Abstract
Context. Red supergiants are the evolved descendants of massive stars with initial masses between 7 and 40 M-circle dot. Their brightness makes them easily detectable in the near infrared, making them useful probes of star formation that occurred several tens of Myr ago. Aims. We investigate the past star formation history of Cygnus OB2, the nearest very massive OB association, using red supergiants as a probe. Our aim is to confirm the evidence, found by previous studies, that star formation in the Cygnus OB2 region started long before the latest burst that gave rise to the dense aggregate of early O-type stars that dominate the appearance of the association at present. Methods. Near-infrared star counts in the Cygnus region reveal moderate evidence for a peak in the areal density of bright, reddened stars approximately coincident with Cygnus OB2. A total of 11 sources are found within a circle of 1 degrees radius centered on the association, of which 4 are non-supergiants based on existing observations. Near-infrared spectroscopy is presented of the remaining seven candidates, including four that have been already classified as M supergiants in the literature. Results. We confirm the presence of seven red supergiants in the region and argue that they are probably physically associated with Cygnus OB2. Their location is roughly coincident with that of the older population identified by previous studies, supporting the scenario in which the main star formation activity in the association has been shifting toward higher Galactic longitudes with time. Their luminosities are compared with the predictions of evolutionary tracks with and without rotation to estimate the mass of their progenitors and ages. In this way, we confirm that massive star formation was already taking place in the area of Cygnus OB2 over 20 Myr ago, and we estimate that the star formation rate in the latest 6 Myr represents a six-fold increase over the massive star formation rate at the time when the progenitors of the current red supergiants were formed. Conclusions. The Cygnus OB2 association has a history of star formation extending into the past for at least about 20 Myr, probably dovetailing with the general history of star formation in the region that gave rise to other associations like the neighboring Cygnus OB9. The sustained massive star formation history also argues for a long lifetime of the giant molecular complex from which Cygnus OB2 formed, whose remnants constitute the present-day Cygnus X complex.
Highlights
The Cygnus OB2 association is one of the prime targets for studies of the upper end of the stellar mass function and of the interaction of massive stars with their surrounding molecular environment (e.g. Knödlseder 2003; Schneider et al 2006; Reipurth & Schneider 2008)
We confirm that massive star formation was already taking place in the area of Cygnus OB2 over 20 Myr ago, and we estimate that the star formation rate in the latest 6 Myr represents a six-fold increase over the massive star formation rate at the time when the progenitors of the current red supergiants were formed
Individual distance estimates for these stars are not available, the association of red supergiants with Cygnus OB2 is indirectly supported by the fact that the areal density of objects selected by the criteria described above peaks at the position of Cygnus OB2 when a broad range of Galactic longitudes is considered
Summary
The Cygnus OB2 association is one of the prime targets for studies of the upper end of the stellar mass function and of the interaction of massive stars with their surrounding molecular environment (e.g. Knödlseder 2003; Schneider et al 2006; Reipurth & Schneider 2008). Comerón & Pasquali 2012; Wright et al 2015, and references therein) show that the star formation history of Cygnus OB2 extends well into the past and that the currently densest and youngest part of the association, where the vast majority of its O stars are found, is contiguous to, and partly overlapping with, an area where B giants and supergiants abound, indicating that many stars in the association have already evolved away from the main sequence and that the main star-forming sites have been shifting with time The study of this older component is made difficult by the effects of stellar evolution, and in particular by the fact that its most massive components have already disappeared as supernovae. By using the most recent synthetic evolutionary tracks we discuss the initial properties and ages of these stars, and are able to extract some conclusions on the distant star formation past of the precursor of the present-day Cygnus OB2 association
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