Ortho-Para Ratio in Water in the Interstellar Medium
Abstract
The water ortho-para ratio (OPR) in the interstellar medium and Solar System materials is often assumed to be related to the formation temperature of water molecules, making it a potentially interesting tracer of the thermal history of the gas. Herschel allowed high-resolution spectroscopic observations of low-energy rotational transitions of both ortho- and para-water. In most cases, the resulting OPR agreed with the high-temperature limit of 3. However, spin temperatures of 24-32 K, were derived in the foreground gas on the line of sight toward Sagittarius B2, at velocities corresponding to the noncircular x2 orbits in the gravitational potential of the Galactic center bar [1,2]. Some very low OPR values were also reported. The first one, in the TW Hya disk [3], was subsequently shown to be highly model depended, and consistent with the high temperature limit within the uncertainties [4]. A very low OPR of 0.1-0.5 was also reported in the Orion Bar photon-dominated region [5]. The corresponding spin temperature, ~ 8-12 K, is much lower the kinetic temperature of the molecular gas in this UV-illuminated region. We have analyzed a more complete set of Herschel observations of water in the Orion Bar, including seven H216O lines and one H218O line, using the Meudon PDR code to derive the gas-phase water abundance and the OPR [6]. The model considers the steep density and temperature gradients present in the region. The model line intensities are in good agreement with the observations, assuming that water molecules form with an OPR corresponding to thermal equilibrium conditions at the local kinetic temperature of the gas, and solely considering gas-phase chemistry. Gas-phase water emission is predicted to arise from a region deep into the cloud, corresponding to a visual extinction of AV∼ 9, with an H216O fractional abundance of ∼ 2 × 10-7. We derive a line-of-sight average OPR of 2.8 ± 0.2, corresponding to a spin temperature of 31-41 K. Consequently, there is no compelling evidence for water ortho-para ratios significantly below the LTE value of 3 to be present in the ISM.
This work was supported by the program PCMI of CNRS/INSU, the LabEx MiChem, and the program DIM ACAV+ of the Ile-de-France Region. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. References [1] Lis, D.C., et al., 2010, A&A, 521, L26 [2] Lis, D.C., et al., 2013, J. Phys. Chem. A, 117, 9661 [3] Hogerheijde, M., et al., 2011, Science, 334, 338 [4] Salinas, V.N., et al., 2016, A&A, 591, A122 [5] Choi, Y., et al. 2014, A&A, 572, L10 [6] Putaud, T., et al. 2019, A&A, in press, arXiv:1908.0268- Publication:
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American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23526104L