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Shining light on PAHs in interstellar ices

Shining light on PAHs in interstellar ices. Jordy Bouwman Sackler Laboratory for Astrophysics, Leiden Observatory. IR absorption. PDR. Bauschlicher et al., ApJ 697, 311, 2009. Wealth of frozen species. CO. H 2 O stretch. H 2 O Lib. H 2 O bend. E.L. Gibb et al., ApJ 536, 347 (2000).

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Shining light on PAHs in interstellar ices

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  1. Shining light on PAHs in interstellar ices Jordy Bouwman Sackler Laboratory for Astrophysics, Leiden Observatory

  2. IR absorption PDR Bauschlicher et al., ApJ 697, 311, 2009

  3. Wealth of frozen species CO H2O stretch H2O Lib. H2O bend E.L. Gibb et al., ApJ 536, 347 (2000) Bouwman et al., A&A, 476, 995 (2007) IR spectrum towards a high mass protostar and laboratory spectrum

  4. IR Emission bands PDR 3.3, 6.2, 7.7, 8.6, and 11.3 mm

  5. PAHs in ices • PAHs are likely to freeze out on cold grains and are incorporated in ices… • How can we study these mixtures spectroscopically? • How do PAHs trapped in ices behaveupon VUV irradiation?

  6. IR spectroscopy • Anthracene:H2O ice 1:70 mixture Bernstein et al., ApJ, 664, 1264 (2007)

  7. PAHs in ices E.L. Gibb et al., ApJ 536, 347 (2000) Bernstein et al., ApJ, 664, 1264 (2007)

  8. Other options? • PAHs are known to be strong UV/VIS absorbers • Common interstellar ice constituents are transparent for l > 240 nm Near UV/VIS spectroscopy!

  9. Sensitive Solid-State Spectrometer J. Bouwman, D.M. Paardekooper, H.M. Cuppen, H. Linnartz, L.J. Allamandola, ApJ, in press (2009)

  10. Pyrene:H2O ice spectrum J. Bouwman et al., Astronomy and Astrophysics, in prep.

  11. Assignment 400 nm band • Not a direct photon product! • Evidence for HCO. radicals

  12. Time and temperature evolution T=11 K T=100 K Ionization is most efficient at low temperatures Photoproduct band growth faster at high temperatures PyH. nm band more prominent at high temperatures J. Bouwman et al., Astronomy and Astrophysics, in prep.

  13. Chemical evolution • VUV irradiation while measuring spectra every 10 s permits real time tracking T=25 K J. Bouwman et al., Astronomy and Astrophysics, in prep.

  14. Reaction rates Reaction scheme: Py = Pyrene Py.+= Pyrene cation PyH. = unknown Pi = Product i kjj = reaction rate jj

  15. Pyrene H2O ice photochemistry • Evidence for PAH-ion mediated (photo-) chemistry • Two different temperature regimes Reaction scheme: Py = Pyrene Py+ = Pyrene cation PyH. = unknown Pi = Product i Kjj = reaction rate jj High T regime low T regime

  16. Astrochemical picture Bernstein, Sandford, Allamandola , Sci. Am. 7, p26(1999)

  17. Conclusions • The setup shows great potential for both spectroscopy and kinetics • PAH reaction paths are temperature dependent • PAHs in H2O ice are readily ionized • Ions are stabilized in water ice and play a previously neglected role in ice chemistry • VLT UV/VIS observations of embedded objects are awaiting data reduction (help)

  18. Acknowledgements Thanks to the Sackler laboratory group: • Harold Linnartz • Herma Cuppen • Lou Allamandola • Claire Romanzin • Arthur Bakker • Daniël Paardekooper • Nadine Wehres • Sergio Ioppolo • Karoliina Isokoski • Karin Öberg • Edith Fayolle • Ankan Das € € €

  19. Trapping of cations • Irradiate to obtain maximum cation signal T=25 K J. Bouwman et al., Astronomy and Astrophysics, in prep.

  20. Normalized pyrene cation decay • Double exponential decay at 10 K t1 = 2 hrs t2 = 351 hrs J. Bouwman et al., Astrophysical Journal, in press (2009)

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