The historical record of massive star formation in Cygnus

Context. The Cygnus region, which dominates the local spiral arm of the Galaxy, is one of the nearest complexes of massive star formation, extending over several hundred parsecs. Its massive stellar content, regions of ongoing star formation, and molecular gas have been studied in detail at virtually all wavelengths. However, little is known of the history of the region beyond the past 10 Myr. Aims. We use the correlations between age, mass and luminosity of red supergiants to explore the history of star formation in Cygnus previous to the formation of the present-day associations. The brightness and spectroscopic characteristics of red supergiants make it easy to identify them and build up a virtually complete sample of such stars at the distance of the Cygnus region, thus providing a record of massive star formation extending several tens of Myr into the past, a period inaccessible through the O and early B stars observable at present. Methods. We have made a selection based on the 2MASS colors of a sample of bright, red stars in an area of 84 square degrees covering the whole present extension of the Cygnus association in the Local Arm. We have obtained spectroscopy in the red visible range allowing an accurate, homogeneous spectral classification as well as a reliable separation between supergiants and other cool stars. Our data are complemented with Gaia Data Release 2 astrometric data. Results. We have identified 29 red supergiants in the area, 17 of which had not been previously classified as supergiants. Twenty-four of the 29 most likely belong to the Cygnus region and four of the remaining to the Perseus arm. We have used their derived luminosities and masses to infer the star formation history of the region. Intense massive star formation activity is found to have started approximately 15 Myr ago, and we find evidence for two other episodes, one taking place between 20 and 30 Myr ago and another one having ended approximately 40 Myr ago. There are small but significant differences between the kinematic properties of red supergiants younger or older then 20 Myr, hinting that stars of the older group were formed outside the precursor of the present Cygnus complex, possibly in the Sagittarius-Carina arm.

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

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.