Ortho-to-para Ratio Research Articles

Radial and Vertical Constraints on the Icy Origin of H2CO in the HD 163296 Protoplanetary Disk

H2CO is a small organic molecule widely detected in protoplanetary disks. As a precursor to grain-surface formation of CH3OH, H2CO is considered an important precursor of O-bearing organic molecules that are locked in ices. Still, since gas-phase reactions can also form H2CO, there remains an open question on the channels by which organics form in disks, and how much the grain versus the gas pathways impact the overall organic reservoir. We present spectrally and spatially resolved Atacama Large Millimeter/submillimeter Array observations of several ortho- and para-H2CO transitions toward the bright protoplanetary disk around the Herbig Ae star HD 163296. We derive column density, excitation temperature, and ortho-to-para ratio (OPR) radial profiles for H2CO, as well as disk-averaged values of N T ∼ 4 × 1012 cm−2, T ex ∼ 20 K, and OPR ∼ 2.7, respectively. We empirically determine the vertical structure of the emission, finding vertical heights of z/r ∼ 0.1. From the profiles, we find a relatively constant OPR ∼ 2.7 with radius, but still consistent with 3.0 among the uncertainties, a secondary increase of N T in the outer disk, and low T ex values that decrease with disk radius. Our resulting radial, vertical, and OPR constraints suggest an increased UV penetration beyond the dust millimeter edge, consistent with an icy origin but also with cold gas-phase chemistry. This Herbig disk contrasts previous results for the T Tauri disk, TW Hya, which had a larger contribution from cold gas-phase chemistry. More observations of other sources are needed to disentangle the dominant formation pathway of H2CO in protoplanetary disks.

Gas-phase Ortho-to-para Ratio of Formaldehyde Formed at Low Temperatures in Laboratory Ices

The ortho-to-para ratio (OPR) of formaldehyde (H2CO) has been used as a probe to estimate the formation temperature of molecules in interstellar, circumstellar, and cometary environments, relying on the assumption that nuclear spin conversion is extremely slow, preserving the OPR from molecular formation. An OPR for H2CO less than 3 corresponds to a spin temperature below 30 K and has been proposed to result from formation at low temperatures within an ice, whereas an OPR of 3 is interpreted as arising from warmer formation in the gas phase. In spite of this common assumption, there is no laboratory evidence in the literature to date in support of it. Here, in the first study of its kind for H2CO, we report rotational spectroscopy measurements of the OPR of H2CO sublimated after its formation in methanol (CH3OH) ice samples that were photolyzed by ultraviolet light at 10, 15, 20, and 40 K. None of the measured OPR values correlated with the ice formation temperature.

Formation of Para-H2O by Vacuum-UV Photolysis of O2 in Solid Hydrogen: Implication for Astrochemistry.

The observation that the ortho to para ratio (OPR) of interstellar H2O is smaller than 3 is an important yet unresolved subject in astronomy. We irradiated O2 embedded in solid H2 at 3 K with vacuum-ultraviolet (VUV) light and observed IR lines associated with para-H2O (denoted as pH2O) and nonrotating H2O-(oH2)n (where oH2 denotes ortho-H2) but no lines associated with ortho-H2O (denoted as oH2O). After maintaining the matrix in darkness for ∼30 h, the amount of pH2O decreased, accompanied by an increase in H2O-(oH2)n via diffusion of oH2. After that, the continuous nuclear-spin conversion from oH2 to para-H2 (denoted as pH2) in solid H2 over time resulted in the conversion of nonrotating H2O-(oH2)n to rotating pH2O in solid pH2. The observation of the formation and conversion of pH2O in our experiment suggests a plausible route in which VUV irradiation of O2 and H2 adsorbed on grain surfaces might be responsible for the smaller OPR of interstellar H2O.

