Article pubs.acs.org/JPCA
Ortho-to-Para Ratio in Interstellar Water on the Sightline toward Sagittarius B2(N) Dariusz C. Lis,*,† Edwin A. Bergin,‡ Peter Schilke,§ and Ewine F. van Dishoeck∥ †
Cahill Center for Astronomy and Astrophysics 301-17, California Institute of Technology, Pasadena, California 91125, United States Department of Astronomy, University of Michigan, 933 Dennison Building, Ann Arbor, Michigan 48109, United States § I. Physicalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany ∥ Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands and Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany ‡
ABSTRACT: The determination of the water ortho-to-para ratio (OPR) is of great interest for studies of the formation and thermal history of water ices in the interstellar medium and protoplanetary disk environments. We present new Herschel observations of the fundamental rotational transitions of orthoand para-water on the sightline toward Sagittarius B2(N), which allow improved estimates of the measurement uncertainties due to instrumental effects and assumptions about the excitation of water molecules. These new measurements, suggesting a spin temperature of 24−32 K, confirm the earlier findings of an OPR below the high-temperature value on the nearby sightline toward Sagittarius B2(M). The exact implications of the low OPR in the galactic center molecular gas remain unclear and will greatly benefit from future laboratory measurements involving water freeze-out and evaporation processes under low-temperature conditions, similar to those present in the galactic interstellar medium. Given the specific conditions in the central region of the Milky Way, akin to those encountered in active Galactic nuclei, gas-phase processes under the influence of strong X-ray and cosmic ray ionization also have to be carefully considered. We summarize some of the latest laboratory measurements and their implications here.
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INTRODUCTION Water is an asymmetric top molecule with two spin isomers, characterized by the total hydrogen nuclear spin of I = 1 (ortho) or I = 0 (para). The energy difference between the ground state rotational levels of ortho- and para-water is 34.2 K. In the high-temperature limit, the relative abundance of the two spin isomers, the ortho-to-para ratio (OPR), is given by the statistical weight ratio, OPR = 3. The OPR then decreases with decreasing temperature.1 The OPR in water has been studied extensively in cometary atmospheres, where values corresponding to spin temperatures of order 30 K are typically derived.1−3 Similar spin temperatures have been derived for other cometary volatiles, such as ammonia, and have often been interpreted as a measure of the temperature, at which molecules formed on the grain surfaces (but see the discussion section below). Since cometary ices are believed to originate from the natal molecular cloud that subsequently collapsed to form the solar nebula, direct measurements of the OPR in the interstellar medium (ISM) are of great interest for understanding the origin of solar system materials and the potential physical mechanisms responsible for the nuclear spin conversion. The HIFI instrument4 aboard the Herschel Space Observatory5 has allowed for the first time high-resolution spectroscopic observations of the fundamental rotational transitions of both ortho- and para-water, and thus direct observational measurements of the water OPR in cold interstellar gas, by means of © XXXX American Chemical Society
absorption spectroscopy toward bright submillimeter dust continuum sources. The HIFI observations trace only gasphase water molecules, and not those frozen as ice on the surface of dust grains. However, these two phases are intimately related, as gas-grain interactions are an integral part of the ISM chemistry, with the gas-phase water abundance defined by a complex balance of freeze-out, thermal, and UV photodesorption processes.6 Early results from the Herschel Science Demonstration Program7,8 showed that, in most cases, the observed OPR in water is consistent with the high temperature limit of 3. However, a lower ratio corresponding to a spin temperature of ∼27 K may be present in the gas on the line of sight toward Sagittarius B2(M), at velocities associated with the molecular gas in the vicinity of the Galactic center (the region referred to in early molecular studies as the “expanding molecular ring”). Observational uncertainties due to instrumental effects and assumptions about the excitation of water molecules have been difficult to estimate quantitatively from these early observations, in order to derive an accurate OPR. Special Issue: A: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 14, 2012 Revised: May 8, 2013
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Subsequent HIFI observations of additional sightlines in the Galactic disk9 have resulted in identification of two additional clouds with an OPR below the high-temperature LTE limit, on the sightlines toward W49N and W33A. A very low OPR, corresponding to a spin temperature of ∼14 K has also been reported in water in the protoplanetary disk of TW Hya.10 We present here new HIFI observations of water absorption on the line of sight toward Sagittarius B2(N), which sample the same gas clouds previously studied toward Sagittarius B2(M).7 These new, independent observations allow us to better ascertain the magnitude of instrumental and excitation effects that limit the accuracy of the determination of the OPR.
