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/H ₂ 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 (H ₂ )≈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.