The Cosmic-Ray Induced Sputtering Process On Icy Grains

TL;DR

This study investigates the efficiency of cosmic-ray induced sputtering of water and carbon monoxide ices in molecular clouds, revealing the dominance of quadratic regimes and the influence of cosmic-ray properties and ice composition.

Contribution

It provides a comprehensive analysis of sputtering regimes for H$_2$O and CO ices considering multiple parameters, highlighting the conditions affecting sputtering efficiency and track formation.

Findings

Quadratic sputtering regime dominates for both ices.

Sputtering rates vary with cosmic-ray flux and grain size.

CO ice exhibits larger track radii and can be affected by lighter CR ions.

Abstract

In molecular cloud cores, the cosmic ray (CR) induced sputtering via CR ion-icy grain collision is one of the desorption processes for ice molecules from mantles around dust grains. The efficiency of this process depends on the incident CR ion properties as well as the physicochemical character of the ice mantle. Our main objective is the examination of the sputtering efficiency for H$_2$O and CO ices found in molecular cloud cores. In the calculation routine, we consider a multi-dimensional parameter space that consists of thirty CR ion types, five different CR ion energy flux distributions, two separate ice mantle components (pure H$_2$O and CO), three ice formation states, and two sputtering regimes (linear and quadratic). We find that the sputtering behavior of H$_2$O and CO ices is dominated by the quadratic regime rather than the linear regime, especially for CO sputtering. The sputtering rate coefficients for H$_2$O and CO ices show distinct variations with respect to the adopted CR ion energy flux as well as the grain size-dependent mantle depth. The maximum radius of the cylindrical latent region is quite sensitive to the effective electronic stopping power. The track radii for CO ice are much bigger than H$_2$O ice values. In contrast to the H$_2$O mantle, even relatively light CR ions ($Z \geq 4$) may lead to a track formation within the CO mantle, depending on ${\rm S_{\rm e,eff}}$. We suggest that the latent track formation threshold can be assumed as a separator between the linear and the quadratic regimes for sputtering.