Fine-structure line deficit in S 140

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 [OI] (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 [CII] 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 [OI] 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 [CII] 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.