Influence of collisional rate coefficients on water vapour excitation

¹ Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Ctra. de Torrejón a Ajalvir km 4, 28850 Madrid, Spain
e-mail: danielf@cab.inta-csic.es
² Observatoire de Paris, LUTH UMR CNRS 8102, 5 place Janssen, 92195 Meudon, France
³ Université Pierre et Marie Curie, LPMAA UMR CNRS 7092, Case 76, 4 place Jussieu, 75252 Paris Cedex 05, France
⁴ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France

Received: 4 June 2012
Accepted: 7 January 1900

Abstract

Context. Water is a key molecule in many astrophysical studies that deal with star or planet forming regions, evolved stars, and galaxies. Its high dipole moment makes this molecule subthermally populated under the typical conditions of most astrophysical objects. This motivated calculation of various sets of collisional rate coefficients (CRC) for H₂O (with He or H₂), which are needed to model its rotational excitation and line emission.

Aims. The most accurate set of CRC are the quantum rates that involve H₂. However, they have been published only recently, and less accurate CRC (quantum with He or quantum classical trajectory (QCT) with H₂) were used in many studies before that. This work aims to underline the impact that the new available set of CRC have on interpretations of water vapour observations.

Methods. We performed accurate non-local, non-LTE radiative transfer calculations using different sets of CRC to predict the line intensities from transitions that involve the lowest energy levels of H₂O (E < 900 K). The results obtained from the different CRC sets were then compared using line intensity ratio statistics.

Results. For the whole range of physical conditions considered in this work, we find that the intensities based on the quantum and QCT CRC are in good agreement. However, at relatively low H₂ volume density (n(H₂)  <  10⁷ cm^-3) and low water abundance (χ(H₂O)  <  10^-6), which corresponds to physical conditions relevant when describing most molecular clouds, we find differences in the predicted line intensities of up to a factor of  ~3 for the bulk of the lines. Most of the recent studies interpreting early Herschel Space Observatory spectra have used the QCT CRC. Our results show that, although the global conclusions from those studies will not be drastically changed, each case has to be considered individually, since depending on the physical conditions, the use of the QCT CRC may lead to a mis-estimate of the water vapour abundance of up to a factor of  ~3. Additionally, the comparison of the quantum state-to-state and thermalised CRC, including the description of the population of the H₂ rotational levels, show that above T_K  ~  100 K, large differences are expected from those two sets for the p–H₂ symmetry. Finally, we find that at low temperature (i.e. T_K < 100 K) modelled line intensities will be differentially affected by the symmetry of the H₂ molecule. If a significant number of H₂O lines is observed, it is then possible to obtain an estimate of the H₂ ortho-to-para ratio from the analysis of the line intensities.