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Article
RESULTS
The two independent observations of the 557 and 1113 GHz lines allow an accurate estimate of the instrumental uncertainties. We have computed the optical depth as a function of velocity from the two spectra and then computed a difference, divided by the average optical depth based on the two estimates. The rms of this quantity, over the velocity ranges used for the subsequent analysis, is 0.075 and 0.054 for the 557 and 1113 GHz lines respectively. Assuming uncorrelated noise, the uncertainty of the individual measurements can be obtained by dividing the quantities above by the square root of 2. Then, the uncertainty of the average, which is used in the subsequent analysis, can by derived by dividing by another square root of 2. The final fractional uncertainties of the optical depth estimates in a single channel are thus 0.038 and 0.037 for the 557 and 1113 GHz lines, respectively. These values are added in quadrature to derive the uncertainty of the OPR in single velocity channels (Figure 2). In the case of the 1669 GHz ortho line, only a single observation using the HIFI 6b mixer band is available (lower panel in Figure 1). The noise level is significantly higher in the 1669 GHz spectrum due to the much higher system temperature in the HEB mixer bands 6−7 compared to the
OBSERVATIONS
Observations of Sagittarius B2(N) presented here (OBSIDs 1342205491, 1342206364, 1342206370, 1342205855, and 1342206498) were carried out between September 2010 and April 2011 using the HIFI instrument in the dual beam switch (DBS) observing mode, as part of the Herschel guaranteed time key program HEXOS: “Herschel/HIFI observations of EXtra-Ordinary Sources: The Orion and Sagittarius B2 starforming regions”.11 The source coordinates are: αJ2000 = 17h47m19.88s, δJ2000 = −28°22′18.40″. The DBS reference beams lie approximately 3′ east and west, perpendicular to the roughly north−south extension of the source. We used the HIFI wide-band spectrometer (WBS) providing a spectral resolution of 1.1 MHz (∼ 0.6 km s−1 at 557 GHz) over a 4 GHz IF bandwidth. The spectra presented here are averages of the H and V polarizations, weighted by the inverse of the system temperature squared, reduced using HIPE12 with pipeline version 8.0-3398. The resulting Level 2 double sideband spectra were deconvolved to an equivalent single sideband spectrum using the HIPE doDeconvolution task and exported to the FITS format for subsequent data reduction and analysis using the IRAM GILDAS software package (http:// www.iram.fr/IRAMFR/GILDAS). The three fundamental rotational transitions of water analyzed here are: the 110−101 and 212−101 ortho-H2O lines at 556.936 and 1669.905 GHz, respectively, as well as the 111− 000 para-H2O line at 1113.343 GHz, which in the subsequent discussion will be referred to by their rest frequencies in GHz. The observed spectra, divided by the corresponding continua, are shown in Figure 1. All spectra have been resampled to the same velocity resolution of 1 km s−1 and are plotted with respect to the Local Standard of Rest velocity (vLSR). In the case of the 557 GHz ortho and 1113 GHz para lines, two independent measurements, using the HIFI mixer bands 1a/ 1b and 4b/5a, respectively, are available. These are shown as the black and red lines in the upper and middle panels in Figure 1, respectively. The two spectra are in very good agreement, confirming excellent stability and calibration of the HIFI spectra. Over a wide range of velocities, the water spectra toward Sagittarius B2(N) are completely saturated and cannot be used to derive the line optical depth and the corresponding column density of water molecules. However, we have identified three velocity ranges, marked in cyan in Figure 1, which can be used for quantitative analysis. These approximately correspond to the velocity ranges where a low water OPR was previously reported toward Sagittarius B2(M).7