Water ortho-to-para ratio in the coma of comet 67P/Churyumov-Gerasimenko

Context. Abundance ratios of the nuclear-spin isomers of H2O and NH3 have been measured in about two dozen comets, with a mean value corresponding to a nuclear-spin temperature of ~30 K. The real meaning of these unequilibrated nuclear-spin abundance ratios is still debated. However, an equilibrated water ortho-to-para ratio of 3 is also commonly observed. Aims. The H channel of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-H) on board Rosetta provided high-resolution 2.5–2.9 μm spectra of H2O vapour in the coma of comet 67P/Churyumov-Gerasimenko (67P), which are suitable for the determination of the ortho-to-para ratio (OPR) of water in this comet. Methods. A large dataset of VIRTIS-H spectra obtained in limb-sounding viewing geometry was analysed, covering heliocentric distances from 1.24 to 2.73 au and altitudes from a few hundred metres to > 100 km. The OPR, together with the H2O rotational temperature and column density, were derived for each spectra using a database of fluorescence synthetic spectra that include both fundamental and hot vibrational water bands. The weak lines of the v1, v1 + v3 − v1 and v2 + v3 − v2 bands in the 2.774–2.910 μm range were used to calculate by how much the strong v3 band centred at 2.67 μm is attenuated due to optical depth effects, expressed by the attenuation factor fatten. Results. Most ortho-to-para ratio determinations are strongly affected by opacity effects, as demonstrated by the observed anti-correlation between the OPR and the column density, and the correlation between the OPR and attenuation factor fatten. Based on both radiative transfer calculations and OPR values obtained in low-opacity conditions, we derive an OPR of 2.94 ± 0.06 for comet 67P. Measured water rotational temperatures show a decrease in gas kinetic temperature with increasing altitude caused by adiabatic cooling. Heliocentric variations are also observed, with warmer temperatures near perihelion. Conclusions. The water ortho-to-para ratio measured in the coma of 67P is consistent with laboratory experiments showing that water vapour that has thermally desorbed from water ice has a statistical value of 3, regardless of the past formation process of water ice.

Measurements of Ortho-to-para Nuclear Spin Conversion of H2 on Low-temperature Carbonaceous Grain Analogs: Diamond-like Carbon and Graphite

Abstract Hydrogen molecules have two nuclear spin isomers: ortho-H2 and para-H2. The ortho-to-para ratio (OPR) is known to affect chemical evolution as well as gas dynamics in space. Therefore, understanding the mechanism of OPR variation in astrophysical environments is important. In this work, the nuclear spin conversion (NSC) processes of H2 molecules on diamond-like carbon and graphite surfaces are investigated experimentally by employing temperature-programmed desorption and resonance-enhanced multiphoton ionization methods. For the diamond-like carbon surface, the NSC time constants were determined at temperatures of 10–18 K and from 3900 ± 800 s at 10 K to 750 ± 40 s at 18 K. Similar NSC time constants and temperature dependence were observed for a graphite surface, indicating that bonding motifs (sp3 or sp2 hybridization) have little effect on the NSC rates.

On the low ortho-to-para H2 ratio in star-forming filaments

The formation of stars and planetary systems is a complex phenomenon that relies on the interplay of multiple physical processes. Nonetheless, it represents a crucial stage for our understanding of the Universe, and in particular of the conditions leading to the formation of key molecules (e.g. water) on comets and planets. Herschel observations demonstrated that stars form in gaseous filamentary structures in which the main constituent is molecular hydrogen (H2). Depending on its nuclear spin H2 can be found in two forms: ‘ortho’ with parallel spins and ‘para’ where the spins are anti-parallel. The relative ratio among these isomers, the ortho-to-para ratio (OPR), plays a crucial role in a variety of processes related to the thermodynamics of star-forming gas and to the fundamental chemistry affecting the deuteration of water in molecular clouds, commonly used to determine the origin of water in Solar System bodies. Here, for the first time, we assess the evolution of the OPR starting from the warm neutral medium by means of state-of-the-art 3D magnetohydrodynamic simulations of turbulent molecular clouds. Our results show that star-forming clouds exhibit a low OPR (≪0.1) already at moderate densities (∼1000 cm−3). We also constrain the cosmic-ray ionisation rate, finding that 10−16 s−1 is the lower limit required to explain the observations of diffuse clouds. Our results represent a step forward in the understanding of the star and planet formation processes providing a robust determination of the chemical initial conditions for both theoretical and observational studies.

