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Register a new userH$_2$ ortho-para spin conversion on inhomogeneous grain surfaces
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We investigate the evolution of the ortho-to-para ratio of overall (gas + ice) H$_2$ via the nuclear spin conversion on grain surfaces coated with water ice under physical conditions that are relevant to star- and planet-forming regions. We utilize the rate equation model that considers adsorption of gaseous H$_2$ on grain surfaces which have a variety of binding sites with a different potential energy depth, thermal hopping, desorption, and the nuclear spin conversion of adsorbed H$_2$. It is found that the spin conversion efficiency depends on the H$_2$ gas density and the surface temperature. As a general trend, enhanced H$_2$ gas density reduces the efficiency, while the temperature dependence is not monotonic; there is a critical surface temperature at which the efficiency is the maximum. At low temperatures, the exchange of gaseous and icy H$_2$ is inefficient (i.e., adsorbed H$_2$ does not desorb and hinders another gaseous H$_2$ to be adsorbed), while at warm temperatures, the residence time of H$_2$ on surfaces is too short for the spin conversion. Additionally, the spin conversion becomes more efficient with lowering the activation barriers for thermal hopping. We discuss whether the spin conversion on surfaces can dominate over that in the gas-phase in star- and planet-forming regions. Finally, we establish a simple but accurate way to implement the H$_2$ spin conversion on grain surfaces in existing astrochemical models.
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The H2 molecule has two nuclear spin isomers, the so-called ortho and para isomers. Nuclear spin conversion (NSC) between these states is forbidden in the gas phase. The energy difference between the lowest ortho and para states is as large as 14.7 meV, corresponding to ~170 K. Therefore, each state of H2 differently affects not only the chemistry but also the macroscopic gas dynamics in space, and thus, the ortho-to-para abundance ratio (OPR) of H2 has significant impacts on various astronomical phenomena. For a long time, the OPR of nascent H2 upon formation on dust grains has been assumed to have a statistical value of three and to gradually equilibrate in the gas phase at the temperature of the circumstances. Recently, NSC of H2 was experimentally revealed to occur on water ice at very low temperatures and thus incorporated into gas-dust chemical models. However, H2 molecules should form well before dust grains are coated by water ice. Information about how the OPR of H2 behaves on bare silicate dust before ice-mantle formation is lacking. Knowing the influence of the OPR of H2 if the OPR changes even on a bare silicate surface within an astronomically meaningful time scale is desirable. We report the first laboratory measurements of NSC of H2 physisorbed on amorphous silicate (Mg2SiO4) at temperatures up to 18 K. The conversion was found to occur very rapidly.
Spectrally-resolved observations of three pure rotational lines of H$_2$, conducted with the EXES instrument on SOFIA toward the classic bow shock HH7, reveal systematic velocity shifts between the S(5) line of ortho-H$_2$ and the two para-H$_2$ lines [S(4) and S(6)] lying immediately above and below it on the rotational ladder. These shifts, reported here for the first time, imply that we are witnessing the conversion of para-H$_2$ to ortho-H$_2$ within a shock wave driven by an outflow from a young stellar object. The observations are in good agreement with the predictions of models for non-dissociative, C-type molecular shocks. They provide a clear demonstration of the chemical changes wrought by interstellar shock waves, in this case the conversion of para-H$_2$ to ortho-H$_2$ in reactive collisions with atomic hydrogen, and provide among the most compelling evidence yet obtained for C-type shocks in which the flow velocity changes continuously.
The formation of stars and planetary systems is a complex phenomenon, which 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. {it Herschel} observations demonstrated that stars form out of gaseous filamentary structures in which the main constituent is molecular hydrogen (H$_2$). Depending on its nuclear spin H$_2$ can be found in two forms: `ortho with parallel spins and `para where the spins are anti-parallel. The relative ratio among these isomers, i.e.,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 formation of water in molecular clouds and its subsequent deuteration, commonly used to determine the origin of water in Solar Systems 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 three-dimensional magneto-hydrodynamic simulations of turbulent molecular clouds. Our results show that star-forming clouds exhibit a low OPR ($ll 0.1$) already at moderate densities ($sim$1000 cm$^{-3}$). We also constrain the cosmic rays ionisation rate, finding that $10^{-16},rm s^{-1}$ 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 process providing a robust determination of the chemical initial conditions for both theoretical and observational studies.
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 2.5pm0.1, 2.7pm0.1, and 2.3pm0.1, 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 2.2pm0.2 and 2.9pm0.2. In contrast, we find an OPR of 3.4pm0.1 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.
(Abridged) We present a large sample of o-H$_2$D$^+$ observations in high-mass star-forming regions and discuss possible empirical correlations with relevant physical quantities to assess its role as a chronometer of star-forming regions through different evolutionary stages. APEX observations of the ground-state transition of o-H$_2$D$^+$ were analysed in a sample of massive clumps selected from ATLASGAL at different evolutionary stages. Column densities and beam-averaged abundances of o-H$_2$D$^+$ with respect to H$_2$, $X$(o-H$_2$D$^+$), were obtained by modelling the spectra under the assumption of local thermodynamic equilibrium. We detect 16 sources in o-H$_2$D$^+$ and find clear correlations between $X$(o-H$_2$D$^+$) and the clump bolometric luminosity and the dust temperature, while only a mild correlation is found with the CO-depletion factor. In addition, we see a clear correlation with the luminosity-to-mass ratio, which is known to trace the evolution of the star formation process. This would indicate that the deuterated forms of H$_3^+$ are more abundant in the early stages of the star formation process and that deuteration is influenced by the time evolution of the clumps. In this respect, our findings would suggest that the $X$(o-H$_2$D$^+$) abundance is mainly affected by the thermal changes rather than density changes in the gas. We have employed these findings together with observations of H$^{13}$CO$^+$, DCO$^+$, and C$^{17}$O to provide an estimate of the cosmic-ray ionisation rate in a sub-sample of eight clumps based on recent analytical work. Our study presents the largest sample of o-H$_2$D$^+$ in star-forming regions to date. The results confirm that the deuteration process is strongly affected by temperature and suggests that o-H$_2$D$^+$ can be considered a reliable chemical clock during the star formation processes, as proved by its strong temporal dependence.
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