Figure 1. HIFI spectra of the 557, 1113, and 1669 GHz fundamental transitions of ortho- and para-water lines toward Sagittarius B2(N), divided by the corresponding continua. The red and black histograms in the upper two panels show independent observations using different HIFI mixer bands (1a and 1b for the 557 GHz line; 4b and 5a for the 1113 GHz line). Only one observation of the 1669 GHz ortho line is available (mixer band 6b). Velocity ranges over which the lines are not completely saturated and the optical depths can be derived are shown in cyan. B
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spectrum is almost completely correlated, and a conservative estimate of the uncertainty in the average OPR obtained based on these data is given by the mean uncertainty for an individual 1 km s−1 channel. In the case of the OPR obtained using the 1113 and 1669 GHz lines, the uncertainty in the average OPR for the two wider intervals is 0.6 times the mean uncertainty in an individual 1 km s−1 channel. For the third, narrow velocity interval, the uncertainty of the average is 0.67 times the mean uncertainty in an individual channel. The resulting mean OPR values for the three velocity intervals, together with the corresponding (2σ) uncertainties, are given in Figure 2. The ortho- and para-water optical depths and column densities were computed under the assumption that the absorption completely covers the continuum and all water molecules are in the corresponding ortho and para rotational ground states. The latter assumption is satisfied under the physical conditions characteristic for the interstellar medium for the 1113 and 1669 GHz transitions but may not always be correct for the 557 GHz ortho line.14 The effect of a nonnegligible excitation temperature is strongest for the 557 GHz ortho-water line, which is always characterized by the weakest background continuum temperature. In this case, the optical depth and the corresponding column density may be underestimated, resulting in a lower OPR derived from the combination of the 557 and 1113 GHz transitions, compared to the 1669 and the 1113 GHz transitions. However, in the case of the Sagittarius B2(N) observations, we do not see a systematic difference between the OPR derived from the two ortho-water lines (upper and lower panels in Figure 2, respectively). In fact, for the −10 km s−1 velocity component, the 1669 GHz line gives a significantly lower OPR compared to the 557 GHz line. This is caused by almost complete saturation of water absorption at these velocities, which makes the resulting optical depth estimates susceptible to baseline errors in the spectra. If we exclude this velocity component, the average OPR toward Sagittarius B2(N), based on the observations of the 557 and 1669 GHz ortho-water lines is 2.34 ± 0.25 (2σ), corresponding to a spin temperature of 24−32 K (see Figure 4 of Mumma et al.1), in excellent agreement with the value previously reported for the nearby sightline toward Sagittarius B2(M). We thus conclude that the HIFI observations do indicate an OPR in water below the high-temperature LTE limit in the gas associated with the region that harbors gas in the noncircular x2 orbits in the gravitational potential of the Galactic center bar.15
SIS mixers used in bands 1−5, not fully compensated by the strength of the continuum, which in Sagittarius B2(N) turns over around 1.5 THz.13 In this case we are thus compelled to use the formalism developed in the earlier study7 and estimate the optical depth uncertainty in a given velocity channel as δτ = exp(τ) × δI/δI0, with δI/δI0 = 0.033 as determined from absorption/emission free channels away from the water line. The water OPR toward Sagittarius B2(N) as a function of LSR velocity is shown in Figure 2. Only velocity channels
Figure 2. Water ortho-to-para optical depth ratio toward Sagittarius B2(N) as a function of velocity (left scale) and the corresponding column density ratio (right scale) derived from the 557 and 1113 GHz spectra (upper panel) and 1669 and 1113 GHz spectra (lower panel). The high-temperature LTE value is marked as a green line, and the average values for the three velocity ranges where the water absorption is not completely saturated are marked in magenta. Uncertainties for individual velocity channels (gray errorbars) and the averages over the three velocity intervals (magenta lines) are 2σ.
