Unlocking Galactic Wolf–Rayet stars with Gaia DR2 – II. Cluster and association membership

Abstract

Galactic Wolf–Rayet (WR) star membership of star-forming regions can be used to constrain the formation environments of massive stars. Here, we utilize Gaia DR2 parallaxes and proper motions to reconsider WR star membership of clusters and associations in the Galactic disc, supplemented by recent near-infrared studies of young massive clusters. We find that only 18–36 per cent of 553 WR stars external to the Galactic Centre region are located in clusters, OB associations or obscured star-forming regions, such that at least 64 per cent of the known disc WR population are isolated, in contrast with only 13 per cent of O stars from the Galactic O star Catalogue. The fraction located in clusters, OB associations or star-forming regions rises to 25–41 per cent from a global census of 663 WR stars including the Galactic Centre region. We use simulations to explore the formation processes of isolated WR stars. Neither runaways, nor low-mass clusters, are numerous enough to account for the low cluster membership fraction. Rapid cluster dissolution is excluded as mass segregation ensures WR stars remain in dense, well-populated environments. Only low-density environments consistently produce WR stars that appeared to be isolated during the WR phase. We therefore conclude that a significant fraction of WR progenitors originate in low-density association-like surroundings which expand over time. We provide distance estimates to clusters and associations host to WR stars, and estimate cluster ages from isochrone fitting.

Far and extreme ultraviolet radiation fields and consequent disc destruction in star-forming regions

“…However, the G 0 and EUV fields are only a factor of 10 lower than in the (sub)virial regions after 10 Myr, despite the local density in the supervirial starforming regions being a factor of 100 lower than at birth. The reason for this is that supervirial star-forming regions dynamically evolve so that the most massive stars sweep up retinues of low-mass stars (Parker et al 2014b;Rate, Crowther & Parker 2020), meaning that the most massive stars will almost exclusively reside in the denser locations of the star-forming regions, where there are lots of low-mass stars that will experience strong radiation fields. However, there are also many low-mass stars that do not reside near to massive stars and so the fraction of discs that survive in supervirial regions can be 25 per cent higher than in the (sub)virial star-forming region (compare the coloured lines with the grey lines in panel h).…”

OB Associations and their origins

“…Questions have been raised over the membership of the star in Vela OB2, since evolutionary models for the star suggest an age of 5.5 ± 1 Myr (Eldridge 2009), while the lower-mass pre-main sequence stars in its vicinity have ages of ∼10 Myr (Jeffries et al 2009). This has led to suggestions of mass transfer within the binary system that might make it appear younger (Jeffries et al 2014), since Hipparcos astrometry suggests the star is a member of the association (Rate et al 2020). de identified a further 92 members of the association, predominantly B-type stars, which Armstrong et al (2018) reduced to 81 after excluding photometric and astrometric contaminants.…”

Studying the ISM at ∼10 pc scale in NGC 7793 with MUSE

“…We observe that WR stars are populating the H ii regions, with the exception of one object (located in the upper left corner and catalogued as WR 3 in Table 1). Rate et al (2020) recently found that isolated WR stars are not an exception, and amount to at least 64% in the Milky Way (as observed in Gaia Collaboration 2018).…”