2017A&A...601L...9G

We report high angular resolution (4.9''x3.0'') 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 0.8mm, 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 4000AU 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≥1mag 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~=10-30K≪T_k) and narrow line-widths (∼2-3km/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 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)~=10⁵-10⁶cm^–3, n(e^–)~=10cm^–3, and T_k~=200K). 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.