Laboratory Determination of the Infrared Band Strengths of Pyrene Frozen in Water Ice: Implications for the Composition of Interstellar Ices

摘要

Broad infrared emission features (e.g., at 3.3, 6.2, 7.7, 8.6, and 11.3microns) from the gas phase interstellar medium have long been attributed topolycyclic aromatic hydrocarbons (PAHs). A significant portion (10%-20%) of theMilky Way's carbon reservoir is locked in PAH molecules, which makes theircharacterization integral to our understanding of astrochemistry. In molecularclouds and the dense envelopes and disks of young stellar objects (YSOs), PAHsare expected to be frozen in the icy mantles of dust grains where they shouldreveal themselves through infrared absorption. To facilitate the search forfrozen interstellar PAHs, laboratory experiments were conducted to determinethe positions and strengths of the bands of pyrene mixed with H2O and D2O ices.The D2O mixtures are used to measure pyrene bands that are masked by the strongbands of H2O, leading to the first laboratory determination of the bandstrength for the CH stretching mode of pyrene in water ice near 3.25 microns.Our infrared band strengths were normalized to experimentally determinedultraviolet band strengths, and we find that they are generally ~50% largerthan those reported by Bouwman et al. based on theoretical strengths. Theseimproved band strengths were used to reexamine YSO spectra published by Boogertet al. to estimate the contribution of frozen PAHs to absorption in the 5-8micron spectral region, taking into account the strength of the 3.25 micron CHstretching mode. It is found that frozen neutral PAHs contain 5%-9% of thecosmic carbon budget, and account for 2%-9% of the unidentified absorption inthe 5-8 micron region.