Unveiling the Remarkable Photodissociation Region of M8

Messier 8 (M8), the Lagoon Nebula, has one of the brightest HII regions and photodissociation regions in the sky, located in the Sagittarius-Carina arm, near our line of sight towards the Galactic Center. We present the bright fine structure lines of [CII], [CI] and multiple rotational transitions of CO observed towards the HII region and the PDR in M8 using SOFIA, IRAM 30 m and APEX telescopes. We explored the morphology of the region by analyzing the spectra of the observed species and by comparing archival infrared, optical and radio images of the nebula with our newly obtained fine structure line and CO data, in particular with the velocity information they provide. Our analysis places the bulk of the molecular material in the background of the nebulosity illuminated by the bright stars 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. Using LTE approach and then non-LTE radiative transfer modeling of different CO transitions, we obtain H_2 densities ranging from ∼ 10^4 to 10^6 cm^-3 and kinetic temperatures of 100 - 150 K in the bright PDR caused by Her 36.