The possible common origin of M 16 and M 17

Abstract

Context. It has been suggested that the well-studied giant HII regions M16 and M17 may have had a common origin, being an example of large-scale triggered star formation. While some features of the distribution of the interstellar medium in the region support this interpretation, no definitive detection of an earlier population of massive stars responsible for the triggering has been made thus far. Aims. We have carried out observations looking for red supergiants in the area covered by a giant shell seen in HI and CO centered on galactic coordinates l ∼ 14 • 5, b ∼ +1 • that peaks near the same radial velocity as the bulk of the emission from both giant HII regions, which are located along the shell. Red supergiants have ages in the range expected for the parent association whose most massive members could have triggered the formation of the shell and of the giant HII regions along its rim. Methods. We have obtained spectroscopy in the visible of a sample of red stars selected on the basis of their infrared colors, whose magnitudes are consistent with them being red supergiants if they are located at the distance of M16 and M17. Spectroscopy is needed to distinguish red supergiants from AGB stars and RGB stars, which are expected to be abundant along the line of sight.Results. Out of a sample of 37 bright red stars, we identify four red supergiants that confirm the existence of massive stars in the age range between ∼ 10 and ∼ 30 Myr in the area. At least three of them have Gaia DR2 parallaxes consistent with them being at the same distance as M16 and M17. Conclusions. The evidence of past massive star formation within the area of the gaseous shell lends support to the idea that it was formed by the combined action of stellar winds and ionizing radiation of the precursors of the current red supergiants. These could be the remnants of a richer population, whose most massive members have exploded already as core-collapse supernovae. The expansion of the shell against the surrounding medium, perhaps combined with the overrun of preexisting clouds, is thus a plausible trigger of the formation of a second generation of stars currently responsible for the ionization of M16 and M17.F. Comerón and J. Torra: On the possible common origin of M16 and M17 selection criteria and our findings, through which we produce a crude estimate of the overall massive star content of the association and the mechanical energy deposited in the bubble, which suggest that the scenario of triggered formation of M16 and M17 is indeed plausible.

The historical record of massive star formation in Cygnus

“…the region, which we can reconstruct thanks to their relationship with their ages. We use the cumulative mass distribution of the 24 red supergiants that we identify in the Cygnus region, following a treatment similar to that presented by Comerón et al (2016) and Comerón & Torra (2018). The number N(M) of red supergiants observed at the present time t = 0 with a mass above a certain value M can be written as…”

Globules and pillars in Cygnus X

“…Globule g4 coincides with the H II region DR18, which hosts a strong IR-source (IRAS 20343+4129, MSX6G080.3624+00.4213) at near-, mid-, and far-IR wavelengths. Comerón & Torra (1999) observed DR18 at IR and optical wavelengths and found an arc-shaped nebula, externally illuminated by a nearby B0.5V star. The globular shape is most likely caused by the object's proximity to the Cyg OB2 cluster (see Fig.…”

Gravitational scattering of stars and clusters and the heating of the Galactic disk

“…(2) Friel (1995) suggested that impact of high velocity clouds on to the Galactic disk could lead to star formation in the Disk gas, a mechanism studied by Comeron & Torra (1992), who related an oblique collision of this kind to the formation of Gould's belt. The cluster formed in the process could retain some kinetic memory of the event, leading to a high z and possibly eccentric Galactic orbits.…”