and C₃H₂ increase infixed proportion with respect to each other, and they vary differently with respect to the exemplars of other chemical groupings, notably HCO⁺ and OH, or HCN, HNC, and CN. The other chemical groups will be the subject of forthcoming papers in this series, beginning with HCN, HNC, and CN. A recent, more comprehensive summary of profiles in OH and HCO⁺ ( Liszt & Lucas (1996)) which might be considered Paper 0 in this series, will appear shortly (Liszt & Lucas, 2000).

The observations and manner of data-taking are discussed in Sect. 2. Sect. 3 is a presentation of the observational results and Sect. 4 is a brief discussion of molecular origins.

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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