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Chemical complexity in local diffuse and translucent clouds : ubiquitous l-C3H and CH3CN, a detection of HC3N and an upper limit on the abundance of CH2CN

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Authors : Liszt, Harvey ; Gerin, Maryvonne ; Beasley, Anthony ; Pety, Jerome
Abstract : We present Jansky Very Large Array observations of 20 - 37 GHz absorption lines from nearby Galactic diffuse molecular gas seen against four cosmologically-distant compact radio continuum sources. The main new observational results are that linear C3H and CH3CN are ubiqitous in the local diffuse molecular interstellar medium at Av < 1 while HC3N was seen only toward B0415 at Av > 4 mag. The linear/cyclic ratio is much larger in C3H than in C3H2 and the ratio CH3CN/HCN is enhanced compared to TMC-1, although not as much as toward the Horsehead Nebula. More consequentially, this work completes a long-term program assessing the abundances of small hydrocarbons (CH, CCH, linear and cyclic C3H and C3H2, and C4H and C4H-) and the CN-bearing species (CN, HCN, HNC, HC3N, HC5N and CH3CN) : their systematics in diffuse molecular gas are presented in detail here. We also observed but did not strongly constrain the abundances of a few oxygen-bearing species, most prominently HNCO. We set limits on the column density of C\HH CN, such that the anion CH2CN- is only viable as a carrier of diffuse interstellar bands if the N(CH2CN)/N(CH2CN-) abundance ratio is much smaller in this species than in any others for which the anion has been observed. We argue that complex organic molecules are not present in clouds meeting a reasonable definition of diffuse molecular gas, ie Av < 1 mag.
Journal : Astrophysical Journal Arxiv