Water delivery from cores to disks: Deuteration as a probe of the prestellar inheritance of H₂O

Abstract

We investigate the delivery of regular and deuterated forms of water from prestellar cores to circumstellar disks. We adopt a semianalytical axisymmetric two-dimensional collapsing core model with post-processing gas-ice astrochemical simulations, in which a layered ice structure is considered. The physical and chemical evolutions are followed until the end of the main accretion phase. When mass averaged over the whole disk, a forming disk has a similar H 2 O abundance and HDO/H 2 O abundance ratio as their precollapse values (within a factor of 2), regardless of time in our models. Consistent with previous studies, our models suggest that interstellar water ice is delivered to forming disks without significant alteration. On the other hand, the local vertically averaged H 2 O ice abundance and HDO/H 2 O ice ratio can differ more, by up to a factor of several, depending on time and distance from a central star. Key parameters for the local variations are the fluence of stellar UV photons en route into the disk and the ice layered structure, the latter of which is mostly established in the prestellar stages. We also find that even if interstellar water ice is destroyed by stellar UV and (partly) reformed prior to disk entry, the HDO/H 2 O ratio in reformed water ice is similar to the original value. This finding indicates that some caution is needed in discussions on the prestellar inheritance of H 2 O based on comparisons between the observationally derived HDO/H 2 O ratio in clouds/cores and that in disks/comets. Alternatively, we propose that the ratio of D 2 O/HDO to HDO/H 2 O better probes the prestellar inheritance of H 2 O. It is also found that icy organics are more enriched in deuterium than water ice in forming disks. The differential deuterium fractionation in water and organics is inherited from the prestellar stages.

ALMA observations of water deuteration: a physical diagnostic of the formation of protostars

“…The lack of pronounced variation between the sources indicate a similar physical and chemical evolution for the systems, with little impact from local variations in, e.g., protostellar luminosity or accretion bursts. This conclusion is compatible with previous studies which modeled the water evolution from the collapse of an isolated pre-stellar cores to circumstellar disk (Visser et al 2009;Cleeves et al 2014;Drozdovskaya et al 2016;Furuya et al 2017).…”

Grain Surface Models and Data for Astrochemistry

“…Typically, the addition of the modified rate approach to the rate equation model, which use the same numerical scheme, slows it by a factor of several due to the performance penalty for accounting for probabilities of reactants to be on the grain surface (Garrod et al 2009). A multiphase model (Furuya et al 2016) without bulk ice chemistry, but with swapping only, takes typically ∼30-60 minutes per trajectory, using a full gas-ice network with deuterium chemistry. It is hence computationally feasible to use such a model to simulate a collapsing core model; tracing 35000 parcels from the prestellar core phase to the circumstellar disk phase results in ∼35000 CPU hours or ∼2 weeks on a ∼100-core machine.…”

Depletion of Heavy Nitrogen in the Cold Gas of Star-forming Regions

“…Photodissociation occurs only in the surface layers (i.e., top four monolayers) in our model, since the rest of ice mantle is assumed to be chemically inert. In other words, photofragments is assumed to immediately recombine in the bulk ice mantle in our model (Furuya et al 2017). According to molecular dynamics (MD) simulations, there are several possible outcomes after a UV photon dissociates water ice; (i) the photofragments are trapped on the surface; (ii) either of the fragments is desorbed into the gas phase; (iii) the fragments recombine and the product is either trapped on the surface or desorbed into the gas phase, etc.…”