Influence of collisional rate coefficients on water vapour excitation

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 H 2 O (with He or H 2 ), which are needed to model its rotational excitation and line emission. Aims. The most accurate set of CRC are the quantum rates that involve H 2 . However, they have been published only recently, and less accurate CRC (quantum with He or quantum classical trajectory (QCT) with H 2 ) 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 H 2 O (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 H 2 volume density (n(H 2 ) < 10 7 cm −3 ) and low water abundance (χ(H 2 O) < 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 H 2 rotational levels, show that above T K ∼ 100 K, large differences are expected from those two sets for the p-H 2 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 H 2 molecule. If a significant number of H 2 O lines is observed, it is then possible to obtain an estimate of the H 2 ortho-to-para ratio from the analysis of the line intensities.

Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN, and HNC isotopologues

“…The averaged excitation temperature is calculated by taking into account the regions of the cloud that contribute to the emerging line profile, the average being given by Eq. ( 2) of Daniel et al (2012).…”

Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars

“…Recently, Dubernet et al (2009) and Daniel et al (2011) published new H 2 O-H 2 collisional rates. Daniel et al (2012) compared these collision rates to those from Faure et al (2007) and found that the line strengths can be affected by up to a factor of 3 for low H 2 O abundance (n H 2 O /n H 2 ∼ 10 −8 ) and low density (n H 2 < 10 7 ) regimes. They also note that when H 2 O excitation is dominated by pumping via the dust radiation field, these differences are attenuated.…”

Uncertainties in water chemistry in disks: An application to TW Hydrae

“…They change within a factor two, while all other high excitation lines stay within 10%. A possible explanation is that the high excitation lines originate close to the inner rim of the outer disk, where the dust temperature is higher (70−110 K) and the level populations could be dominated by IR pumping (Daniel et al 2012). A possibility to quantify this radiative pumping is to compare the excitation temperature of a lines upper level T ex with the temperature of the radiation field at that wavelength T rad and the local gas temperature T gas .…”