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Title: Laboratory determination of the infrared band strengths of pyrene frozen in water ice: Implications for the composition of interstellar ices

Broad infrared emission features (e.g., at 3.3, 6.2, 7.7, 8.6, and 11.3 μm) from the gas phase interstellar medium have long been attributed to polycyclic aromatic hydrocarbons (PAHs). A significant portion (10%-20%) of the Milky Way's carbon reservoir is locked in PAH molecules, which makes their characterization integral to our understanding of astrochemistry. In molecular clouds and the dense envelopes and disks of young stellar objects (YSOs), PAHs are expected to be frozen in the icy mantles of dust grains where they should reveal themselves through infrared absorption. To facilitate the search for frozen interstellar PAHs, laboratory experiments were conducted to determine the positions and strengths of the bands of pyrene mixed with H{sub 2}O and D{sub 2}O ices. The D{sub 2}O mixtures are used to measure pyrene bands that are masked by the strong bands of H{sub 2}O, leading to the first laboratory determination of the band strength for the CH stretching mode of pyrene in water ice near 3.25 μm. Our infrared band strengths were normalized to experimentally determined ultraviolet band strengths, and we find that they are generally ∼50% larger than those reported by Bouwman et al. based on theoretical strengths. These improved band strengths were usedmore » to reexamine YSO spectra published by Boogert et al. to estimate the contribution of frozen PAHs to absorption in the 5-8 μm spectral region, taking into account the strength of the 3.25 μm CH stretching mode. It is found that frozen neutral PAHs contain 5%-9% of the cosmic carbon budget and account for 2%-9% of the unidentified absorption in the 5-8 μm region.« less
Authors:
 [1] ; ;  [2] ;  [3] ;  [4] ;  [5] ;  [6]
  1. New York Center for Astrobiology and Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180 (United States)
  2. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 (United States)
  3. Infrared Processing and Analysis Center, Mail Code 100-22, California Institute of Technology, Pasadena, CA 91125 (United States)
  4. Department of Chemistry, University of California Irvine, Irvine, CA 92697-2025 (United States)
  5. Space Science Division, Mail Stop 245-6, NASA Ames Research Center, Moffett Field, CA 94035 (United States)
  6. NASA Goddard Space Flight Center, Exoplanets and Stellar Astrophysics Laboratory, Code 667, Greenbelt, MD 20771 (United States)
Publication Date:
OSTI Identifier:
22357239
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 784; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; ABUNDANCE; CARBON; DUSTS; EMISSION; HEAVY WATER; ICE; MILKY WAY; MOLECULES; POLYCYCLIC AROMATIC HYDROCARBONS; PROTOSTARS; PYRENE; SPECTRA; STARS; ULTRAVIOLET RADIATION