Deuterium-enriched water ties planet-forming disks to comets and protostars

Water is a fundamental molecule in the star and planet formation process, essential for catalysing the growth of solid material and the formation of planetesimals within disks^1,2. However, the water snowline and the HDO:H₂O ratio within proto-planetary disks have not been well characterized because water only sublimates at roughly 160 K (ref. ³), meaning that most water is frozen out onto dust grains and that the water snowline radii are less than 10 AU (astronomical units)^4,5. The sun-like protostar V883 Ori (M_* = 1.3 M_⊙)⁶ is undergoing an accretion burst⁷, increasing its luminosity to roughly 200 L_⊙ (ref. ⁸), and previous observations suggested that its water snowline is 40-120 AU in radius^6,9,10. Here we report the direct detection of gas phase water (HDO and [Formula: see text]) from the disk of V883 Ori. We measure a midplane water snowline radius of approximately 80 AU, comparable to the scale of the Kuiper Belt, and detect water out to a radius of roughly 160 AU. We then measure the HDO:H₂O ratio of the disk to be (2.26 ± 0.63) × 10^-3. This ratio is comparable to those of protostellar envelopes and comets, and exceeds that of Earth's oceans by 3.1σ. We conclude that disks directly inherit water from the star-forming cloud and this water becomes incorporated into large icy bodies, such as comets, without substantial chemical alteration.

Radical reactions on interstellar icy dust grains: Experimental investigations of elementary processes

Molecular clouds (MCs) in space are the birthplace of various molecular species. Chemical reactions occurring on the cryogenic surfaces of cosmic icy dust grains have been considered to play important roles in the formation of these species. Radical reactions are crucial because they often have low barriers and thus proceed even at low temperatures such as ∼10 K. Since the 2000s, laboratory experiments conducted under low-temperature, high-vacuum conditions that mimic MC environments have revealed the elementary physicochemical processes on icy dust grains. In this review, experiments conducted by our group in this context are explored, with a focus on radical reactions on the surface of icy dust analogues, leading to the formation of astronomically abundant molecules such as H₂, H₂O, H₂CO, and CH₃OH and deuterium fractionation processes. The development of highly sensitive, non-destructive methods for detecting adsorbates and their utilization for clarifying the behavior of free radicals on ice, which contribute to the formation of complex organic molecules, are also described.

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.

The nuclear-spin-forbidden rovibrational transitions of water from first principles

The water molecule occurs in two nuclear-spin isomers that differ by the value of the total nuclear spin of the hydrogen atoms, i.e., I = 0 for para-H2O and I = 1 for ortho-H2O. Spectroscopic transitions between rovibrational states of ortho and para water are extremely weak due to the tiny hyperfine nuclear-spin –rotation interaction of only ∼30 kHz and, so far, have not been observed. We report the first comprehensive theoretical investigation of the hyperfine effects and ortho–para transitions in [Formula: see text]O due to nuclear-spin –rotation and spin–spin interactions. We also present the details of our newly developed general variational approach to the simulation of hyperfine effects in polyatomic molecules. Our results for water suggest that the strongest ortho–para transitions with room-temperature intensities on the order of 10−31 cm/molecule are about an order of magnitude larger than previously predicted values and should be detectable in the mid-infrared ν2 and near-infrared 2 ν1 + ν2 and ν1 + ν2 + ν3 bands by current spectroscopy experiments.

Spectroscopic determination of interconversion rates among three nuclear spin isomers of methane in crystalline II

We measured infrared absorption spectra of crystalline II of CH₄ and succeeded in detecting a prominent Q(2) peak in the ν₃ vibrational region by rapid cooling after annealing as well as previously reported rovibrational and librational-vibrational peaks. The integral intensities of the R(0), R(1), and Q(2) peaks were found to show biexponential dependence on time. This clearly demonstrates the interconversion among the three nuclear-spin isomers occupying low-lying rotational levels. The two relaxation rates obtained by biexponential fitting were (0.48, 2.3), (1.1, 4.1), (2.3, 5.1), and (3.4, 15.3) in units of inverse hour (h^-1) at 5.2, 6.0, 6.5, and 7.0 K, respectively.

Rotational Motion and Nuclear Spin Interconversion of H2O Encapsulated in C60 Appearing in the Low-Temperature Heat Capacity

The heat capacity of H₂O encapsulated in fullerene C₆₀ is determined for the first time at temperatures between 0.6 and 200 K. The water molecule in H₂O@C₆₀ undergoes quantum rotation at low temperature, and the ortho-H₂O and para-H₂O isomers are identified by labeling the rotational energy levels with the nuclear spin states. A rounded heat capacity maximum is observed at ∼2 K after rapid cooling due to splitting of the rotational J _KaKc = 1₀₁ ground state of ortho-H₂O. This anomalous feature decreases in magnitude over time, reflecting the conversion of ortho-H₂O to para-H₂O. Time-dependent heat capacity measurements at constant temperature reveal three nuclear spin conversion processes: a thermally activated transition with E_a ≈ 3.2 meV and two temperature-independent tunneling processes with time constants of τ₁ ≈ 1.5 h and τ₂ ≈ 11 h.

A magnetically focused molecular beam source for deposition of spin-polarised molecular surface layers

Separating molecular spin isomers is a challenging task, with potential applications in various fields ranging from astrochemistry to magnetic resonance imaging. A new promising method for spin-isomer separation is magnetic focusing, a method which was shown to be capable of producing a molecular beam of ortho-water. Here, we present results from a modified magnetic focusing apparatus and show that it can be used to separate the spin isomers of acetylene and methane. From the measured focused profiles of the molecular beams and a numerical simulation analysis, we provide estimations for the spin purity and the significantly improved molecular flux obtained with the new setup. Finally, we discuss the spin-relaxation conditions which will be needed to apply this new source for measuring nuclear magnetic resonance signals of a single surface layer.

Determining the Humidity-Dependent Ortho-to- Para Ratio of Water Vapor at Room Temperature Using Terahertz Spectroscopy

The origin of the water spin isomers observed under various physico-chemical conditions is of great interest, including that of H₂O molecules in the gas phase. Here, terahertz time-domain spectroscopy (THz-TDS) was used to study the humidity-dependent ortho-to- para (O/P) ratio of water vapor at room temperature. The relative contents of para and ortho molecules were obtained by fitting the absorption lines of water vapor showing the relationship between the spin isomer contents and humidity. Larger O/P ratios with values of ∼3.2 were observed at lower humidity (<20%) due to the stronger attractive forces of para molecules. The concentration of the ortho isomers then began to decrease at higher humidity (>20%) due to the preferential formation of dimers and clusters at increasing concentrations. Thus, the ratio gradually decreased with increasing humidity.

Confinement Effects on the Nuclear Spin Isomer Conversion of H2O

The mechanism for interconversion between the nuclear spin isomers (NSI) of H₂O remains shrouded in uncertainties. The temperature dependence displayed by NSI interconversion rates for H₂O isolated in an argon matrix provides evidence that confinement effects are responsible for the dramatic increase in their kinetics with respect to the gas phase, providing new pathways for o-H₂O↔p-H₂O conversion in endohedral compounds. This reveals intramolecular aspects of the interconversion mechanism which may improve methodologies for the separation and storage of NSI en route to applications ranging from magnetic resonance spectroscopy and imaging to interpretations of spin temperatures in the interstellar medium.