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DISCUSSION The OPR in water derived on the two sightlines toward Sagittarius B2, corresponding to a spin temperature of 24−32 K, is similar to the values measured in cometary atmospheres. A natural explanation, following the arguments used in the solar system literature, would thus be that this spin temperature corresponds to some physical temperature at which the water molecules formed, either in the gas phase or on grain surfaces. In the gas phase, water molecules are believed to form with an OPR of 3, for example, through dissociative recombination of H3O+, and the nuclear spin conversion is generally expected to be slow (see, e.g., recent quantum relaxation calculations16). Nevertheless, given the very long time scales associated with the interstellar medium, an ortho-to-para conversion via gasphase reactions has to be carefully considered. For example, proton exchange reactions with both H+ and H3+ can drive the OPR to the equilibrium at the gas temperature.17−19 The time
where the absorption is not completely saturated (shown in cyan in Figure 1) are shown. Error bars are 2σ in individual 1 km s−1 velocity channels. To estimate the uncertainty of the average OPR in each velocity interval, we smoothed the 557, 1113, and 1669 GHz spectra from 1 km s−1 velocity resolution to 5 and 10 km s−1 resolution and investigated how the noise in the resulting spectra varies with the channel width. For completely uncorrelated channels, the expected improvement is a factor of square root of 5 and square root of 10, respectively. However, the noise in the 557 GHz spectrum only goes down by a factor of 0.9 when resampling from 1 to 5 km s−1 and does not improve further when resampling to 10 km s−1. In the case of the 1113 and 1669 GHz lines, the rms improves by a factor 0.67 and 0.6 when resampling to 5 km s−1 and 10 km s−1, respectively. We thus conclude that the noise in the 557 GHz C
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K show a high nuclear spin temperature, suggesting that the nuclear spin temperatures of gaseous water molecules thermally desorbed from ice do not necessarily reflect the surface temperature at which water molecules condensed or formed under some laboratory conditions. In a separate experiment, the ortho−para conversion of single water molecules trapped inside closed fluerene cages was investigated.26 The existence of metastable ortho-water molecules was demonstrated, and interconversion of ortho and para spin isomers was tracked in real time, showing a near-exponential behavior with a time constant of 12 h at a temperature of 3.5 K. While these latest laboratory studies may not precisely reproduce the conditions in the interstellar space, significant progress is being made, which will ultimately lead to a generally accepted interpretation of the values of the OPR measured in the ISM and solar system materials.
scale for this process depends on the local cosmic ray or X-ray ionization rate and the gas density. Assuming a gas density of 104 cm−3, a fractional abundance of protonated ions of 10−8, and a rate coefficient of 10−9 cm−3 s−1, we estimate the time scale of the ortho−para equilibration to be of order 3 × 105 years in moderately dense interstellar clouds. If the water molecules can last for such a long time in the gaseous state (i.e., are not destroyed by photons or other reactions) than the observed OPR may indeed reflect the local gas temperature. Given the long time scale for the gas-phase nuclear spin conversion, the formation of water molecules via grain-surface processes, along with the corresponding mechanisms that release the molecules into the gas, need to be considered as well. Both laboratory measurements of water ices under conditions similar to those encountered in the interstellar space, as well as theoretical simulations, are of key importance to provide the quantitative information. The first question to address is whether the OPR indeed reflects the grain temperature at the time of ice formation, or whether it can be modified as the temperature changes. In a recent study,20 single water molecules, prepared as pure para-H2O, were isolated in solid Ar matrices at 4 K. Infrared spectra of the water vapor above the ice were then obtained upon heating to 260− 280 K, which show thermal OPR values. This suggests that spin conversion of water clusters is fast in Ar matricesthe OPR retains no history of the formation temperature, but instead reflects the current grain/ice temperature. Studies of water molecules sublimating below 250 K are not feasible in the laboratory due to the low water vapor pressure. Nevertheless, matrix isolation results of single water molecules show that at temperatures as low as 30 K spin conversion of single water molecules and dimers proceed within minutes.20 As a result, long time stability of para-water molecules in ices at higher temperatures seems unlikely, and the conclusion that the observed ortho-para ratio of ices in comets reflects their formation temperatures is in doubt. Another possibility is that the desorption process modifies the OPR. At low grain temperatures, only nonthermal processes can get molecules off the grains. The leading process to do so is photodesorption. The UV photons dissociate a water ice molecule, and the resulting fragments can either be trapped, recombine, or escape from the surface. Recent laboratory data and molecular dynamics calculations of water ice photodesorption21,22 indicate that the probability to produce a gas-phase water molecule per incident UV is about 10−3, with the probability to produce H + OH a factor of 2 higher. Water molecules are released into the gas phase via two mechanisms: about 50% desorbs as recombined H2O, and the other 50% consists of H2O “kicked out” from the surface (an energetic H atom released by photodissociation kicks out surrounding water molecules). The recombination mechanism would reset the OPR to the statistical value of 3, while the kickout mechanism should preserve the OPR value acquired on the grain surface. Subsequent calculations23,24 have shown a weak dependence of the H2O photodesorption probability on the ice temperature, with different mechanisms showing somewhat different temperature behavior. The other process to get molecules off the grains is thermal evaporation at higher grain temperatures. Measurements of nuclear spin states and rotational temperatures of thermally desorbed water molecules from amorphous solid water (ASW) have also been carried out.25 Spectra of desorbed water molecules measured at ∼150 K from vapor-deposited ASW at 8
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada, and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France, and the US. Consortium members are: Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiologa (CSICINTA); Sweden: Chalmers University of Technology − MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University − Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. Support for this work was provided by NASA through an award issued by JPL/Caltech.