The TW Hya Rosetta Stone Project. II. Spatially Resolved Emission of Formaldehyde Hints at Low-temperature Gas-phase Formation

Abstract Formaldehyde (H2CO) is an important precursor to organics like methanol (CH3OH). It is important to understand the conditions that produce H2CO and prebiotic molecules during star and planet formation. H2CO possesses both gas-phase and solid-state formation pathways, involving either UV-produced radical precursors or CO ice and cold (≲20 K) dust grains. To understand which pathway dominates, gaseous H2CO's ortho-to-para ratio (OPR) has been used as a probe, with a value of 3 indicating “warm” conditions and <3 linked to cold formation in the solid state. We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of multiple ortho- and para-H2CO transitions in the TW Hya protoplanetary disk to test H2CO formation theories during planet formation. We find disk-averaged rotational temperatures and column densities of 33 ± 2 K, (1.1 ± 0.1) × 1012 cm−2 and 25 ± 2 K, (4.4 ± 0.3) × 1011 cm−2 for ortho- and para-H2CO, respectively, and an OPR of 2.49 ± 0.23. A radially resolved analysis shows that the observed H2CO emits mostly at rotational temperatures of 30–40 K, corresponding to a layer with z/R ≥ 0.25. The OPR is consistent with 3 within 60 au, the extent of the pebble disk, and decreases beyond 60 au to 2.0 ± 0.5. The latter corresponds to a spin temperature of 12 K, well below the rotational temperature. The combination of relatively uniform emitting conditions, a radial gradient in the OPR, and recent laboratory experiments and theory on OPR ratios after sublimation, led us to speculate that gas-phase formation is responsible for the observed H2CO across the TW Hya disk.

Highresolution spectroscopic probing of ortho and para nuclear-spin isomers of heavy water in the gas phase

Abstract Doubly deuterated water (D2O) exhibits two distinct nuclear spin-isomers, ortho and para, which have not much been studied and there is hardly any information about the spin-exchange processes of D2O. Here, we investigated the nuclear-spin quantum states of D2O in gas phase by evaluating the ortho-to-para ratio (OPR) using high-resolution cavity ring-down spectroscopy coupled with quantum cascade laser in the mid-IR spectral region at 7.8 μm. We experimentally achieved an OPR of (1.95 ± 0.16):1 for D2O spin-isomers in the isolated gaseous phase. We further analyzed the OPR of the residual D2O in a 1:1 mixture of D2O and H2O, and observed indications of potential alteration of thermodynamic probability of ortho and para nuclear-spin states of D2O associated with the D-H exchange reaction in the system. Our findings pave the way to probe directly the spin behaviour of D2O molecules in the gas phase and thus fostering exploration of spin-dependent chemistry in a wide range of chemical and biological systems.

Water ortho–para conversion by microwave background radiation in space

ABSTRACT A theoretical model of water ortho–para conversion induced by blackbody radiation in space is developed. The model is based on two main ingredients: the mixing of water ortho and para states by a hyperfine spin-rotation interaction in the molecule and the interruption of this mixing by surrounding blackbody radiation. The model predicts the lifetime of water spin isomers τ = 2.7 Myr for radiation with a temperature of 100 K and τ = 1.3 Gyr for microwave background radiation. The time dependence of the ortho-to-para ratio (OPR) of water molecules interacting with microwave background radiation is determined. The temperature dependence of the stationary OPR predicted by the model is found to be different from the usually accepted OPR for water spin isomers.

