Spatially resolved images of reactive ions in the Orion Bar

Abstract : We report high angular resolution (4.9" x 3.0") images of reactive ions SH+, HOC+, and SO+ toward the Orion Bar photodissociation region (PDR). We used ALMA-ACA to map several rotational lines at 0.8 mm, complemented with multi-line observations obtained with the IRAM 30m telescope. The SH+ and HOC+ emission is restricted to a narrow layer of 2"- to 10"-width (~800 to 4000 AU depending on the assumed PDR geometry) that follows the vibrationally excited H2^* emission. Both ions efficiently form very close to the H/H2 transition zone, at a depth of A_V < 1 mag into the neutral cloud, where abundant C+, S+, and H2^* coexist. SO+ peaks slightly deeper into the cloud. The observed ions have low rotational temperatures (T_rot~10-30 K << T_k) and narrow line-widths (~2-3 km/s), a factor of ~2 narrower that those of the lighter reactive ion CH+. This is consistent with the higher reactivity and faster radiative pumping rates of CH+ compared to the heavier ions, which are driven relatively faster toward smaller velocity dispersion by elastic collisions and toward lower T_rot by inelastic collisions. We estimate column densities and average physical conditions from a non-LTE excitation model (n(H2)~10^5-10^6 cm^-3, n(e^-)~10 cm^-3, and T_k~200 K). Regardless of the excitation details, SH+ and HOC+ clearly trace the most exposed layers of the UV-irradiated molecular cloud surface, whereas SO+ arises from slightly more shielded layers.
Type de document :
Article dans une revue
Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 601, <10.1051/0004-6361/201730716>
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Contributeur : Marie-Paule Pomies <>
Soumis le : mardi 2 mai 2017 - 08:26:07
Dernière modification le : jeudi 8 juin 2017 - 01:11:20





Javier R. Goicoechea, S. Cuadrado, J. Pety, E. Bron, J. H. Black, et al.. Spatially resolved images of reactive ions in the Orion Bar. Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 601, <10.1051/0004-6361/201730716>. <hal-01516644>



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