1 Instituto de Ciencias de Materiales de Madrid (CSIC), 28049 Madrid, Spain
2 Institut de Radioastronomie Millimétrique, 38406 Saint-Martin d’Hères, France
3 LERMA, Obs. de Paris, PSL Research University, CNRS, Sorbonne Universiteés, 75005, UPMC Univ. Paris 06, ENS, France
4 Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
5 OASU/LAB-UMR5804, CNRS, Université Bordeaux, 33615 Pessac, France
6 Observatorio Astronómico Nacional (IGN). Apartado 112, 28803 Alcalá de Henares, Spain
Received: 1 March 2017
Accepted: 21 April 2017
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 H2* emission. Both ions efficiently form very close to the H/H2 transition zone, at a depth of AV ≲ 1 mag into the neutral cloud, where abundant C+, S2* coexist. SO+ peaks slightly deeper into the cloud. The observed ions have low rotational temperatures (Trot ≈ 10−30 K ≪ Tk) 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 more quickly toward smaller velocity dispersion by elastic collisions and toward lower Trot 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 Tk ≈ 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.
Key words: astrochemistry / line: identification / ISM: clouds / (ISM:) photon-dominated region (PDR)
This paper makes use of the following ALMA data: ADS/JAO.ALMA#2012.1.00352.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ.
© ESO, 2017