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

Lim, Beomdu; Nazé, Yaël; Gosset, Éric; Rauw, Grégor

doi:10.1093/mnras/stz2548

Cited by 35 publications

(41 citation statements)

References 79 publications

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“…The distance to the Carina nebula and its parts has a long literature that extends over at least fifty years (see Table A.1) Before Gaia DR2 the most reliable result was the geometric value of of 2350 ± 50 pc based on the 3D expansion of the Homunculus nebula around η Car. The Gaia DR2 results from different authors (Cantat-Gaudin et al 2018;Binder & Povich 2018;Shull & Danforth 2019;Kuhn et al 2019;Lim et al 2019;Zucker et al 2020) and those cited in this paper are consistent with that value, indicating that Trumpler 14, Trumpler 16 W, and η Car are at the same or similar distance. The only discrepant Gaia DR2 paper is Davidson et al (2018), which places Trumpler 14 450 ± 200 pc beyond Trumpler 16, but that work does not properly take into account the spatial correlations and systematic errors in the data.…”

Section: The Carina Nebula Associationsupporting

confidence: 88%

The Villafranca catalog of Galactic OB groups

Apellániz

Bellido

Barbá

et al. 2020

A&A

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Context. The spectral classifications of the Galactic O-Star Spectroscopic Survey (GOSSS) and the astrometric and photometric data from Gaia have significantly improved our ability to measure distances and determine memberships of stellar groups (clusters, associations, or parts thereof) with OB stars. In the near future, the situation will be further improved thanks to subsequent Gaia data releases and new photometric and spectroscopic surveys. Aims. We initiated a program to identify and determine the membership of Galactic stellar groups with OB stars and measure distances to them. Given the data currently available, we started with the identification and distance determinations of groups with O stars. In this paper, we concentrate on groups that contain stars with the earliest spectral subtypes. Methods. We used GOSSS to select Galactic stellar groups with O2–O3.5 stars and the method described in paper 0 of this series, which combines Gaia DR2 G + GBP + GRP photometry, positions, proper motions, and parallaxes to assign robust memberships and measure distances. We also included Collinder 419 and NGC 2264, the clusters cited in that paper, to generate our first list of 16 O-type Galactic stellar groups. Results. We derived distances, determined the membership, and analyzed the structure of sixteen Galactic stellar groups with O stars, Villafranca O-001 to Villafranca O-016, including the fourteen groups with the earliest-O-type optically accessible stars known in the Milky Way. We compared our distance with previous results from the literature and establish that the best consistency is with (the small number of) VLBI parallaxes and the worst is with kinematic distances. Our results indicate that very massive stars can form in relatively low-mass clusters or even in near-isolation, as is the case for the Bajamar star in the North America nebula. This lends support to the hierarchical scenario of star formation, where some stars are born in well-defined bound clusters but others are born in associations that are unbound from the beginning: groups of newborn stars come in many shapes and sizes. We propose that HD 64 568 and HD 64 315 AB could have been ejected simultaneously from Haffner 18 (Villafranca O-012 S). Our results are consistent with a difference of ≈20 μas in the Gaia DR2 parallax zero point between bright and faint stars.

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Section: The Carina Nebula Associationsupporting

confidence: 88%

The Villafranca catalog of Galactic OB groups

Apellániz

Bellido

Barbá

et al. 2020

A&A

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“…While the scatter is significant, the slope of the fit to the OB associations is similar to the slope of the size-linewidth relation of Larson (1981) of 0.38 and the slope of 0.39 determined for the substructures of Cyg OB2 and Car OB1 by Lim et al (2019). Lim et al (2019) suggest that the closeness of OB associations to the size-linewidth relation derived for molecular clouds is an indication that star formation across gravitationally-unbound turbulent clouds may be a better explanation for the formation of OB associations rather than the expansion of clusters. However, given the large degree of scatter in Figure 3, we are unable to draw any substantial conclusions regarding the origin of OB associations from this relation alone.…”

Section: Properties Of Observed Ob Associationssupporting

confidence: 70%

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

Ward

Kruijssen

Rix

2020

Monthly Notices of the Royal Astronomical Society

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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 lowdensity side of this distribution, rather than as the remnants of expanding clusters. We utilise 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 localised expansion from sub-clusters 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.

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“…Gaia DR2 parallaxes allowed Berlanas et al (2019) to extend this work to the third dimension, revealing that the known OB stars constitute at least two separate groups along the line-of-sight. Lim et al (2019) studied the spatial distribution of high-mass members of Cyg OB2 and suggest the association is comprised of two dense clusters and a lower-density halo, though the clus-ters do not appear to be kinematically distinct and there is no evidence for such clusters in the spatial distribution of lowmass stars (e.g., Wright et al 2014a). Kiminki et al (2007) performed the first kinematic study of Cyg OB2, measuring radial velocities for 146 OB stars over a 6 year period, allowing them to identify binaries and measure systemic velocities.…”

Section: Structure and Kinematicsmentioning

confidence: 99%

OB Associations and their origins

Wright

2020

New Astronomy Reviews

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OB associations are unbound groups of young stars made prominent by their bright OB members, and have long been thought to be the expanded remnants of dense star clusters. They have been important in astrophysics for over a century thanks to their luminous massive stars, though their low-mass members have not been well studied until the last couple of decades. This has changed thanks to data from X-ray observations, spectroscopic surveys and astrometry from Gaia that allows their full stellar content to be identified and their dynamics to be studied, which in turn is leading to changes in our understanding of these systems and their origins, with the old picture of Blaauw (1964a) now being superseded. It is clear now that OB associations have considerably more substructure than once envisioned, both spatially, kinematically and temporally. These changes have implications for the star formation process, the formation and evolution of planetary systems, and the build-up of stellar populations across galaxies.

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

Cited by 35 publications

References 79 publications

The Villafranca catalog of Galactic OB groups

The Villafranca catalog of Galactic OB groups

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

OB Associations and their origins

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