The lack of fast rotators in Cyg OB2. I. Insights from spectral reclassification of its B0 population

TL;DR

This study addresses why Cygnus OB2 lacks fast-rotating O-type stars. By reclassifying the B0 population with rotation-aware tools and measuring projected rotational velocities, the authors find that about one-fifth of B0 stars are actually late-O, and only a small number exceed 200 km s−1, with just one new O-type star above this threshold. Even after accounting for misclassifications, the fast-rotator tail remains absent, suggesting a combination of youth, formation conditions, and dynamical processes (ejection as runaways) shape the angular-momentum distribution. These results have important implications for massive-star evolution, binary interaction timelines, and the role of environment in setting initial rotation speeds, motivating broader, high-resolution surveys beyond the cluster core.

Abstract

Context. Cygnus OB2, in the Cygnus X complex -- one of the most active star-forming regions of the Galaxy -- hosts hundreds of O- and B-type stars at different evolutionary stages. This association provides a unique laboratory to study massive star evolution and dynamics. However, despite extensive studies, the absence of a fast-rotating group ($v\sin{i}>200\,\mathrm{km\,s^{-1}}$) among the O-type population of Cygnus OB2 challenges current models of massive star evolution. Aims. Stellar rotation strongly impacts spectral line shapes of O-type stars, and high rotation can potentially lead to misclassifications. We investigate whether some stars in Cygnus OB2, classified at low spectral resolution as B0, are actually rapidly rotating late-O types. Such cases could explain the observed lack of fast rotators in Cygnus OB2. Methods. Accounting for rotation, we reclassified the known B0 population in Cygnus OB2, using the MGB tool and both the new and pre-existing optical spectroscopy. Finally, we computed the projected rotational velocities using iacob-broad. Results. About $19\,\%$ of the initial B0 population in Cygnus OB2 are, in fact, late-O types. Only six stars in the entire dataset show $v\sin{i}>200\,\mathrm{km\,s^{-1}}$, with just one new O-type star exceeding this threshold. Conclusions. In our study of Cygnus OB2, we continue to find a notable lack of fast rotators among its O-type population. We propose a combination of three factors as the most likely explanation: (i) the young age of Cygnus OB2 may imply that fast rotators have not been produced yet due to binary interactions; (ii) fast rotators may have been dynamically ejected from the core as runaway stars; and (iii) local star formation conditions may hinder binary formation (reducing spin-up interactions) or result in slower rotational velocities at birth.

Figures (9)

• Figure 1: Comparison of projected rotational velocities resulting from iacob-broad, using the Fourier transform (FT) and the goodness-of-fit (GOF) methods. The dashed lines indicate deviations of $20\,km\per s$ or $20\,\%$ (whichever is the largest) from the 1:1 correlation. The gray-shaded square denotes rotational velocities below the $\mathrm{c/R}$ threshold, estimated for a spectral resolution of $\mathrm{R\!=\!5000}$. The green squares show those $v\sin{i}$ in which $\mathrm{GOF(\textit{v}_{mac})\!=\!0}$ is used.
• Figure 2: Computed projected rotational velocities for the star sample in this work. Different shades of blue represent the corresponding spectral types (O- or B-type) after our spectral reclassification.
• Figure 3: Projected rotational velocities for the O-type population in Cygnus OB2. Left: histogram representing all $v\sin{i}$ values for the Cygnus OB2 O-type stars (in orange), combining results from this study with those from 2020AA...642A.168B. The rotational velocities computed in this work for O stars are plotted in blue. Middle: $v\sin{i}$ data for O-type stars with spectroscopic masses $\mathrm{M\!<\!32\,\mathrm{M_\odot}}$2020AA...642A.168B. Right: $v\sin{i}$ results from 2022AA...665A.150H for Galactic O-type stars with $\mathrm{M\!<\!32\,\mathrm{M_\odot}}$ (gray histogram). The cyan overlay merges the blue and violet histograms from the middle panel.
• Figure 4: Distribution of projected rotational velocities for O-type stars $v\sin{i}$ across different regions of the local Universe. The histograms in black, green, purple, and brown represent the $v\sin{i}$ measurements for Galactic stars in the solar neighborhood 2022AA...665A.150H, 30 Doradus 2013AA...560A..29R, Carina OB1 2025AA...695A.248B, and NGC 346 2019AA...626A..50D, respectively. The orange histogram displays the rotational velocity distribution for Cygnus OB2, integrating O-type stars from 2020AA...642A.168B with those newly identified in our reclassification of the B0 sample.
• Figure 5: Projected rotational velocities from the O+ B0 population of Cygnus OB2 (shown in pink), compared with the $v\sin{i}$ distribution of Galactic O stars 2022AA...665A.150H. The Cygnus OB2 distribution contains results obtained by 2020AA...642A.168B (O-type stars) and all the $v\sin{i}$ computed in this work (new O and B0 stars).