2007A&A...470..749B

HCOOH is one of the more common species in interstellar ices with abundances of 1-5% with respect to solid H₂O. With the launch of the Spitzer Space Telescope new infrared spectra have become available of interstellar ices in different environments. So far systematic laboratory studies on HCOOH-containing interstellar ice analogues are lacking. This study aims at characterizing the HCOOH spectral features in astrophysically relevant ice mixtures in order to interpret astronomical data. The ices are grown under high vacuum conditions and spectra are recorded in transmission using a Fourier transform infrared spectrometer. Pure HCOOH ices deposited at 15 K and 145 K are studied, as well as binary and tertiary mixtures containing H₂O, CO, CO₂ and CH₃OH. The mixture concentrations are varied from 50:50% to ∼10:90% for HCOOH:H₂O. Binary mixtures of HCOOH:X and tertiary mixtures of HCOOH:H₂O:X with X=CO, CO₂, and CH₃OH, are studied for concentrations of ∼10:90% and ∼7:67:26%, respectively. Pure HCOOH ice spectra show broad bands which split around 120 K due to the conversion of a dimer to a chain-structure. Broad single component bands are found for mixtures with H₂O. Additional spectral components are present in mixtures with CO, CO₂ and CH₃OH. The resulting peak position, full width at half maximum and band strength depend strongly on ice structure, temperature, matrix constituents and the HCOOH concentration. Comparison of the solid HCOOH 5.9, 7.2, and 8.1 µm features with astronomical data toward the low mass source HH 46 and high mass source W 33A shows that spectra of binary mixtures do not reproduce the observed ice features. However, our tertiary mixtures especially with CH₃OH match the astronomical data very well. Thus interstellar HCOOH is most likely present in tertiary or more complex mixtures with H₂O, CH₃OH and potentially also CO or CO₂, providing constraints on its formation.