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.

(ISM:) photon-dominated region (PDR)

line: identification

ISM: clouds

astrochemistry

Author

J. R. Goicoechea

CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)

S. Cuadrado

CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)

J. Pety

Sorbonne University

Institut de RadioAstronomie Millimetrique (IRAM)

E. Bron

Sorbonne University

CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)

John H Black

Astronomy and Plasmaphysics

J. Cernicharo

CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)

E. Chapillon

Institut de RadioAstronomie Millimetrique (IRAM)

Observatoire Aquitain des Sciences de l'Univers, OASU

A. Fuente

Spanish National Observatory (OAN)

M. Gerin

Sorbonne University

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 601 Article Number: L9, pp. 1-9 L9

Subject Categories

Astronomy, Astrophysics and Cosmology

Roots

Basic sciences

Infrastructure

Onsala Space Observatory

DOI

10.1051/0004-6361/201730716

More information

Latest update

9/6/2018 1