The water line emission and ortho-to-para ratio in the Orion Bar photon-dominated region

Context. The ortho-to-para ratio (OPR) of water in the interstellar medium (ISM) is often assumed to be related to the formation temperature of water molecules, making it a potentially interesting tracer of the thermal history of interstellar gas. Aims. A very low OPR of 0.1–0.5 was previously reported in the Orion Bar photon-dominated region (PDR), based on observations of two optically thin H218O lines which were analyzed by using a single-slab large velocity gradient (LVG) model. The corresponding spin temperature does not coincide with the kinetic temperature of the molecular gas in this UV-illuminated region. This was interpreted as an indication of water molecules being formed on cold icy grains which were subsequently released by UV photodesorption. Methods. A more complete set of water observations in the Orion Bar, including seven H216O lines and one H218O line, carried out using Herschel/HIFI instrument, was reanalyzed using the Meudon PDR code to derive gas-phase water abundance and the OPR. The model takes into account the steep density and temperature gradients present in the region. Results. The model line intensities are in good agreement with the observations assuming that water molecules formed with an OPR corresponding to thermal equilibrium conditions at the local kinetic temperature of the gas and when solely considering gas-phase chemistry and water gas-grain exchanges through adsorption and desorption. Gas-phase water is predicted to arise from a region deep into the cloud, corresponding to a visual extinction of AV ~ 9, with a H216O fractional abundance of ~2 × 10−7 and column density of (1.4 ± 0.8) × 1015 cm−2 for a total cloud depth of AV = 15. A line-of-sight average OPR of 2.8 ± 0.2 is derived. Conclusions. The observational data are consistent with a nuclear spin isomer repartition corresponding to the thermal equilibrium at a temperature of 36 ± 2 K, much higher than the spin temperature previously reported for this region and close to the gas kinetic temperature in the water-emitting gas.

Comet 66P/du Toit: not a near-Earth main belt comet

Context. Main belt comets (MBCs) are a peculiar class of volatile-containing objects with comet-like morphology and asteroid-like orbits. However, MBCs are challenging targets to study remotely due to their small sizes and the relatively large distance they are from the Sun and the Earth. Recently, a number of weakly active short-period comets have been identified that might originate in the asteroid main belt. Among all of the known candidates, comet 66P/du Toit has been suggested to have one of the highest probabilities of coming from the main belt. Aims. The main goal of this study is to investigate the physical properties of 66P via spectroscopic and imaging observations to constrain its formation conditions. In particular, the isotopic abundance ratio and the ortho-to-para ratio (OPR) of gaseous species can be derived via high-resolution spectroscopy, which is sensitive to the formation temperature of the nucleus. Methods. We obtained medium and high-resolution spectra of 66P from 300–2500 nm with the X-shooter and the UVES instruments at the Very Large Telescope in July 2018. We also obtained a series of narrow-band images of 66P to monitor the gas and dust activity between May and July 2018 with TRAPPIST-South. In addition, we applied a dust model to characterize the dust coma of 66P and performed dynamical simulations to study the orbital evolution of 66P. Results. We derive the OPR of ammonia (NH3) in 66P to be 1.08 ± 0.06, which corresponds to a nuclear spin temperature of ~34 K. We compute the production rates of OH, NH, CN, C3, and C2 radicals and measure the dust proxy, Afρ. The dust analysis reveals that the coma can be best-fit with an anisotropic model and the peak dust production rate is about 55 kg s−1 at the perihelion distance of 1.29 au. Dynamical simulations show that 66P is moderately asteroidal with the capture time, tcap ~ 104 yr. Conclusions. Our observations demonstrate that the measured physical properties of 66P are consistent with typical short-period comets and differ significantly from other MBCs. Therefore, 66P is unlikely to have a main belt origin.

