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

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 [Formula: see text] 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 [Formula: see text] 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 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 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.