Density and infrared band strength of interstellar carbon monoxide (CO) ice analogues

TL;DR

This study experimentally investigates the density and infrared band strength of interstellar CO ice analogues, providing new measurements and a methodology applicable to other ice components under simulated interstellar conditions.

Contribution

It introduces a method to determine ice densities at various temperatures and revises the infrared band strength value for CO ice used in astrophysical observations.

Findings

CO ice density varies minimally with temperature above 14 K.

A new infrared band strength value for CO ice at 20 K is proposed.

The methodology can be applied to other ice components for density estimation.

Abstract

The motivation to study experimentally CO ice under mimicked interstellar conditions is supported by the large CO gas abundances and ubiquitous presence of CO in icy grain mantles. Upon irradiation in its pure ice form, this highly stable species presents a limited ion and photon-induced chemistry, and an efficient non-thermal desorption. Using infrared spectroscopy, single laser interference, and quadrupole mass spectrometry during CO ice deposition, the CO ice density was estimated as a function of deposition temperature. Only minor variations in the density were found. The proposed methodology can be used to obtain the density of other ice components at various deposition temperatures provided that this value of the density is known for one of these temperatures, which is typically the temperature corresponding to the crystalline form. The apparent tendency of the CO ice density to decrease at deposition temperatures below 14 K is in line with recently published colorimetric measurements. This work allowed to revisit the value of the infrared band strength needed for calculation of the CO ice column density in infrared observations, $8.7 \times 10^{-18} ~ {\rm cm ~ molecule}^{-1}$ at 20 K deposition temperature.