Photochemistry of polycyclic aromatic hydrocarbons in cosmic water ice

I. Mid-IR spectroscopy and photoproducts

¹ Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: bouwman@strw.leidenuniv.nl
² NASA-Ames Research Center, Space Science Division, Mail Stop 245-6, Moffett Field, CA 94035, USA

Received: 27 May 2010
Accepted: 23 September 2010

Abstract

Context. Polycyclic aromatic hydrocarbons (PAHs) are known to be abundantly present in photon-dominated regions (PDRs), as evidenced by their ubiquitous mid-IR emission bands. Towards dense clouds, however, their IR emission bands are strongly suppressed. It is here where molecules are known to reside on very cold grains (T ≤ 30 K) in the form of interstellar ices. Therefore, it is likely that non-volatile species, such as PAHs, also freeze out on grains. Such icy grains act as catalytic sites and, upon vacuum ultraviolet (VUV) irradiation, chemical reactions are initiated. In the study presented here, these reactions and the resulting photoproducts are investigated for PAH containing water ices.

Aims. The aim of this work is to monitor vacuum ultraviolet induced chemical reactions of PAHs in cosmic ice through their IR signatures, to characterize the families of species formed in these reactions, and to apply the results to astronomical observations.

Methods. Mid-infrared Fourier transform absorption spectroscopic measurements ranging from 6500 to 450 cm^-1 are performed on freshly deposited and vacuum ultraviolet processed PAH containing cosmic H₂O ices at low temperatures.

Results. The mid-IR spectroscopy of anthracene, pyrene and benzo[ghi]perylene containing H₂O ice is reported. Band strengths of the neutral PAH modes in H₂O ice are derived. Additionally, spectra of vacuum ultraviolet processed PAH containing H₂O ices are presented. These spectra are compared to spectra measured in VUV processed PAH:argon matrix isolation studies. It is concluded that the parent PAH species is ionized in H₂O ice and that other photoproducts, mainly more complex PAH derivatives, also form. The importance of PAHs and their PAH:H₂O photoproducts in astronomical mid-infrared spectroscopic studies, in particular in the 5−8 μm region, is discussed. As a test-case, the VUV photolyzed PAH:H₂O laboratory spectra are compared to a high resolution ISO-SWS spectrum of the high-mass embedded protostar W33A and to a Spitzer spectrum of the low-mass Young Stellar Object (YSO) RNO 91. For these objects, an upper limit of 2–3% with respect to H₂O ice is derived for the contribution of PAHs and PAH:H₂O photoproducts to the absorbance in the 5−8 μm region towards these objects.