Water deuteration and ortho-to-para nuclear spin ratio of H₂in molecular clouds formed via the accumulation of H I gas

Abstract

We investigate the water deuteration ratio and ortho-to-para nuclear spin ratio of H 2 (OPR(H 2 )) during the formation and early evolution of a molecular cloud, following the scenario that accretion flows sweep and accumulate H i gas to form molecular clouds.We follow the physical evolution of post-shock materials using a one-dimensional shock model, combined with post-processing gasice chemistry simulations. This approach allows us to study the evolution of the OPR(H 2 ) and water deuteration ratio without an arbitrary assumption of the initial molecular abundances, including the initial OPR(H 2 ). When the conversion of hydrogen into H 2 is almost complete the OPR(H 2 ) is already much smaller than the statistical value of three because of the spin conversion in the gas phase. As the gas accumulates, the OPR(H 2 ) decreases in a non-equilibrium manner. We find that water ice can be deuterium-poor at the end of its main formation stage in the cloud, compared to water vapor observed in the vicinity of low-mass protostars where water ice is sublimated. If this is the case, the enrichment of deuterium in water should mostly occur at somewhat later evolutionary stages of star formation, i.e., cold prestellar/protostellar cores. The main mechanism to suppress water ice deuteration in the cloud is the cycle of photodissociation and reformation of water ice, which efficiently removes deuterium from water ice chemistry. The removal efficiency depends on the main formation pathway of water ice. The OPR(H 2 ) plays a minor role in water ice deuteration at the main formation stage of water ice.

Chemical fractionation of deuterium in the protosolar nebula

“…Recent deuterium chemistry models that consider this are those of Taquet, Charnley & Sipilä (2014) and Furuya et al (2015). The former considers circumstellar envelopes in a physically detailed way and confirms that D/H is high for surface and low for bulk ice species.…”

“…This is consistent with observations (e.g., Whittet et al 2001) and some models (Garrod & Pauly 2011;Taquet, Charnley & Sipilä 2014;Hocuk & Cazaux 2015). Ice appears later than in the model of Furuya et al (2015). This is largely because the simulation of Furuya et al starts at an earlier point in cloud evolution and takes longer before the onset of rapid core collapse.…”

“…More than 90 per cent of ice molecules are deposited when AV > 4 mag and ortho/para ratio is around 10 −3 , according to the models of Sipilä, Caselli & Harju (2013) and Furuya et al (2015).…”

“…This means that we consider a medium where refractory elements are significantly depleted, while the ice forming species are not. This approach is similar to that of Furuya et al (2015). Self-shielding of HD, and its mutual shielding by H2 was added (Le Petit, Roueff & Le Bourlot 2002;Wolcott-Green & Haiman 2011), in addition to the shielding of gaseous and adsorbed H2, CO, and N2.…”

“…Photodesorption yield Y ph was taken to be 3×10 −4 for interstellar photons (Arasa et al 2015;Furuya et al 2015) and a 1.5 times lower value for cosmic-ray-induced photons (Kalvāns 2015c). Eq.…”