Are PAHs precursors of small hydrocarbons in photo-dissociation regions? The Horsehead case

By Jérome Pety, D. Teyssier, D. Fossé, M. Gerin, È. Roueff, Alain Abergel, É. Habart, J. C. O. D. ParisENS, Iram, Iemcsic and 4 more. 2005

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We present maps at high spatial and spectral resolution in emission lines of CCH, c-C3H2, C4H, 12CO and C18O of the edge of the Horsehead nebula obtained with the IRAM Plateau de Bure Interferometer (PdBI). The edge of the Horsehead nebula is a one-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on. All hydrocarbons are detected at high signal-to-noise ratio in the PDR where intense emission is seen both in the H2 ro-vibrational lines and in the PAH mid-infrared bands. C18O peaks farther away from the cloud edge. Our observations demonstrate that CCH, c-C3H2 and C4H are present in UV-irradiated molecular gas, with abundances nearly as high as in dense, well-shielded molecular cores. PDR models i) need a large density gradient at the PDR edge to correctly reproduce the offset between the hydrocarbons and H2 peaks; and ii) fail to reproduce the hydrocarbon abundances. We propose that a new formation path of carbon chains, in addition to gas phase chemistry, should be considered in PDRs: because of intense UV-irradiation, large aromatic molecules and small carbon grains may fragment and feed the interstellar medium with small carbon clusters and molecules in significant amounts.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/2082342499238733a1145d69d9e5d23622898edd

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Citations in the corpus

A 2 Millimeter Spectral Line Survey of the Starburst Galaxy NGC 253 (2006, background): “High abundances of hydrocarbon radicals, such as C2H and C3H2, are observed toward PDRs (Fossé et al. 2000; Fuente et al. 1993, 2003; Pety et al. 2005).”
The global dust SED: tracing the nature and evolution of dust with DustEM (2010, background): “The photo-fragmentation of the smaller, aromatic-rich grains could then be an important source of molecular aromatic species such as PAH s and molec- ular fragments (e.g., Jones 1990; Duley 2000; Petrie et al. 2003; Pety et al. 2005; Jones 2005, 2009).”
The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point (2013, background): “In PDRs EUV/UV photo-processing (a-C:H→ a-C) progressively destroys the aliphatic component that binds the growing aromatic domains, eventually leading to photo-fragmentation and the release of daughter species (e.g., Pety et al. 2005, and as discussed in Paper III)16.”
The Molecular Universe (2011, background): “The small hydrocarbon radicals “released” in the fragmentation may also set off a rich and interesting chemistry that may well contribute to the rich inventory of hydrocarbon radicals in photodissociation regions (Pety et al. 2005).”
The Calibration of Mid-Infrared Star Formation Rate Indicators (2007, background): “…2002; Haas, Klaas & Bianchi 2002; Boselli, Lequeux & Gavazzi 2004; Peeters, Spoon & Tielens 2004; Wu et al. 2005; Mattioda et al. 2005) and/or destruction/fragmentation of the 8 µm emission carriers (Boulanger et al. 1988, 1990; Helou, Ryter & Soifer 1991; Houck et al. 2004; Pety et al. 2005).”
Star Formation in NGC 5194 (M51a): The Panchromatic View from GALEX to Spitzer (2005, background): “…Hydrocarbons (PAH, Leger & Puget (1984)), large molecules transiently heated by single UV and optical photons (Sellgren, Luan & Werner 1990), and which can be destroyed, fragmented, or ionized by harsh UV photon fields (Boulanger et al. 1988, 1990; Helou, Ryter & Soifer 1991; Pety et al. 2005).”