Abstract
The study of the ortho-to-para ratio of assorted gas-phase interstellar molecules such as H2 ,H 2O, NH3 ,a nd H2O + has gained interest in recent years, based partially on new spectral observations of light hydrides by the Herschel Space Observatory. Although these ratios can yield valuable information about the thermal history of the interstellar cloud where the molecules are found, an understanding of how the ratios are determined involves a number of often poorly studied processes, which can include both gas-phase and grain-surface reactions. In this article, we consider the processes that determine the ortho-to-para ratio of the molecular ion H2O + in diffuse interstellar clouds and attempt to reproduce an unusual observed ratio for this ion. In addition to the study of ortho-to- para ratios, we look carefully at current uncertainties in the gas-phase formation of large neutral molecules in cold dense interstellar clouds via ion-neutral radiative association and dissociative recombination, among other processes.
Highlights
In addition to the study of ortho-topara ratios, we look carefully at current uncertainties in the gas-phase formation of large neutral molecules in cold dense interstellar clouds via ion-neutral radiative association and dissociative recombination, among other processes
In a final attempt to reproduce the observed ortho-to-para ratio (OPR) in diffuse clouds toward the galactic center, Tanaka et al [10] studied the possibility that, as the ortho and para forms of H2O+ radiatively relax down their respective ladders following their formation in the exothermic reaction between OH+ and H2, radiative transitions can occur between ortho and para states, especially for high rotational levels that are close to degeneracy
The observed value is, perhaps serendipitously, in agreement with the value we obtain for the case in which the destruction of the water cation by dissociative recombination is more rapid than thermalization of the spin, and the formation occurs via reaction (8) by the mechanism of H hopping rather than by scrambling
Summary
The gas was found to be mainly atomic with the exception of hydrogen, which is roughly evenly divided between atomic and molecular forms, while only a few other diatomic molecules were detected This picture has changed first with absorption studies using distant quasars as continuous millimeter-wave sources [5] and more recently with far-infrared (sometimes referred to as the THz region) absorption studies with the Herschel Space Observatory against continuua in this region of the spectrum, such as put out by hot cores [6]. The water cation (H2O+), detected in the spiral arms, was originally measured to have an ortho-to-para ratio of 4.8 ± 0.5, quite different from the normal value of 3.0 [6] This unusual ratio has been of interest for some time, and we discussed it in a preliminary manner in a previous conference proceeding in this series [7]. An alternative interpretation is that the water cation is produced in the outer regions of the clouds, which are richer in atomic hydrogen than the inner regions [8]
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