H 2CO in IC 443G: Unusual Excitation and a Diagnostic of Shock Astrochemistry
Abstract
Formaldehyde (H2CO) has been detected in IC 443G, a clump of molecular gas impacted by the shock driven by the IC 443 SNR. The excitation of H2CO is found to be unusual: the rotation temperature within K(-1) ladders (about 23 K) is much less than the rotation temperature between K(-1) ladders (above about 150 K). Detailed LVG and microturbulent radiative transfer analyses of eight observed transitions (four para, four ortho) show that IC 443G is very warm (above 300 K) and very dense. The two-temperature excitation of H2CO cannot be explained by radiative excitation, but can be produced by cool electrons at low kinetic temperature, acting together with hot neutral colliders. The H2CO results (and new SO results) are compared with abundance predictions of both dissociative and nondissociative shock models. When filling factors are taken into account, the H2CO abundance is consistent with a nondissociative shock but not a dissociative one. However, the observed properties of H2CO and SO, taken together with other results for HCO(+), indicate that IC 443G is a highly complex region consisting of multiple shocks (both dissociative and nondissociative) propagating through a highly clumped medium containing a magnetic field and significant ionization. Observations of eight selected molecular species may help to clarify the various shock parameters in IC 443G.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1991
- DOI:
- 10.1086/170639
- Bibcode:
- 1991ApJ...381..173T
- Keywords:
-
- Cosmochemistry;
- Formaldehyde;
- Interstellar Gas;
- Molecular Clouds;
- Shock Waves;
- Stellar Composition;
- Astrophysics;
- INTERSTELLAR: MOLECULES;
- NEBULAE: INDIVIDUAL ALPHANUMERIC: IC 443