Unveiling the remarkable photodissociation region of M8
  Messier 8 is one of the brightest HII regions in the sky. Using SOFIA, APEX
12 m and IRAM 30 m telescopes, we have performed a comprehensive imaging survey
of the emission from the [CII], [CI] and CO isotopologues within 1.3 $\times$
1.3 pc around the dominant Herschel 36 (Her 36) stellar system. To further
explore the morphology of the region we compare archival infrared, optical and
radio images of the nebula with our newly obtained [CII] and CO data. We
perform a quantitative analysis, using both LTE and non-LTE methods to
determine the abundances of some of the observed species as well as kinetic
temperatures and volume densities. Bright CO, [CII] and [CI] emission has been
found towards the HII region and the photodissociation region (PDR) in M8. Our
analysis places the bulk of the molecular material in the background of the
nebulosity illuminated by the bright stellar systems Her 36 and 9 Sagitarii.
Since the emission from all observed atomic and molecular tracers peaks at or
close to the position of Her 36, we conclude that the star is still physically
close to its natal dense cloud core and heats it. A veil of warm gas moves away
from Her 36 toward the Sun and its associated dust contributes to the
foreground extinction in the region. One of the most prominent star forming
regions in M8, the Hourglass Nebula, is particularly bright due to cracks in
this veil close to Her 36. By using radiative transfer modeling of different
transitions of CO isotopologues, we obtain H$_{2}$ densities ranging from $\sim
10^{4} - 10^{6}$ cm$^{-3}$ and kinetic temperatures of 100 $-$ 150 K in the
bright PDR caused by Her 36.
