Spatially resolved images of reactive ions in the Orion Bar

Journal article, 2017

We report high angular resolution (4.9" X 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 30m telescope. The SH+ and HOC+ emission is restricted to a narrow layer of 2" - to 10" -width (approximate to 800 to 4000AU depending on the assumed PDR geometry) that follows the vibrationally excited H-2(*) emission. Both ions efficiently form very close to the H/H-2 transition zone, at a depth of AV less than or similar to 1 mag into the neutral cloud, where abundant C+, S+, and H-2(*) coexist. SO+ peaks slightly deeper into the cloud. The observed ions have low rotational temperatures (T-rot approximate to 10-30K << T-k) and narrow line-widths (similar to 2-3 km s(-1)), a factor of similar or equal to 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(H-2) approximate to 10(5)-10(6) cm(-3), n(e(-)) approximate to 10 cm(-3), and T-k approximate to 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.