Astronomy and Astrophysics – Astronomy
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.300p&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Astronomy and Astrophysics
Astronomy
Scientific paper
Observational studies of photo-dissociation regions (PDRs) have shown that gas-phase small hydrocarbons are more abundant than predicted by models, especially in the layers of PDRs where polycyclic aromatic hydrocarbon (PAH) molecules emit in the aromatic infrared bands (AIBs, Pety et al., A&A 2005). This strongly indicates that additional chemical pathways play a role in the formation of hydrocarbons. Several studies have shown that PAHs are formed by photo-fragmentation of very small dust particles called evaporating very small grains (eVSGs, Rapacioli et al., A&A 2005; Berné et al., A&A 2007; Pilleri et al., A&A 2011). In this work, we combine IR and mm observations to investigate if the production of small hydrocarbons could be related to this process. We obtained high-spatial-resolution spectro-imagery data at different wavelengths to trace the emission of PAHs, eVSGs and hydrocarbons in the north-west PDR of the reflection nebula NGC 7023. This PDR is a good template for such study since the emission of all these species can be spatially resolved at both IR and mm wavelengths. We present new mm interferometric and single-dish observations obtained at the IRAM 30m telescope and at the Plateau de Bure interferometer of C_2H and C_3H_2. We compare these results with near- and mid-IR data obtained with Spitzer and AKARI to search for a link between the emission of small hydrocarbons and the chemical transition between eVSGs and PAHs. The comparison of the AIB emission with the interferometric maps shows that the emission of small hydrocarbons peaks at the transition between eVSGs and PAHs, where eVSGs are destroyed to produce free PAHs. In the same regions, we report a strong intensity decrease of the aliphatic 3.4 μm emission band relative to the 3.3 μm aromatic feature. This suggests that aliphatic CH_3 sidegroups of PAHs/eVSGs are released in the gas-phase, possibly influencing the chemistry of small hydrocarbons. The observations presented here are consistent with eVSGs being a mixture of PAH clusters of various sizes, aliphatic groups and/or super-hydrogenated PAHs. We conclude that the photo-dissociation of PAHs and eVSGs play an important role in the chemistry of PDRs.
Boulanger Francois
Fuente Asuncion
Gerin Maryvonne
Joblin Ch.
Montillaud J.
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