Cyg OB2 Lacks Fast Rotators: Spectral Reclassification Reveals B0 Stars May Be Rapidly Rotating Late-O Types

The puzzling absence of rapidly rotating stars within the Cygnus OB2 association, a prolific stellar nursery in our galaxy, challenges current understandings of how massive stars evolve. D. Galán-Diéguez, S. R. Berlanas, and A. Herrero, alongside colleagues including M. Abdul-Masih, D. J. Lennon, and C. Martínez-Sebastián, investigated whether misclassification of stars could account for this anomaly. The team meticulously reclassified B0-type stars within Cygnus OB2, considering the effects of stellar rotation on observed spectra, and employed advanced techniques to determine their true classifications. Their results reveal that a significant proportion of previously identified B0 stars are, in fact, rapidly rotating late-O types, yet still demonstrate a striking overall lack of extremely fast rotators, suggesting that the dynamics and conditions within this star-forming region play a crucial role in shaping the rotational properties of massive stars.

Researchers sought to determine if the distribution of stellar rotation in Cygnus OB2 differs from that observed in other star-forming regions. The team employed rigorous statistical techniques, including bootstrap resampling, to confirm a genuine deficit of fast rotators, strengthening the evidence for unusual stellar evolution in this region. The analysis carefully considered data grouping to avoid artificial patterns and accurately estimate the range of possible rotational velocities. Researchers reclassified stars previously identified as B0-types, suspecting some might be rapidly rotating late-O types. The team employed sophisticated spectroscopic analysis, utilizing both existing and newly acquired data, to account for the effects of stellar rotation on spectral features. Results reveal that approximately nineteen per cent of the initially classified B0 stars are, in fact, late-O types, demonstrating the potential for misclassification due to rapid rotation.

This reclassification is significant because stellar rotation broadens spectral lines, making accurate classification challenging. Researchers reclassified stars initially identified as B0-types, seeking to understand the previously observed lack of fast-rotating O-type stars. The team utilized both existing and newly acquired spectroscopic data, accounting for the influence of rotation on spectral characteristics. Results demonstrate that approximately nineteen per cent of the stars originally classified as B0 are, in fact, late-O types. This reclassification is significant because late-O stars are typically less massive and more prone to rapid rotation.

Further analysis revealed that only six stars exhibit rotational velocities exceeding 200 kilometres per second, with just one newly identified O-type star reaching velocities above this threshold. Researchers reclassified a portion of stars previously identified as B0-type, discovering that approximately nineteen per cent are, in fact, late-O types. Further analysis revealed a continued scarcity of rapidly rotating O-type stars within the sample, with only six stars exhibiting rotational velocities exceeding 200 kilometres per second and just one newly identified O-type star reaching this threshold. These findings reinforce the existing puzzle surrounding the unexpectedly low number of fast rotators among massive stars, challenging current models of stellar evolution.

The researchers propose a combination of factors to explain this observation, suggesting the young age of Cygnus OB2 may not yet have allowed sufficient time for the production of rapidly rotating stars through binary interactions. They also posit that fast rotators may have been ejected from the core of the association as runaway stars, or that local conditions during star formation may hinder the development of rapid rotation. This work contributes to a growing understanding of the forces shaping the evolution of massive stars and their rotational characteristics.