Redistribution of ices between grain populations in protostellar envelopes

Only the coldest grains get ices

Engineering Research Institute “Ventspils International Radio Astronomy Centre” of Ventspils University of Applied Sciences, Inženieru 101, Ventspils, LV-3601, Latvia

^★ Corresponding author; juris.kalvans@venta.lv

Received: 8 October 2024
Accepted: 23 December 2024

Abstract

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.