High Spectral Resolution SOFIA/EXES Observations of C2H2 toward Orion IRc2

Abstract We present high spectral resolution observations from 12.96 to 13.33 microns toward Orion IRc2 using the mid-infrared spectrograph, Echelon-Cross-Echelle Spectrograph (EXES), at Stratospheric Observatory for Infrared Astronomy (SOFIA). These observations probe the physical and chemical conditions of the Orion hot core, which is sampled by a bright, compact, mid-infrared background continuum source in the region, IRc2. All 10 of the rovibrational C2H2 transitions expected in our spectral coverage are detected with high signal-to-noise ratios (S/Ns), yielding continuous coverage of the R-branch lines from J = 9–8 to J = 18–17, including both ortho and para species. Eight of these rovibrational transitions are newly reported detections. The isotopologue, 13CCH2, is clearly detected with a high S/N. This enabled a direct measurement of the 12C/13C isotopic ratio for the Orion hot core of 14 ± 1 and an estimated maximum value of 21. We also detected several HCN rovibrational lines. The ortho and para C2H2 ladders are clearly separate, and tracing two different temperatures, 226 K and 164 K, respectively, with a non-equilibrium ortho to para ratio (OPR) of 1.7 ± 0.1. Additionally, the ortho and para V LSR values differ by about 1.8 ± 0.2 km s−1, while the mean line widths differ by 0.7 ± 0.2 km s−1, suggesting that these species are not uniformly mixed along the line of sight to IRc2. We propose that the abnormally low C2H2 OPR could be a remnant from an earlier, colder phase, before the density enhancement (now the hot core) was impacted by shocks generated from an explosive event 500 years ago.

Multiple Paths of Deuterium Fractionation in Protoplanetary Disks

Abstract We investigate deuterium chemistry coupled with the nuclear spin-state chemistry of H2 and in protoplanetary disks. Multiple paths of deuterium fractionation are found; exchange reactions with D atoms, such as HCO+ + D, are effective in addition to those with HD. In a disk model with grain sizes appropriate for dark clouds, the freeze-out of molecules is severe in the outer midplane, while the disk surface is shielded from UV radiation. Gaseous molecules, including DCO+, thus become abundant at the disk surface, which tends to make their column density distribution relatively flat. If the dust grains have grown to millimeter size, the freeze-out rate of neutral species is reduced and the abundances of gaseous molecules, including DCO+ and N2D+, are enhanced in the cold midplane. Turbulent diffusion transports D atoms and radicals at the disk surface to the midplane, and stable ice species in the midplane to the disk surface. The effects of turbulence on chemistry are thus multifold; while DCO+ and N2D+ abundances increase or decrease depending on the regions, HCN and DCN in the gas and ice are greatly reduced at the innermost radii, compared to the model without turbulence. When cosmic rays penetrate the disk, the ortho-to-para ratio (OPR) of H2 is found to be thermal in the disk, except in the cold (≲10 K) midplane. We also analyze the OPR of and H2D+, as well as the main reactions of H2D+, DCO+, and N2D+, in order to analytically derive their abundances in the cold midplane.

The ortho-to-para ratio of H2Cl+: Quasi-classical trajectory calculations and new simulations in light of new observations

