The [O I] fine structure line profiles in Mon R2 and M17 SW: The puzzling nature of cold foreground material identified by [¹²C II] self-absorption

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
² Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, 1270398 Antofagasta, Chile
³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USA
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁵ Institute of Optical Sensor Systems, German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Centro de Astro-Ingeniería UC, Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Avda Vicuña Mackenna 4860, Macul, Santiago, Chile

^★ Corresponding author; guevara@ph1.uni-koeln.de

Received: 26 April 2024
Accepted: 16 September 2024

Abstract

Context. Recent studies of the optical depth comparing [¹²C II] and [¹³C II] line profiles in Galactic star-forming regions have revealed strong self-absorption in [¹²C II] by low excitation foreground material. This implies a high column density for C⁺, corresponding to equivalent A_V values of a few (up to about 10) mag.

Aims. As the nature and origin of such a great column of cold C⁺ foreground gas are difficult to determine, it is essential to constrain the physical conditions of this material.

Methods. We conducted high-resolution observations of [O I] 63 μm and [O I] 145 μm lines in M17 SW and Mon R2. The [O I] 145 μm transition traces warm PDR-material, while the [O I] 63 μm line traces the foreground material, as manifested by the absorption dips.

Results. A comparison of both [O I] line profiles with [C II] isotopic lines confirm warm PDR-origin background emission and a significant column of cold foreground material, causing the self-absorption to be visible in the [¹²C II] and [O I] 63 μm profiles. In M17 SW, the C⁺ and O⁰ column densities are comparable for both layers. Mon R2 exhibits larger O⁰ columns compared to C⁺, indicating additional material where the carbon is neutral or in molecular form. Small-scale spatial variations in the foreground absorption profiles and the large column density (~10¹⁸ cm⁻²) of the foreground material suggest the emission is coming from high-density regions associated with the cloud complex – and not a uniform diffuse foreground cloud.

Conclusions. The analysis confirms that the previously detected intense [C II] foreground absorption is attributable to a large column of low-excitation dense atomic material, where carbon is ionized and oxygen is in a neutral atomic form.