Cause and effects of the massive star formation in Messier 8 East

¹ Max-Planck Institute for Radio Astronomy, Auf dem Hügel, 53121 Bonn, Germany
e-mail: mtiwari@mpifr-bonn.mpg.de
² Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
³ INAF-Istituto di Radioastronomia, and Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy
⁴ Korea Astronomy and Space Science Institute Daedeokdae-ro 776, Yuseong-gu Daejeon 34055, Republic of Korea
⁵ Instituto de Radioastronomía Milimétrica, Avenida Divina Pastora 7, 18012 Granada, Spain
⁶ European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla 7630355, Santiago, Chile

Received: 9 July 2020
Accepted: 16 October 2020

Abstract

Context. Messier 8 (M8), one of the brightest H II 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 H II 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 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 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 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) 8 μm 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 8 μm emission, while N₂H⁺ traces the cool and dense gas following the emission distribution of the APEX Telescope Large Area Survey of the Galaxy 870 μm 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 II region giving rise to an IF that is moving at a speed ≥0.26 km s⁻¹ 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 30 K for the warm and cool gas components, respectively, and constrain the H₂ volume densities to be in the range of 10⁴–10⁶ cm⁻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.