Evolution towards the critical limit and the origin of Be stars

By S. Ekström, G. Meynet, Andre Maeder, F. Barblan. 2007

173 citations recorded by the source corpus.

Context. More and more evidence leads to considering classical Be stars as rotating close to the critical velocity. If so, then the question that arises is the origin of this high surface velocity. Aims. We determine which mechanisms accelerate the surface of single stars during the main sequence evolution. We study their dependence on the metallicity and derive the frequency of stars with different surface velocities in clusters of various ages and metallicities. Methods. We have computed 112 stellar models of four different initial masses between 3 and 60 M� , at four different metallicities between 0 and 0.020, and with seven different values of the ratio Ω/Ωcrit between 0.1 and 0.99. For all the models, computations were performed until either the end of the main sequence evolution or until the critical limit was reached. Results. The evolution of surface velocities during the main sequence lifetime results from an interplay between meridional circulation (bringing angular momentum to the surface) and mass loss by stellar winds (removing it). The dependence on metallicity of these two mechanisms plays a key role in determining, for each metallicity, a limiting range of initial masses (spectral types) for stars able to reach or at least approach the critical limit. Present models predict a higher frequency of fast rotating stars in clusters with ages between 10 and 25 Myr. This is the range of ages where most of Be stars are observed. To reproduce the observed frequencies of Be stars, it is necessary to first assume that the Be star phenomenon already occurs for stars with υ/υcrit ≥ 0.7 and, second, that the fraction of fast rotators on the zero-age main sequence is higher at lower metallicities. Depending on the stage at which the star becomes a Be star, it may present either larger or less enrichments in nitrogen at the surface.

Subfields: Astronomy and Astrophysics, Education, Instrumentation

View Paper: https://www.semanticscholar.org/paper/fa94cfac3edebca69708a3ecc95889d7897a10a1

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Citations in the corpus

Effects of rotation on the evolution of primordial stars (2008, methodology): “With such a coupling, the mass loss becomes much larger and the evolution may be strongly altered (Ekström et al. 2008a).”
A STELLAR ROTATION CENSUS OF B STARS: FROM ZAMS TO TAMS (2010, methodology): “…rapid rotating Be stars: 1) they were born as rapid rotators; 2) angular momentum redistribution with evolution causes their outer layers to spin up (Ekström et al. 2008); and 3) they are the mass gainers that were spun up by a past episode of Roche lobe overflow in an interacting binary…”
Rotational velocities of A-type stars - IV. Evolution of rotational velocities (2012, methodology): “Figure 16a displays the comparison of the observationally inferred evolution of the v/vcrit ratio with those obtained by Ekström et al. (2008) for three solar mass stars with different initial v/vcrit ratios.”
THE ROTATION RATES OF MASSIVE STARS: THE ROLE OF BINARY INTERACTION THROUGH TIDES, MASS TRANSFER, AND MERGERS (2012, methodology): “We refer the reader to the excellent discussion by Ekström et al. (2008b), which describes this effect in detailed models of single stars that allow for differential rotation.”
Classical Be stars (2013, methodology): “The works by Ekström et al. (2008) and Granada et al. (2013) investigated the angular momentum evolution of B stars and their link to Be stars.”