2020A&A...644A..25T

Context. Messier 8 (M8), one of the brightest HII regions in our Galaxy, is powered by massive O-type stars and is associated with recent and ongoing massive star formation. Two prominent massive star-forming regions associated with M8 are M8-Main, the particularly bright part of the large-scale HII region (mainly) ionized by the stellar system Herschel 36 (Her 36) and M8 East (M8 E), which is mainly powered by a deeply embedded young stellar object (YSO), the bright infrared (IR) source M8E-IR.
Aims. We study the interaction of the massive star-forming region M8 E with its surroundings using observations of assorted diffuse and dense gas tracers that allow quantifying the kinetic temperatures and volume densities in this region. With a multiwavelength view of M8 E, we investigate the cause of star formation. Moreover, we compare the star-forming environments of M8-Main and M8 E, based on their physical conditions and the abundances of the various observed species toward them.
Methods. We used the Institut de Radioastronomia Millimetrica 30m telescope to perform an imaging spectroscopy survey of the ∼1pc scale molecular environment of M8E-IR and also performed deep integrations toward the source itself. We imaged and analyzed data for the J=1-0 rotational transitions of ¹²CO, ¹³CO, N₂H⁺, HCN, H¹³CN, HCO⁺, H¹³CO⁺, HNC, and HN¹³C observed for the first time toward M8 E. To visualize the distribution of the dense and diffuse gas in M8 E, we compared our velocity-integrated intensity maps of ¹²CO, ¹³CO, and N₂H⁺ with ancillary data taken at IR and submillimeter wavelengths. We used techniques that assume local thermodynamic equilibrium (LTE) and non-LTE to determine column densities of the observed species and constrain the physical conditions of the gas that causes their emission. Examining the class 0/ I and classII YSO populations in M8 E, allows us to explore the observed ionization front (IF) as seen in the high resolution Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) 8µm emission image. The difference between the ages of the YSOs and their distribution in M8 E were used to estimate the speed of the IF.
Results. We find that ¹²CO probes the warm diffuse gas also traced by the GLIMPSE 8µm emission, while N₂H⁺ traces the cool and dense gas following the emission distribution of the APEX Telescope Large Area Survey of the Galaxy 870µm dust continuum. We find that the star-formation in M8 E appears to be triggered by the earlier formed stellar cluster NGC 6530, which powers an HII region giving rise to an IF that is moving at a speed ≥0.26km/s across M8 E. Based on our qualitative and quantitative analysis, the J=1-0 transition lines of N₂H⁺ and HN¹³C appear to be more direct tracers of dense molecular gas than the J=1-0 transition lines of HCN and HCO⁺. We derive temperatures of 80 and 30K for the warm and cool gas components, respectively, and constrain the H₂ volume densities to be in the range of 10⁴-10⁶cm^–3. Comparison of the observed abundances of various species reflects the fact that M8 E is at an earlier stage of massive star formation than M8-Main.