Multi-hydrogenated species with proper symmetry properties can present different spin configurations, and thus exist under different spin symmetry forms, labeled as para and ortho for two-hydrogen molecules. We investigated here the ortho-to-para ratio (OPR) of H$_2$Cl$^+$ in the light of new observations performed in the z=0.89 absorber toward the lensed quasar PKS 1830-211 with the Atacama Large Millimeter/submillimeter Array (ALMA). Two independent lines of sight were observed, to the southwest (SW) and northeast (NE) images of the quasar, with OPR values found to be $3.15 \pm 0.13$ and $3.1 \pm 0.5$ in each region, respectively, in agreement with a spin statistical weight of 3:1. An OPR of 3:1 for a molecule containing two identical hydrogen nuclei can refer to either a statistical result or a high-temperature limit depending on the reaction mechanism leading to its formation. It is thus crucial to identify rigorously how OPRs are produced in order to constrain the information that these probes can provide. To understand the production of the H$_2$Cl$^+$ OPR, we undertook a careful theoretical study of the reaction mechanisms involved with the aid of quasi-classical trajectory calculations on a new global potential energy surface fit to a large number of high-level ab initio data. Our study shows that the major formation reaction for H$_2$Cl$^+$ produces this ion via a hydrogen abstraction rather than a scrambling mechanism. Such a mechanism leads to a 3:1 OPR, which is not changed by destruction and possible thermalization reactions for H$_2$Cl$^+$ and is thus likely to be the cause of observed 3:1 OPR ratios, contrary to the normal assumption of scrambling.

Fluorescent H2 Emission Lines from the Reflection Nebula NGC 7023 Observed with IGRINS

Abstract We have analyzed the temperature, velocity, and density of H2 gas in NGC 7023 with a high-resolution near-infrared spectrum of the northwestern filament of the reflection nebula. By observing NGC 7023 in the H and K bands at R ≃ 45,000 with the Immersion GRating INfrared Spectrograph, we detected 68 H2 emission lines within the 1″ × 15″ slit. The diagnostic ratio of 2-1 S(1)/1-0 S(1) is 0.41−0.56. In addition, the estimated ortho-to-para ratio (OPR) is 1.63−1.82, indicating that the H2 emission transitions in the observed region arise mostly from gas excited by UV fluorescence. Gradients in the temperature, velocity, and OPR within the observed area imply motion of the photodissociation region (PDR) relative to the molecular cloud. In addition, we derive the column density of H2 from the observed emission lines and compare these results with PDR models in the literature covering a range of densities and incident UV field intensities. The notable difference between PDR model predictions and the observed data, in high rotational J levels of ν = 1, is that the predicted formation temperature for newly formed H2 should be lower than that of the model predictions. To investigate the density distribution, we combine pixels in 1″ × 1″ areas and derive the density distribution at the 0.002 pc scale. The derived gradient of density suggests that NGC 7023 has a clumpy structure, including a high clump density of ∼105 cm−3 with a size smaller than ∼5 × 10−3 pc embedded in lower-density regions of 103–104 cm−3.

The ortho-to-para ratio of interstellar NH2: quasi-classical trajectory calculations and new simulations

Based on recent $Herschel$ results, the ortho-to-para ratio (OPR) of NH$_2$ has been measured towards the following high-mass star-forming regions: W31C (G10.6-0.4), W49N (G43.2-0.1), W51 (G49.5-0.4), and G34.3+0.1. The OPR at thermal equilibrium ranges from the statistical limit of three at high temperatures to infinity as the temperature tends toward zero, unlike the case of H$_{2}$. Depending on the position observed along the lines-of-sight, the OPR was found to lie either slightly below the high temperature limit of three (in the range $2.2-2.9$) or above this limit ($\sim3.5$, $\gtrsim 4.2$, and $\gtrsim 5.0$). In low temperature interstellar gas, where the H$_{2}$ is para-enriched, our nearly pure gas-phase astrochemical models with nuclear-spin chemistry can account for anomalously low observed NH$_2$-OPR values. We have tentatively explained OPR values larger than three by assuming that spin thermalization of NH$_2$ can proceed at least partially by H-atom exchange collisions with atomic hydrogen, thus increasing the OPR with decreasing temperature. In this paper, we present quasi-classical trajectory calculations of the H-exchange reaction NH$_2$ + H, which show the reaction to proceed without a barrier, confirming that the H-exchange will be efficient in the temperature range of interest. With the inclusion of this process, our models suggest both that OPR values below three arise in regions with temperatures $\gtrsim20-25$~K, depending on time, and values above three but lower than the thermal limit arise at still lower temperatures.

