Astrochemistry

Redistribution of Ices Between Grain Populations in Protostellar Envelopes. Only the Coldest Grains Get Ices

By Keith Cowing
Status Report
astro-ph.GA
February 3, 2025
Filed under , , , , , , , , , ,
Redistribution of Ices Between Grain Populations in Protostellar Envelopes. Only the Coldest Grains Get Ices
Results of Model Ice_small, where all ice is on the surface of a = 0.037 µm grains (top) and Model Ice_large with ice on the 0.232 µm grains (bottom). Left: Ice thickness (black lines) changes with Td (red). The thicker lines show larger grains. Right: Ice (solid lines) and gas-phase (dotted lines) abundances of the major ice species relative to H2. This is to be compared with Model Standard in Fig. 2. — astro-ph.GA

Context. Matter that falls onto a protoplanetary disk (PPD) from a protostellar envelope is heated before it cools again. This induces sublimation and subsequent re-adsorption of ices that accumulated during the prestellar phase.

Aims. We explore the fate of ices on multiple-sized dust grains in a parcel of infalling matter. Methods. A comprehensive kinetic chemical model using five grain-size bins with different temperatures was applied for an infalling parcel. The parcel was heated to 150 K and then cooled over a total timescale of 20 kyr. Effects on ice loss and re-accumulation by the changed gas density, the maximum temperature, the irradiation intensity, the size-dependent grain temperature trend, and the distribution of the ice mass among the grain-size bins were investigated.

Results. A massive selective redistribution of ices exclusively onto the surface of the coldest grain-size bin occurs in all models. The redistribution starts already during the heating stage, where ices that are sublimated from warmer grains re-adsorb onto colder grains before complete sublimation. During the cooling stage, the sublimated molecules re-freeze again onto the coldest grains. In the case of full sublimation, this re-adsorption is delayed and occurs at lower temperatures because a bare grain surface has lower molecular desorption energies in our model.

Conclusions. Most protostellar envelope grains enter the PPD ice poor (bare). Ices are carried by a single coldest grain-size bin, here representing 12 % of the total grain surface area. This bare ice-grain dualism can affect the rate of the grain coagulation. The ice components are stratified on the grains according to their sublimation temperatures.

Juris Kalvāns

Comments: Accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA); Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2501.15609 [astro-ph.GA] (or arXiv:2501.15609v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2501.15609
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Submission history
From: Juris Kalvans Dr.phys.
[v1] Sun, 26 Jan 2025 17:30:50 UTC (2,039 KB)
https://arxiv.org/abs/2501.15609
Astrobiology, Astrochemistry,

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