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.

Not all stars form in clusters – Gaia-DR2 uncovers the origin of OB associations

ABSTRACT Historically, it has often been asserted that most stars form in compact clusters. In this scenario, present-day gravitationally unbound OB associations are the result of the expansion of initially gravitationally bound star clusters. However, this paradigm is inconsistent with recent results, both theoretical and observational, that instead favour a hierarchical picture of star formation in which stars are formed across a continuous distribution of gas densities and most OB associations never were bound clusters. Instead they are formed in situ as the low-density side of this distribution, rather than as the remnants of expanding clusters. We utilize the second Gaia data release to quantify the degree to which OB associations are undergoing expansion and, therefore, whether OB associations are the product of expanding clusters, or whether they were born in situ, as the large-scale globally unbound associations that we see today. We find that the observed kinematic properties of associations are consistent with highly substructured velocity fields and additionally require some degree of localized expansion from subclusters within the association. While most present-day OB associations do exhibit low levels of expansion, there is no significant correlation between radial velocity and radius. Therefore, the large-scale structure of associations is not set by the expansion of clusters, rather it is a relic of the molecular gas cloud from which the association was formed. This finding is inconsistent with a monolithic model of association formation and instead favours a hierarchical model, in which OB associations form in situ, following the fractal structure of the gas from which they form.

The dynamics of the γ Vel cluster and nearby Vela OB2 association

ABSTRACT The kinematics of low-mass stars in nearby OB associations can provide clues about their origins and evolution. Combining the precise positions, proper motions, and parallaxes given in the second Gaia Data Release with radial-velocity measurements obtained with the Hermes spectrograph at the Anglo-Australian Telescope, we have an opportunity to study in detail the kinematics of low-mass stars belonging to the nearby γ Vel cluster and the Vela OB2 association it is projected against. The presence of lithium is used to confirm the youth of our targets. We separate our sample into the cluster and association populations based on the Gaia-ESO Survey membership probabilities their parallaxes, and kinematics. We find strong evidence for expansion in the OB association population with at least 4σ significance along all three axes, though the expansion is notably anisotropic. We discuss these results in the context of cluster and association dispersal theories.

Internal motions in OB associations with Gaia DR2

ABSTRACT We study the motions inside 28 OB associations with the use of Gaia DR2 proper motions. The average velocity dispersion calculated for 28 OB associations including more than 20 stars with Gaia DR2 proper motion is σv = 4.5 km s−1. The median virial and stellar masses of OB associations are Mvir = 8.9 × 105 and Mst = 8.1 × 103 M⊙, respectively. The median star-formation efficiency in parent giant molecular clouds appears to be ϵ = 1.2 per cent. Gaia DR2 proper motions confirm the expansion in the Per OB1, Car OB1, and Sgr OB1 associations found earlier with Gaia DR1 data. We also detect the expansion in Gem OB1, Ori OB1, and Sco OB1 associations, which became possible for the first time now when analysed with Gaia DR2 proper motions. The analysis of the distribution of OB stars in the Per OB1 association shows the presence of a shell-like structure with the radius of 40 pc. Probably, the expansion of the Per OB1 association started with the velocity greater than the present-day expansion velocity equal to 5.0 ± 1.7 km s−1.

A Gaia view of the two OB associations Cygnus OB2 and Carina OB1: the signature of their formation process

ABSTRACT OB associations are the prime star-forming sites in galaxies. However, the detailed formation process of such stellar systems still remains a mystery. In this context, identifying the presence of substructures may help in tracing the footprints of their formation process. Here, we present a kinematic study of the two massive OB associations Cygnus OB2 and Carina OB1 using the precise astrometry from the Gaia Data Release 2 and radial velocities. From the parallaxes of stars, these OB associations are confirmed to be genuine stellar systems. Both Cygnus OB2 and Carina OB1 are composed of a few dense clusters and a halo which have different kinematic properties: the clusters occupy regions of 5–8 parsecs in diameter and display small dispersions in proper motion, while the haloes spread over tens of parsecs with two to three times larger dispersions in proper motion. This is reminiscent of the so-called line width–size relation of molecular clouds related to turbulence. Considering that the kinematics and structural features were inherited from those of their natal clouds would then imply that the formation of OB associations may result from structure formation driven by supersonic turbulence, rather than from the dynamical evolution of individual embedded clusters.