The fine structure line deficit in S 140
  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. We mapped the fine structure lines of [OI] (63
{\mu}m) and [CII] (158 {\mu}m) 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.
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 seven due to self-absorption. The external cloud interface
forms a second PDR at an inclination of 80-85 degrees illuminated by an 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, rather 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 why IRS 1 shows no associated fine-structure line peak,
while IRS 2 does. 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.
