The abundance of C3H2 and other small hydrocarbons in the diffuse
  interstellar medium
  Hydrocarbons are ubiquitous in the interstellar medium, observed in diverse
environments ranging from diffuse to molecular dark clouds and strong
photon-dominated regions near HII regions. Recently, two broad diffuse
interstellar bands (DIBs) at 4881{\AA} and 5450{\AA} 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)/EB-V = 4 \times 1014 cm-2 mag-1. Here we present new
measurements of N(l-C3H2) based on simultaneous 18-21 GHz VLA absorption
profiles of cyclic and linear C3H2 taken along sightlines toward extragalactic
radiocontinuum background sources with foreground Galactic reddening EB-V = 0.1
- 1.6 mag. We find that N(l-C3H2)/N(c-C3H2) ? 1/15 - 1/40 and N(l-C3H2)/EB-V ?
2 \pm 1 \times 1011 cm-2 mag-1, so that the column densities of l-C3H2 needed
to explain the diffuse interstellar bands are some three orders of magnitude
higher than what is observed. We also find N(C4H)/EB-V < 1.3 \times 1013 cm-2
mag-1 and N(C4H-)/EB-V < 1 \times 1011 cm-2 mag-1 (3?). Using available data
for CH and C2H 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.
