Observations of C3H2 in the diffuse interstellar medium.
Abstract
We report the detection of the hydrocarbon ring molecule cyclopropenylidene C_3_H_2_ in the diffuse interstellar medium in the lines of sight toward strong extragalactic (3C 123, NRAO 150, 3C 111) and galactic continuum sources (Sgr A and W 49/1). Both the ortho and para transitions at 18.3 and 21.6 GHz have been observed. The results are analyzed using statistical equilibrium calculations. C_3_H_2_ absorption measurements appear to be a very sensitive tracer of the low-density interstellar medium, making direct estimates of the density along most of the lines of sight possible for the first time. Densities of about a few 10^3^ cm^-3^ are determined for all lines of sight. The typical fractional abundance of C_3_H_2_ derived for the diffuse gas is comparable to that measured in dark clouds, [C_3_H_2_]/[H_2_] ~ 10^-9^-10^-8^. An important finding is the detection of C_3_H_2_ at velocities where H I is observed but for which no other molecular species have been reported in absorption. Broad H I features break up into several C_3_H_2_ absorption components, revealing the clumpiness of the cold diffuse interstellar medium. The fact that C_3_H_2_ is observed in lines of sight with low extinction, A_V_ ~ 1 mag, elucidates the ability of this ring molecule to exist in UV exposed environments. Finally, some remarks about the chemistry of cyclopropenylidene and other carbon ring molecules are given.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- November 1988
- Bibcode:
- 1988A&A...206..108C
- Keywords:
-
- Abundance;
- Cyclic Hydrocarbons;
- Interstellar Chemistry;
- Interstellar Matter;
- Dark Matter;
- H I Regions;
- Interstellar Extinction;
- Optical Thickness;
- Radio Sources (Astronomy);
- Astrophysics