The Villafranca catalog of Galactic OB groups

Abstract

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.

Spectroscopic characterization of the known O-star population in Cygnus OB2

“…Both Cyg OB2 #8 and #22 have been traditionally treated as multiple stars but as already pointed out by Bica et al (2003), they should be considered as two clusters likely constituting the core of Cygnus OB2 and hosting the brightest stars of the area. A recent analysis by Maíz Apellániz et al (2020) has confirmed their cluster nature, and showed that both are at a distance of 1.7 kpc (the same distance as obtained by Berlanas et al 2019, see Sect. 4.5) and have similar proper motions.…”

High-sensitivity radio study of the non-thermal stellar bow shock EB27

“…Regarding the star, the latest results from the Gaia mission 1 (Gaia Collaboration et al 2016;Collaboration et al 2020) provided the most accurate astrometric measurements to date: RA, Dec(J2000)= 20:33:36.076, +43:59:07.38; parallax Π = 0.582 ± 0.012 mas (thus d = 1.72 kpc); proper motions (µα cos δ, µ δ ) = (−2.59 ± 0.01, +0.73 ± 0.01) mas yr −1 . The distance we obtained is significantly larger than the value of 1.32-1.4 kpc typically used in the literature for this object; however, this value is in great agreement with the value of ≈ 1.7 kpc found more recently by other authors for Cyg OB2 (Berlanas et al 2019;Maíz Apellániz et al 2020).…”

Distance of Hi-GAL sources

“…When studying a particular region, we recommend comparing the velocity and spatial distribution of the Hi-GAL sources with the systemic velocity of the region and its morphology, respectively. Finally, a path to improvement will be to update the distance (up to 4 kpc) of the nearby optical H ii regions and star forming regions (e.g., Cantat-Gaudin & Anders 2020; Zucker et al 2020;Maíz Apellániz et al 2020) used in the process and the inclusion of recent molecular cloud distance catalogs (such as Chen et al 2020), both based on the recent Gaia-DR2 data.…”