Fine-structure line deficit in S 140^⋆

¹ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
e-mail: ossk@ph1.uni-koeln.de
² SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
³ Kapteyn Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, 400005 Mumbai, India
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 31 March 2015
Accepted: 2 June 2015

Abstract

Aims. We try to understand the gas heating and cooling in the S 140 star-forming region by spatially and spectrally resolving the distribution of the main cooling lines with GREAT/SOFIA and combining our data with existing ground-based and Herschel observations that trace the energy input and the density and temperature structure of the source.

Methods. We mapped the fine-structure lines of [O i] (63 μm) and [C ii] (158 μm) and the rotational transitions of CO 13−12 and 16−15 with GREAT/SOFIA and analyzed the spatial and velocity structure to assign the emission to individual heating sources. We measure the optical depth of the [C ii] line and perform radiative transfer computations for all observed transitions. By comparing the line intensities with the far-infrared continuum we can assess the total cooling budget and measure the gas heating efficiency.

Results. The main emission of fine-structure lines in S 140 stems from a 8.3′′ region close to the infrared source IRS 2 that is not prominent at any other wavelength. It can be explained by a photon-dominated region (PDR) structure around the embedded cluster if we assume that the [O i] line intensity is reduced by a factor of seven owing to self-absorption. The external cloud interface forms a second PDR at an inclination of 80−85 degrees illuminated by a UV field of 60 times the standard interstellar radiation field. The main radiation source in the cloud, IRS 1, is not prominent at all in the fine-structure lines. We measure line-to-continuum cooling ratios below 10^-4, i.e. values lower than in any other Galactic source, actually matching the far-IR line deficit seen in ULIRGs. In particular, the low intensity of the [C ii] line can only be modeled by an extreme excitation gradient in the gas around IRS 1. We found no explanation for why IRS 1 shows no associated fine-structure line peak, while IRS 2 does.

Conclusions. The inner part of S 140 mimics the far-IR line deficit in ULIRGs thereby providing a template that may lead to a future model.