Other
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.177g&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Other
Scientific paper
Predominantly heated by the UV photons, and weakly shielded from the ambient interstellar radiation field (ISRF), the diffuse interstellar medium (ISM) has long been thought to physically and chemically behave like a photo-dissociation region (PDR). Yet, the observations reveal an unexpected chemical richness, and the derived abundances of many species considerably exceed those predicted by PDR-type steady state chemical models. Since the production pathways of these species are protected by highly endo-energetic reactions, it has been proposed that these chemical discrepancies are nothing else but a signature of the effect of another energy source, like magnetized turbulence. Among all species, CH^+ and SH^+ (the first and the most recent species detected in the ISM) are particularly interesting, since the energies involved in the hydrogenations of C^+ and S^+ are large and differ by more than a factor of 2 (4640 K and 9860 K respectively). In the framework of the Herschel-HIFI key program PRISMAS, we carried out a simultaneous analysis of CH^+, 13CH^+, and SH^+ observed in absorption toward remote star forming regions in the Galactic disk. We find that CH^+ and SH^+ have specific dynamical and chemical signatures that strongly constrain their formation processes : (1) the linewidth distributions, when compared to those of HCN, HNC, and CN, show that CH^+ and SH^+ are formed in regions with the same dynamical properties ; and (2) the predictions of PDR-type models fail by several orders of magnitude to reproduce both the observed abundances and their ratio. We therefore investigate those results in the framework of the TDR (Turbulent Dissipation Regions) model where the carbon and sulfur hydrogenations are activated and driven by the dissipation of turbulent energy in the diffuse ISM through the ion-neutral friction and the gas viscosity. In TDRs, both the dynamical and chemical signatures of CH^+ and SH^+ can be reproduced and could be explained as two independent measurements of the ion-neutral velocity drift in the dissipative regions of turbulence.
de Luca Massimo
Falgarone Edith
Gerin Maryvonne
Godard Benjamin
Pineau des Forêts Guillaume
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