Comparative Chemistry of Diffuse Clouds II : CN , HCN , HNC , CH 3 CN & N

Using the Plateau de Bure interferometer, we observed the λ3mm absorption lines of CN, HCN and HNC from some of the diffuse clouds which lie toward our well-studied sample of compact extragalactic mm-wave continuum sources. The column densities of these species all vary by a factor of about fifty and are prominent in only a limited subset of the clouds seen in the most ubiquitous species such as OH,HCO + , C2H and C3H2. We searched unsuccessfully 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 densities vary strongly and non-linearly with N(HCO +), for example, which probably best represents their variation with H2 as well. But their abundances are very tightly and linearly coupled to each other, varying in fixed proportion, as is the case for OH and HCO + , and (only slightly more loosely) for C2H 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(HCN) = 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 molecular 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 more abundant. It seems likely that the sequence of features with increasing column densities of the CN-HCN family or CnHm-family molecules in diffuse gas actually represents a series of gas parcels of increasingly higher H2-fraction, number density, and molecular abundances, occurring over a relatively narrow interval of total hydrogen column density. Our experiment seems to have caught many molecules in the act of turning on – and turning each other on – in the diffuse interstellar medium.