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 4881Å and 5450Å were attributed to the linear version of propynylidene l -C _ { 3 } H _ { 2 } , a species whose more stable cyclic conformer c -C _ { 3 } H _ { 2 } 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 -C _ { 3 } H _ { 2 } ) /E _ { B - V } = 4 \times 10 ^ { 14 } ~ { } \mbox { $ { ~ { } { cm } ^ { -2 } } $ } mag ^ { -1 } .
Here we present new measurements of N ( l -C _ { 3 } H _ { 2 } ) based on simultaneous 18-21 GHz VLA absorption profiles of cyclic and linear C _ { 3 } H _ { 2 } taken along sightlines toward extragalactic radiocontinuum background sources with foreground Galactic reddening E _ { B - V } = 0.1 - 1.6 mag .
We find that N ( l -C _ { 3 } H _ { 2 } ) /N ( c -C _ { 3 } H _ { 2 } ) \approx 1 / 15 - 1 / 40 and N ( l -C _ { 3 } H _ { 2 } ) /E _ { B - V } \approx 2 \pm 1 \times 10 ^ { 11 } \mbox { $ { ~ { } { cm } ^ { -2 } } $ } mag ^ { -1 } , so that the column densities of l -C _ { 3 } H _ { 2 } needed to explain the diffuse interstellar bands are some three orders of magnitude higher than what is observed .
We also find N ( C _ { 4 } H ) /E _ { B - V } < 1.3 \times 10 ^ { 13 } \mbox { $ { ~ { } { cm } ^ { -2 } } $ } mag ^ { -1 } and N ( C _ { 4 } H ^ { - } ) /E _ { B - V } < 1 \times 10 ^ { 11 } \mbox { $ { ~ { } { cm } ^ { -2 } } $ } mag ^ { -1 } ( 3 \sigma ) .
Using available data for CH and C _ { 2 } 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 .