Comparative chemistry of diffuse clouds II. CN, HCN, HNC, CH3CN & N2H+

Using the Plateau de Bure interferometer. we observed the lambda3 mm absorp tion lines of CN, HCN and HNC from some of the diffuse clouds which lie tow ard our well-studied sample of compact extragalactic mm-wave continuum sour ces. The column densities of these species all vary by a factor of about fi fty and are prominent in only a limited subset of the clouds seen in the mo st ubiquitous species such as OH. HCO+, C2H and C3H2. We searched unsuccess fully for CH3CN and N2H+, which are underabundant compared to dark clouds, by factors of at least 10 and 100, respectively. The CN-HCN-HNC column dens ities vary strongly and non-linearly with N(HCO+), for example, which proba bly best represents their variation with Ha as well. But their abundances a re very tightly and linearly coupled to each other. varying in fixed propor tion, as is the case for OH and HCO+ and (only slightly more loosely) for C 2H and C3H2 Having: measured one, it is hardly necessary to observe the two others in this group. We find <N(HNC)/N(HCN)> = 0.21 +/- 0.05, <N(CN)/N(HC N)> = 6.8 +/- 1. Such a small N(HNC)/N(HCN) ratio is typical of warmer gas in darker, denser environments. and is consistent with the notion of molecu lar formation in warmer media. The 6.8:1:0.21 ratio in diffuse gas is very different from TMC-1 (6.8:4.5:4.5) where HCN and HNC are relatively much mo re abundant. It seems likely that the sequence of features with increasing column densities of the CN-HCN family or CnHm-family molecules in diffuse g as actually represents a series of gas parcels of increasingly higher H-2-f raction, number density, and molecular abundances, occurring over a relativ ely narrow interval of total hydrogen column density. Our experiment seems to have caught many molecules in the act of turning on and turning each oth er on - in the diffuse interstellar medium.