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REFERENCES
(1) Mumma, M. J.; Weaver, H. A.; Larson, H. P. The Ortho-Para Ratio of Water Vapor in Comet p/Halley. Astron. Astrophys. 1987, 187, 419−424. (2) Crovisier, J.; Leech, K.; Bockelée-Morvan, D.; Brooke, T. Y.; Hanner, M. S.; Altieri, B.; Keller, H. U.; Lellouch, E. The spectrum of Comet Hale-Bopp (C/1995 01) observed with the Infrared Space Observatory at 2.9 AU from the Sun. Science 1997, 275, 1904−1907. (3) Kawakita, H.; Watanabe, J.-I.; Furusho, R.; Fuse, T.; Capria, M. T.; De Sanctis, M. C.; Cremonese, G. Spin Temperatures of Ammonia and Water Molecules in Comets. Astrophys. J. 2004, 601, 1152−1158. (4) de Graauw, T.; Helmich, F. P.; Phillips, T. G.; Stutzki, J.; Caux, E.; Whyborn, N. D.; Dieleman, P.; Roelfsema, P. R.; Aarts, H.; Assendorp, R.; et al. The Herschel-Heterodyne Instrument for the FarInfrared (HIFI). Astron. Astrophys. 2010, 518, L6. (5) Pilbratt, G. L.; Riedinger, J. R.; Passvogel, T.; Crone, G.; Doyle, D.; Gageur, U.; Heras, A. M.; Jewell, C.; Metcalfe, L.; Ott, S.; et al. Herschel Space Observatory. An ESA Facility for Far-Infrared and Submillimetre Astronomy. Astron. Astrophys. 2010, 518, L1. (6) Hollenbach, D.; Kaufman, M. J.; Bergin, E. A.; Melnick, G. J. Water, O2, and Ice in Molecular Clouds. Astrophys. J. 2009, 690, 1497−1521.
D
dx.doi.org/10.1021/jp312333n | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Encapsulated in a Fullerene Cage. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 12894−12898.
(7) Lis, D. C.; Phillips, T. G.; Goldsmith, P. F.; Neufeld, D. A.; Herbst, E.; Comito, C.; Schilke, P.; Müller, H. S. P.; Bergin, E. A.; Gerin, M.; et al. Herschel/HIFI Measurements of the Ortho/Para Ratio in Water towards Sagittarius B2(M) and W31C. Astron. Astrophys. 2010, L26. (8) Emprechtinger, M.; Lis, D. C.; Bell, T.; Phillips, T. G.; Schilke, P.; Comito, C.; Rolffs, R.; van der Tak, F.; Ceccarelli, C.; Aarts, H.; et al. The Distribution of Water in the High-Mass Star-Forming Region NGC 6334 I. Astron. Astrophys. 2010, 521, L28. (9) Flagey, N.; Goldsmith, P. F.; Lis, D. C.; Gerin, M.; Neufeld, D.; Sonnentrucker, P.; De Luca, M.; Godard, B.; Goicoechea, J. R.; Monje, R.; et al. Water Absorption in Galactic Translucent Clouds: Conditions and History of the Gas Derived from Herschel/HIFI PRISMAS Observations. Astrophys. J. 2013, 762, 11. (10) Hogerheijde, M. R.; Bergin, E. A.; Brinch, C.; Cleeves, L. I.; Fogel, J. K. J.; Blake, G. A.; Dominik, C.; Lis, D. C.; Melnick, G.; Neufeld, D. A.; et al. Detection of the Water Reservoir in a Forming Planetary System. Science 2011, 334, 338−340. (11) Bergin, E. A.; Phillips, T. G.; Comito, C.; Crockett, N. R.; Lis, D. C.; Schilke, P.; Wang, S.; Bell, T. A.; Blake, G. A.; Bumble, B.; et al. Herschel Observations of EXtra-Ordinary Sources (HEXOS): The Present and Future of Spectral Surveys with Herschel/HIFI. Astron. Astrophys. 