On the kinematic evolution of young local associations and the Sco-Cen complex

Context: Over the last decade, several groups of young (mainly low-mass) stars have been discovered in the solar neighbourhood (closer than ~100 pc), thanks to cross-correlation between X-ray, optical spectroscopy and kinematic data. These young local associations -- including an important fraction whose members are Hipparcos stars -- offer insights into the star formation process in low-density environments, shed light on the substellar domain, and could have played an important role in the recent history of the local interstellar medium. Aims: To study the kinematic evolution of young local associations and their relation to other young stellar groups and structures in the local interstellar medium, thus casting new light on recent star formation processes in the solar neighbourhood. Methods: We compiled the data published in the literature for young local associations. Using a realistic Galactic potential we integrated the orbits for these associations and the Sco-Cen complex back in time. Results: Combining these data with the spatial structure of the Local Bubble and the spiral structure of the Galaxy, we propose a recent history of star formation in the solar neighbourhood. We suggest that both the Sco-Cen complex and young local associations originated as a result of the impact of the inner spiral arm shock wave against a giant molecular cloud. The core of the giant molecular cloud formed the Sco-Cen complex, and some small cloudlets in a halo around the giant molecular cloud formed young local associations several million years later. We also propose a supernova in young local associations a few million years ago as the most likely candidate to have reheated the Local Bubble to its present temperature.Comment: 27 pages, 13 figures, 14 tables. Accepted for publication in Astronomy & Astrophysic

On the possible common origin of M16 and M17

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. 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 $l \sim 14^\circ 5$, $b\sim +1^\circ$ 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. 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 $\sim 10$ and $\sim 30$~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. 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.Comment: 7 pages, 3 figures; accepted for publication in Astronomy and Astrophysic

A new Wolf-Rayet star in Cygnus

We report the discovery of a new Wolf-Rayet star in the direction of Cygnus. The star is strongly reddened but quite bright in the infrared, with J = 9.22, H = 8.08 and K = 7.09 (2MASS). On the basis of its H + K spectrum, we have classified WR 142a a WC8 star. We have estimated its properties using as a reference those of other WC8 stars in the solar neighbourhood as well as those of WR 135, whose near-infrared spectrum is remarkably similar. We thus obtain a foreground reddening of A(V) = 8.1 mag, M(J) = -4.3, log(L/Lo) = 5.0 - 5.2, R = 0.8 Ro, T = 125,000 K, M = 7.9 - 9.7 Mo, and a mass loss of (1.4 - 2.3)e-05 Mo/yr. The derived distance modulus, DM = 11.2 +/- 0.7 mag, places it in a region occupied by several OB associations in the Cygnus arm, and particularly in the outskirts of both Cygnus OB2 and Cygnus OB9. The position in the sky alone does not allow us to unambiguously assign the star to either association, but based on the much richer massive star content of Cygnus OB2 membership in this latter association appears to be more likely