ON THE INFERENCE OF THE COSMIC-RAY IONIZATION RATE ζ FROM THE HCO+-to-DCO+ ABUNDANCE RATIO: THE EFFECT OF NUCLEAR SPIN

ABSTRACT The chemistry of dense interstellar regions was analyzed using a time-dependent gas–grain astrochemical simulation and a new chemical network that incorporates deuterated chemistry, taking into account nuclear spin states for the hydrogen chemistry and its deuterated isotopologues. With this new network, the utility of the [HCO+]/[DCO+] abundance ratio as a probe of the cosmic-ray ionization rate has been re-examined, with special attention paid to the effect of the initial value of the ortho-to-para ratio (OPR) of molecular hydrogen. After discussing the use of the probe for cold cores, we compare our results with previous theoretical and observational results for a molecular cloud close to the supernova remnant W51C, which is thought to have an enhanced cosmic-ray ionization rate ζ caused by the nearby γ-ray source. In addition, we attempt to use our approach to estimate the cosmic-ray ionization rate for L1174, a dense core with an embedded star. Beyond the previously known sensitivity of [HCO+]/[DCO+] to ζ, we demonstrate its additional dependence on the initial OPR and, secondarily, on the age of the source, its temperature, and its density. We conclude that the usefulness of the [HCO+]/[DCO+] abundance ratio in constraining the cosmic-ray ionization rate in dense regions increases with the age of the source and the ionization rate as the ratio becomes far less sensitive to the initial value of the OPR.

Ortho-to-para ratio of NH2

We have used the Herschel-HIFI instrument to observe both nuclear spin symmetries of amidogen (NH2) towards the high-mass star-forming regions W31C (G10.6-0.4), W49N (G43.2-0.1), W51 (G49.5-0.4) and G34.3+0.1. The aim is to investigate the ratio of nuclear spin types, the ortho-to-para ratio (OPR), of NH2. The excited NH2 transitions are used to construct radiative transfer models of the hot cores and surrounding envelopes in order to investigate the excitation and possible emission of the ground state rotational transitions of ortho-NH2 N_(K_a,K_c} J=1_(1,1) 3/2 - 0_(0,0) 1/2 and para-NH2 2_(1,2) 5/2 - 1_(0,1) 3/2$ used in the OPR calculations. Our best estimate of the average OPR in the envelopes lie above the high temperature limit of three for W49N, specifically 3.5 with formal errors of \pm0.1, but for W31C, W51, and G34.3+0.1 we find lower values of 2.5\pm0.1, 2.7\pm0.1, and 2.3\pm0.1, respectively. Such low values are strictly forbidden in thermodynamical equilibrium since the OPR is expected to increase above three at low temperatures. In the translucent interstellar gas towards W31C, where the excitation effects are low, we find similar values between 2.2\pm0.2 and 2.9\pm0.2. In contrast, we find an OPR of 3.4\pm0.1 in the dense and cold filament connected to W51, and also two lower limits of >4.2 and >5.0 in two other translucent gas components towards W31C and W49N. At low temperatures (T \lesssim 50 K) the OPR of H2 is <10^-1, far lower than the terrestrial laboratory normal value of three. In such a "para-enriched H2" gas, our astrochemical models can reproduce the variations of the observed OPR, both below and above the thermodynamical equilibrium value, by considering nuclear-spin gas-phase chemistry. The models suggest that values below three arise in regions with temperatures >20-25 K, depending on time, and values above three at lower temperatures.

A non-equilibrium ortho-to-para ratio of water in the Orion PDR

Context. The ortho-to-para ratio (OPR) of H2O is thought to be sensitive to the temperature of water formation. The OPR of H2O is thus useful for studying the formation mechanism of water.

Ortho-to-Para Ratio in Interstellar Water on the Sightline toward Sagittarius B2(N)

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 ortho- and 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.