2012ApJ...753L..28L

Hydrocarbons are ubiquitous in the interstellar medium, observed in diverse environments ranging from diffuse to molecular dark clouds and strong photon-dominated regions near H II regions. Recently, two broad diffuse interstellar bands (DIBs) at 4881 Å and 5450 Å were attributed to the linear version of propynylidene l-C₃H₂, a species whose more stable cyclic conformer c-C₃H₂ has been widely observed in the diffuse interstellar medium at radio wavelengths. This attribution has already been criticized on the basis of indirect plausibility arguments because the required column densities are quite large, N(l-C₃H₂)/E _ B - V_=4x10¹⁴/cm2/mag. Here we present new measurements of N(l-C₃H₂) based on simultaneous 18-21 GHz Very Large Array absorption profiles of cyclic and linear C₃H₂ taken along sight lines toward extragalactic radio-continuum background sources with foreground Galactic reddening E_ B - V_= 0.1-1.6 mag. We find that N(l-C₃H₂)/N(c-C₃H₂) ~1/15-1/40 and N(l-C₃H₂)/E_ B - V_~(2±1)x10¹¹/cm2/mag, so that the column densities of l-C₃H₂ needed to explain the DIBs are some three orders of magnitude higher than what is observed. We also find N(C₄H)/E _ B - V_<1.3x10¹³/cm2/mag and N(C₄H^–)/E_B - V_<1x10¹¹/cm2/mag (3σ). Using available data for CH and C₂ H we compare the abundances of small hydrocarbons in diffuse and dark clouds as a guide to their ability to contribute as DIB carriers over a wide range of conditions in the interstellar medium.