摘要：Context.Messier 8 (M8), one of the brightest H IIregions 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 H IIregion (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 Radioastronomía Millimétrica 30 m telescope to perform an imaging spectroscopy survey of the ~1 pc scale molecular environment of M8E-IR and also performed deep integrations toward the source itself. We imaged and analyzed data for theJ= 1 → 0 rotational transitions of12CO,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 of12CO,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 class II 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 that12CO 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 H IIregion giving rise to an IF that is moving at a speed ≥0.26 km s−1across M8 E. Based on our qualitative and quantitative analysis, theJ= 1 → 0 transition lines of N2H+and HN13C appear to be more direct tracers of dense molecular gas than theJ= 1 → 0 transition lines of HCN and HCO+. We derive temperatures of 80 and 30 K for the warm and cool gas components, respectively, and constrain the H2volume 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.
关键词：Key wordsenISM: abundancesISM: moleculessubmillimeter: ISMstars: formationISM: individual objects: M 8
