The lack of fast rotators in Cyg OB2

I. Insights from spectral reclassification of its B0 population

¹ Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n E-38205 La Laguna Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna E-38206 La Laguna Tenerife, Spain
³ Gran Telescopio Canarias (GRANTECAN), c/ Cuesta de San José s/n E-38712 Breña Baja La Palma, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 September 2025
Accepted: 16 October 2025

Abstract

Context. Cygnus OB2, located within 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 rich association offers a unique opportunity to study the evolution and dynamic interactions of massive stars. However, despite extensive studies, a notable absence of a fast-rotating group (v sin i > 200 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, with high rotational velocities potentially leading to misclassifications. This study investigates 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. Considering the effects of 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. We find that approximately 19% of the initial B0 population in Cygnus OB2 are, in fact, late-O types. Further analysis shows that only six stars in the entire dataset have projected rotational velocities above 200 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.