2010, 521, L20. (12) Ott, S. The Herschel Data Processing System − HIPE and Pipelines − Up and Running Since the Start of the Mission. Astron. Soc. Pacific Conf. Series 2010, 434, 139−142. (13) Lis, D. C.; Schilke, P.; Bergin, E. A.; Emprechtinger, M. The HEXOS Team. Hot, Metastable Hydronium Ion in the Galactic centre: Formation Pumping in X-ray Irradiated Gas? Philos. Trans. R. Soc., A 2012, 370, 5162−5173. (14) Emprechtinger, M.; Lis, D. C.; Rollfs, R.; Schilke, P.; Monje, R. R.; Comito, C.; Ceccarelli, C.; Neufeld, D. The Abundance, Ortho/ Para Ratio, and Deuteration of Water in the High-Mass Star Forming Region NGC 6334 I. Astrophys. J. 2012, 765, 61. (15) Binney, J.; Gerhard, O. E.; Stark, A. A.; Bally, J.; Uchida, K. I. Understanding the Kinematics of Galactic Centre Gas. Mon. Not. R. Astron. Soc. 1991, 252, 210−218. (16) Cacciani, P.; Cosléou, J.; Khelkhal, M. Nuclear Spin Conversion in H2O. Phys. Rev. A: At., Mol., Opt. Phys. 2012, 85, 012521. (17) Dalgarno, A.; Black, J. H.; Weisheit, J. C. Ortho-Para Transitions in H2 and the Fractionation of HD. Astrophys. J., Lett. 1973, 14, 77−79. (18) Flower, D. R.; Watt, G. D. On the Ortho-H2/Para-H2 Ratio in Molecular Clouds. Mon. Not. R. Astron. Soc. 1984, 209, 25−31. (19) Le Bourlot, J. Ammonia Formation and the Ortho-to-Para Ratio of H2 in Dark Clouds. Astron. Astrophys. 1991, 242, 235−240. (20) Sliter, R.; Gish, M.; Vilesov, A. F. Fast Nuclear Spin Conversion in Water Clusters and Ices: A Matrix Isolation Study. J. Phys. Chem. 2011, 115, 9682−9688. (21) Ö berg, K. I.; Linnartz, H.; Visser, R.; van Dishoeck, E. F. Photodesorption of Ices. II. H2O and D2O. Astrophys. J. 2009, 693, 1209−1218. (22) Andersson, S.; van Dishoeck, E. F. Photodesorption of Water Ice. A Molecular Dynamics Study. Astron. Astrophys. 2008, 491, 907− 916. (23) Arasa, C.; Andersson, S.; Cuppen, H. M.; van Dishoeck, E. F.; Kroes, G.-J. Molecular Dynamics Simulations of the Ice Temperature Dependence of Water Ice Photodesorption. J. Chem. Phys. 201, 132, 184510. (24) Arasa, C.; Andersson, S.; Cuppen, H. M.; van Dishoeck, E. F.; Kroes, G. J. Molecular Dynamics Simulations of D2O Ice Photodesorption. J. Chem. Phys. 2011, 134, 164503. (25) Hama, T.; Watanabe, N.; Kouchi, A.; Yokoyama, M. Spin Temperature of Water Molecules Desorbed from the Surfaces of Amorphous Solid Water, Vapor-deposited and Produced from Photolysis of a CH4/O2 Solid Mixture. Astrophys. J. Lett. 2011, 738, L15. (26) Beduz, C.; Carravetta, M.; Chen, J. Y.-C.; Concistrè, M.; Denning, M.; Frunzi, M.; Horsewill, A. J.; Johannessen, O. G.; Lawler, R.; Lei, X.; et al. Quantum Rotation of Ortho and Para-Water E
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