Dust Absorption towards Supernova Remnant W44

Supernova remnants (SNRs) can strongly affect the chemical composition of the interstellar dust. In this paper we investigate to what degree the dust and ices are modified by observing four stars expected to be absorbed by a giant molecular cloud interacting with SNR W44, using medium-resolution spectroscopy in 2-5 $\mu$m. Absorption from H2O ice around 3.0 $\mu$m and aliphatic hydrocarbon dust around 3.4 $\mu$m were detected towards two stars, while probable CO ice at 4.67 $\mu$m towards one of them. Millimeter gas-phase CO J = 1-0 lines and three-dimensional dust extinction maps show that the dense molecular gas associated with W44 dominates (> 60%) the total interstellar extinction (A_K ~ 2.6) along these two sightlines. The H2O ice column densities are a factor of 1.5-3 lower than nearby MCs at similar extinctions, possibly because of the destruction of ice by shocks and cosmic rays (CRs) from W44, consistent with the low CO ice abundance relative to H2O (< 12%). One of the sightlines shows an unusually strong 3.4 $\mu$m aliphatic hydrocarbon absorption. If the carriers are located in diffuse dust along the sightline, unrelated to W44, its strength is ~ 4 times larger than those typically observed for diffuse dust clouds. Alternatively, the carriers may be enhanced in the W44 environment. We discuss several possible explanations, including shock formation of aliphatic hydrocarbons in diffuse clouds associated with W44, contribution from aliphatic hydrocarbons in shocked and CR-bombarded molecular clouds, and changes in the extinction law due to the SNR interaction.