Effects of CO₂ on H₂O band profiles and band strengths in mixed H₂O:CO₂ ices

¹ Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: oberg@strw.leidenuniv.nl
² Department of Physics, Scottish Universities Physics Alliance (SUPA), University of Strathclyde, John Anderson Building, 107 Rottenrow East, Glasgow G4 ONG, Scotland
³ AURA/NOAO-South, Casilla 603, La Serena, S. A. Chile

Received: 22 June 2006
Accepted: 17 October 2006

Abstract

Context.H₂O is the most abundant component of astrophysical ices. In most lines of sight it is not possible to fit both the H₂O 3 μm stretching, the 6 μm bending intensities with a single pure H₂O spectrum. Recent Spitzer observations have revealed CO₂ ice in high abundances and it has been suggested that CO₂ mixed into H₂O ice can affect the positions, shapes and relative strengths of the 3 μm and 6 μm bands.

Aims.We investigate whether the discrepancy in intensity between H₂O bands in interstellar clouds and star forming regions can be explained by CO₂ mixed into the observed H₂O ice affecting the bands differently.

Methods.Laboratory infrared transmission spectroscopy is used to record spectra of H₂O:CO₂ ice mixtures at astrophysically relevant temperatures and composition ratios.

Results.The H₂O peak profiles and band strengths are significantly different in H₂O:CO₂ ice mixtures compared to pure H₂O ice. The ratio between the strengths of the 3 μm and 6 μm bands drops linearly with CO₂ concentration such that it is 50% lower in a 1:1 mixture compared to pure H₂O ice. In all H₂O:CO₂ mixtures, a strong free-OH stretching band appears around 2.73 μm, which can be used to put an upper limit on the CO₂ concentration in the H₂O ice. The H₂O bending mode profile also changes drastically with CO₂ concentration; the broad pure H₂O band gives way to two narrow bands as the CO₂ concentration is increased. This makes it crucial to constrain the environment of H₂O ice to enable correct assignments of other species contributing to the interstellar 6 μm absorption band. The amount of CO₂ present in the H₂O ice of B5:IRS1 is estimated by simultaneously comparing the H₂O stretching and bending regions and the CO₂ bending mode to laboratory spectra of H₂O, CO₂, H₂O:CO₂ and HCOOH.