On the Ortho:Para Ratio of H+ 3 in Diffuse Molecular Clouds
Abstract
The excitation temperature T 01 derived from the relative intensities of the J = 0 (para) and J = 1 (ortho) rotational levels of H2 has been assumed to be an accurate measure of the kinetic temperature in interstellar environments. In diffuse molecular clouds, the average value of T 01 is ~70 K. However, the excitation temperature T(H+ 3) derived from the (J, K) = (1, 1) (para) and (1, 0) (ortho) rotational levels of H+ 3 has been observed to be ~30 K in the same types of environments. In this work, we present observations of H+ 3 in three additional diffuse cloud sight lines for which H2 measurements are available, showing that in four of five cases T 01 and T(H+ 3) are discrepant. We then examine the thermalization mechanisms for the ortho:para ratios of H+ 3 and H2, concluding that indeed T 01 is an accurate measure of the cloud kinetic temperature, while the ortho:para ratio of H+ 3 need not be thermal. By constructing a steady-state chemical model taking into account the nuclear spin dependence of reactions involving H+ 3, we show that the ortho:para ratio of H+ 3 in diffuse molecular clouds is likely governed by a competition between dissociative recombination with electrons and thermalization via reactive collisions with H2.
Based in part on observations made with ESO Telescopes at the La Silla or Paranal Observatories under program ID 384.C-0618.- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 2011
- DOI:
- 10.1088/0004-637X/729/1/15
- arXiv:
- arXiv:1101.4641
- Bibcode:
- 2011ApJ...729...15C
- Keywords:
-
- astrochemistry;
- ISM: molecules;
- molecular processes;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 13 pages, 8 figures, 5 tables, accepted for publication in ApJ