Spatially resolved images of reactive ions in the Orion Bar^⋆,⋆⋆

¹ Instituto de Ciencias de Materiales de Madrid (CSIC), 28049 Madrid, Spain
e-mail: jr.goicoechea@icmm.csic.es
² Institut de Radioastronomie Millimétrique, 38406 Saint-Martin d’Hères, France
³ LERMA, Obs. de Paris, PSL Research University, CNRS, Sorbonne Universiteés, 75005, UPMC Univ. Paris 06, ENS, France
⁴ Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁵ OASU/LAB-UMR5804, CNRS, Université Bordeaux, 33615 Pessac, France
⁶ Observatorio Astronómico Nacional (IGN). Apartado 112, 28803 Alcalá de Henares, Spain

Received: 1 March 2017
Accepted: 21 April 2017

Abstract

We report high angular resolution (4.9′′ × 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 30 m 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⁺, S₂^* 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^-1), 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 more quickly toward smaller velocity dispersion by elastic collisions and toward lower T_rot by inelastic collisions. We estimate column densities and average physical conditions from an excitation model (n(H₂) ≈ 10⁵−10⁶ 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.