Dust Absorption toward Supernova Remnant W44

Abstract 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 μ m. Absorption from H 2 O ice around 3.0 μ m and aliphatic hydrocarbon dust around 3.4 μ m were detected toward two stars, while probable CO ice at 4.67 μ m was detected toward one of them. Millimeter gas-phase CO J = 1–0 lines and three-dimensional (3D) 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 sight lines. The H 2 O ice column densities are a factor of 1.5–3 lower than nearby molecular clouds 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 H 2 O (≲12%). One of the sight lines shows an unusually strong 3.4 μ m aliphatic hydrocarbon absorption. If the carriers are located in diffuse dust along the sight line, unrelated to W44, their strength is ∼4 times larger than that 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.