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Title:THE KEY ROLE OF NUCLEAR-SPIN ASTROCHEMISTRY
Author(s):Le Gal, Romane
Contributor(s):Persson, Carina; Muller, Sebastien; Talbi, Dahbia; Guo, Hua; Xie, Changjian; Herbst, Eric
Subject(s):Mini-symposium: ALMA's Molecular View
Abstract:Thanks to the new spectroscopic windows opened by the recent_x000d_ generation of telescopes, a large number of molecular lines have been_x000d_ detected. In particular, nuclear-spin astrochemistry has gained_x000d_ interest owing to numerous ortho-to-para ratio (OPR) measurements for species including H$_3^+$, CH$_2$, C$_3$H$_2$, H$_2$O, NH$_3$, NH$_2$, H$_2$S,_x000d_ H$_2$CS, H$_2$O$^+$ and H$_2$Cl$^+$. Any multi-hydrogenated species_x000d_ can indeed present different spin configurations, if some of their_x000d_ hydrogen nuclei are identical, and the species thus exist in distinguishable_x000d_ forms, such as ortho and para. In thermal equilibrium, OPRs are only_x000d_ functions of the temperature and since spontaneous conversion between_x000d_ ortho and para states is extremely slow in comparison with typical_x000d_ molecular cloud lifetimes, OPRs were commonly believed to reflect a_x000d_ ``formation temperature''.~However, observed OPRs are not always consistent with their thermal equilibrium values, as for the NH$_3$ and NH$_2$ cases.~It is thus crucial to understand how interstellar OPRs are formed to constrain the information such new probes can provide.~This involves a comprehensive analysis of the processes governing the interstellar nuclear-spin chemistry, including the formation and possible conversions of the different spin symmetries both in the gas and solid phases. If well understood, OPRs might afford new powerful astrophysical diagnostics on the chemical_x000d_ and physical conditions of their environments, and in particular could trace their thermal history. In this context, observations of non-thermal values for the OPR of the radical NH$_2$ toward four high-mass star-forming regionsfootnote{Persson et al. 2016, A&A,_x000d_ 586, A128}, and a 3:1 value measured for the H$_2$Cl$^+$ OPR toward_x000d_ diffusefootnote{Neufeld et al. 2016, ApJ, 807, 54} and denser gas,_x000d_ led us to develop detailed studies of the mechanisms involved in_x000d_ obtaining such OPRs with the aid of quasi-classical trajectory_x000d_ calculationsfootnote{Le Gal et al. 2016, A&A, 596, A35 and Le Gal et_x000d_ al., in prep}. We will present these new promising results,_x000d_ improving our understanding of the interstellar medium.
Issue Date:6/21/2017
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:APS
Genre:CONFERENCE PAPER/PRESENTATION
Type:Text
Language:English
URI:http://hdl.handle.net/2142/97091
DOI:10.15278/isms.2017.WF04
Date Available in IDEALS:2017-07-27


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