Surprisingly weak fine-structure line emission from S 140

¹ I. Physikalisches Institut der Universität zu Köln, Germany
² SRON Netherlands Institute for Space Research, Groningen, The Netherlands
³ Kapteyn Astronomical Institute, University of Groningen, The Netherlands
⁴ Tata Institute of Fundamental Research, Mumbai, India

Abstract

GREAT observations of the [Oi] and [Cii] fine-structure lines in S 140 show very low intensities towards the embedded sources. We measure line-to-continuum cooling ratios of 10⁻⁵ − 10⁻⁴, i.e. values matching the far-IR line deficit seen in ULIRGs. Normal PDR models fail to explain the combination of source size, line ratios, and absolute line intensities. A match is, however, possible when modelling the sources in terms of inside-out PDRs. The small beam-filling of PDRs in a dense homogeneous medium can lead to a line deficit compared to the infrared continuum, emitted from a much larger region.