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 Title: Physics And Chemistry In UV Illuminated Regions: The Horsehead Case Author(s): Guzman, Viviana V.; Pety, Jérôme; Gratier, Pierre; Goicoechea, Javier; Gerin, Maryvonne; Roueff, Evelyne Subject(s): Mini-symposium: Astronomical Molecular Spectroscopy in the Age of ALMA Abstract: Molecular lines are used to trace the structure of the interstellar medium and the physical conditions of the gas in different environments, from protoplanetary disks to high-z galaxies. To fully benefit from the diagnostic power of molecular lines, the formation and destruction paths of the molecules, including the interplay between gas-phase and grain surface chemistry, must be quantitatively understood. Well-defined sets of observations of simple template sources are key to benchmark the theoretical models. In this context the PDR of the Horsehead mane is a particularly interesting case because it has a simple geometry (almost 1D, viewed edge-on) and the density profile across the PDR is well constrained. In this talk, I will summarize our recent results on the ISM physics and chemistry in the Horsehead, from a complete and unbiased line survey at 1, 2 and 3mm performed with the IRAM-30m telescope, where approximately 30 species (plus their isotopologues) are detected with up to 7 atoms. I will show the importance of the interplay between the solid and gas phase chemistry in the formation of (complex) organic molecules, like H$_2$CO, CH$_3$OH, and CH$_3$CN, which reveal that photo-desorption of ices is an efficient mechanism to release molecules into the gas phase. The case of CH$_3$CN is especially surprising, as it is 40 times more abundant in the warm ($T_\mathrm{kin}\sim60$~K) UV-illuminated edge of the nebula, than in the shielded and colder ($T_\mathrm{kin}\sim20$~K) inner layers. I will show that complex molecules, such as HCOOH, CH$_2$CO, CH$_3$CHO, and CH$_3$CCH are easily detected in the PDR. I will also discuss new diagnostics of the UV-illuminated gas, like CF$^+$ (for which we recently resolved its hyperfine structure for the first time), which is observable from the ground, and we propose it can be used as a proxy of C$^+$. I will finish by reporting the first detection of a new molecule, recently confirmed to be the small hydrocarbon C$_3$H$^+$, which shows that photo-erosion of PAHs is needed to explain the enhanced abundance of other small hydrocarbons, like C$_3$H and C$_3$H$_2$. Issue Date: 2014-06-19 Publisher: International Symposium on Molecular Spectroscopy Citation Info: Guzman, V.V.; Roueff, E.; Gerin, M.; Goicoechea, J.; Gratier, P.; Pety, J. PHYSICS AND CHEMISTRY IN UV ILLUMINATED REGIONS: THE HORSEHEAD CASE. Proceedings of the International Symposium on Molecular Spectroscopy, Urbana, IL, June 16-21, 2014. DOI: 10.15278/isms.2014.RF05 Genre: CONFERENCE PAPER/PRESENTATION Type: Text Language: English URI: http://hdl.handle.net/2142/51074 DOI: https://doi.org/10.15278/isms.2014.RF05 Rights Information: Copyright 2014 by the authors. Licensed under a Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/ Date Available in IDEALS: 2014-09-172015-04-14
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