Collisional excitation of O2 by H2: the validity of LTE models in interpreting O2 observations
1 LOMC – UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP 1123, 76063 Le Havre, France
2 Department of Optics and Spectroscopy, Tomsk State University, 36 Lenin av., 634050 Tomsk, Russia
3 Université de Bordeaux, Institut des Sciences Moléculaires, CNRS UMR 5255, 33405 Talence Cedex, France
4 Radboud University Nijmegen, Institute for Molecules and Materials, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
Received: 7 April 2014
Accepted: 28 May 2014
Context. Oxygen molecules (O2) are of particular interest because of their crucial role in astrochemisty. Modelling of O2 molecular emission spectra from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species.
Aims. Rotational excitation of O2(X3Σ−) by H2 is investigated theoretically and experimentally and we check the validity of the local thermodynamic equilibrium (LTE) approach for interpreting O2 observations.
Methods. Using a new ab initio potential energy surface, collisional excitation of O2 is studied using a full close-coupling approach. The theoretical calculations are validated by comparison with crossed beam scattering experiments. We also performed calculations for the excitation of O2 from a large velocity gradient (LVG) radiative transfer code using the new rate coefficients.
Results. State-to-state rate coefficients between the 27 lowest levels of O2 were calculated for temperatures ranging from 5 K to 150 K. The critical densities of the O2 lines are found to be at ≳104 cm-3 for temperatures higher than 50 K. This value is slightly larger than the one previously determined using previous He rate coefficients.
Conclusions. The new rate coefficients will help in interpreting O2 emission lines observed where LTE conditions are not fully fulfilled and enable an accurate determination of the O2 abundance in the interstellar medium.
Key words: molecular processes / molecular data / ISM: abundances / ISM: molecules
© ESO, 2014