Unveiling the remarkable photodissociation region of M8

TL;DR

This study provides a detailed multi-wavelength analysis of the M8 nebula, revealing the distribution, physical conditions, and the influence of Her 36 on its surrounding molecular cloud and PDR.

Contribution

It offers the first comprehensive imaging survey of [CII], [CI], and CO isotopologues in M8, combining observations and modeling to elucidate the nebula's structure and star formation activity.

Findings

Bright emission from [CII], [CI], and CO indicates active PDR and HII regions.

Her 36 remains close to its natal dense cloud core, heating it.

Derived densities range from 10^4 to 10^6 cm^-3 with temperatures of 100-150 K.

Abstract

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.