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 [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.