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

Abstract

Context. Cygnus OB2 provides a unique insight into the high-mass stellar content in one of the largest groups of young massive stars in our Galaxy. Although several studies of its massive population have been carried out over the last decades, an extensive spectroscopic study of the whole known O-star population in the association is still lacking. Aims. We aim to carry out a spectroscopic characterization of all the currently known O stars in Cygnus OB2, determining the distribution of rotational velocities and accurate stellar parameters to obtain an improved view of the evolutionary status of the region. Methods. Based on existing and new optical spectroscopy, we performed a detailed quantitative spectroscopic analysis of all the known O-type stars identified in the association. For this purpose, we used the user-friendly iacob-broad and iacob-gbat automatized tools, FASTWIND stellar models, and astrometry provided by the Gaia second data release. Results. We created the most complete spectroscopic census of O stars carried out so far in Cygnus OB2 using already existing and new spectroscopy. We present the spectra for 78 O-type stars, from which we identify new binary systems, obtain the distribution of rotational velocities, and determine the main stellar parameters for all the stars in the region that have not been detected as double-line spectroscopic binaries. We also derive radii, luminosities, and masses for those stars with reliable Gaia astrometry, in addition to creating the Hertzsprung-Russell Diagram to interpret the evolutionary status of the association. Finally, we inspect the dynamical state of the population and identify runaway candidates. Conclusions. Our spectroscopic analysis of the O-star population in Cygnus OB2 has led to the discovery of two new binary systems and the determination of the main stellar parameters, including rotational velocities, luminosities, masses, and radii for all identified stars. This work has shown the improvement reached when using accurate spectroscopic parameters and astrometry for the interpretation of the evolutionary status of a population, revealing, in the case of Cygnus OB2, at least two star-forming bursts at ~3 and ~5 Myr. We find an apparent deficit of very fast rotators in the distribution of rotational velocities. The inspection of the dynamical distribution of the sample has allowed us to identify nine O stars with peculiar proper motions and discuss a possible dynamical ejection scenario or past supernova explosions in the region.

Revisiting the Cygnus OB associations

“…The Cygnus region of the Galactic plane is home to a considerable amount of recent star formation, including the massive Cygnus X star-forming region, thousands of OB stars, H II regions, supernova remnants and nine OB associations (Mel'Nik & Efremov 1995;Reipurth & Schneider 2008). Since the study of Massey & Thompson (1991), the highly reddened OB star population of Cygnus OB2 has been studied by many authors, most recently by Wright, Drew & Mohr-Smith (2015) and Berlanas et al (2020). The association is home to approximately 70 O-type stars with ages between 1 and 7 Myr and a total mass of 1.65 x 10 4 M (Wright et al 2015).…”

The Gaia-ESO Survey: The analysis of the hot-star spectra

“…We mainly based the analysis on the Si iii λ4552 line since metallic lines do not suffer from strong Stark broadening nor from nebular contamination. However, for the few cases where this line is too weak we used nebular free or weakly contaminated He i lines (He i λ4713, λ4471 and/or λ4387, see Ramírez-Agudelo et al 2013;Berlanas et al 2020). Typical uncertainties in v sin i and v macro are of the order of 10 -20%.…”

The nature of the Cygnus extreme B supergiant 2MASS J20395358+4222505

“…Later, Hanson ( 2003 ) places it at a shorter distance of 1.5 kpc, that Negueruela et al ( 2008 ) find consistent with their data. 10 More recently, Rygl et al ( 2012 ) find a similar distance of 8 Interestingly, a similar situation arose in DR2 for Cyg OB2#12, the blue hypergiant in the region (see Berlanas et al 2018 ) but it has been solved in eDR3 9 DR2 data give μ α = −2.109 ± 0.451 mas a −1 and μ δ = 0.073 ± 0.533 mas a −1 (Berlanas et al 2020 ). 10 Clark et al ( 2012 ) prefer the distance of 1.75 kpc for Cyg OB2 #12 in their analysis.…”