Populations of rotating stars

Abstract

Context. Even though it is broadly accepted that single Be stars are rapidly rotating stars surrounded by a flat rotating circumstellar disk, there is still a debate about how fast these stars rotate and also about the mechanisms involved in the angular-momentum and mass input in the disk. Aims. We study the properties of stars that rotate near their critical-rotation rate and investigate the properties of the disks formed by equatorial mass ejections. Methods. We used the most recent Geneva stellar evolutionary tracks for rapidly rotating stars that reach the critical limit and used a simple model for the disk structure. Results. We obtain that for a 9 M star at solar metallicity, the minimum average velocity during the main-sequence (MS) phase to reach the critical velocity is around 330 km s −1 , whereas it would be 390 km s −1 at the metallicity of the Small Magellanic Cloud (SMC). Red giants or supergiants originating from very rapid rotators rotate six times faster and show N/C ratios three times higher than those originating from slowly rotating stars. This difference becomes stronger at lower metallicity. It might therefore be very interesting to study the red giants in clusters that show a large number of Be stars on the MS band. On the basis of our single-star models, we show that the observed Be-star fraction with cluster age is compatible with the existence of a temperature-dependent lower limit in the velocity rate required for a star to become a Be star. The mass, extension, and diffusion time of the disks produced when the star is losing mass at the critical velocity, obtained from simple parametrized expressions, are estimated to be between 9.4 × 10 −12 and 1.4 × 10 −7 M (3 × 10 −6 to 4.7 × 10 −2 times the mass of the Earth), 2000 and 6500 R , and 10 and 30 yr. These values are not too far from those estimated for disks around Be-type stars. At a given metallicity, the mass and the extension of the disk increase with the initial mass and with age on the MS phase. Denser disks are expected in low-metallicity regions.

Circumstellar discs in Galactic centre clusters: Disc-bearing B-type stars in the Quintuplet and Arches clusters

“…The simulations naturally predict denser, more extended, and more massive discs around higher-mass stars, which could explain why our sample is dominated by mid-to late B-type stars. For rapidly rotating stars in the mass range 2−9 M , Granada et al (2013) find modelled disc extents of 10-30 AU and disc masses of 10 −8 −10 −11 M reaching the same order of magnitude as our lower disc mass limit (M gas+dust ∼ 10 −8 M ) estimated for the Arches and Quintuplet excess emission sources (Sect. 5.2.2).…”

“…Both are related to the B[e] and Be phenomena. In single Be stars, equatorial mass loss due to rapid rotation close to the critical rotation velocity removes angular momentum from the surface of the star (Granada et al 2013). Although absolute numbers of expected rapid rotators with optically thick discs are not known, theoretical models predict a fraction of a few per cent at cluster ages of a few Myr (Granada et al 2013, see their Fig.…”

“…As a consequence of the initial rotation profile, an early adjustment phase prohibits the generation of critically rotating stellar surfaces during the early main-sequence phase (see Granada et al 2013 for details), as would be required to explain the very young B-star disc population in the GC clusters.…”

Evolution of single B-type stars with a large angular momentum content

“…This is the same internal rotational profile obtained after the sharp decrease in the surface velocity of our models with a flat profile at the ZAMS. Therefore, we propose to use the grids presented by Georgy et al (2013; assuming that the rotational rate at the ZAMS is the rate when the quasiequilibrium has been reached) together with our new models of very rapidly rotating stars throughout the MS, to study how rotating stellar populations evolve in time, as we have already done in Granada et al (2013), by using the new Geneva population synthesis code (SYCLIST, for synthetic clusters, isochrones, and stellar tracks) presented by Georgy et al (2014). We studied how the fraction of stars with surface velocities above a given limit varies in clusters of various ages.…”

“…This was achieved by the numerous improvements that were implemented in the Geneva stellar evolution code (Ekström et al 2012), which enable computing models of rotating stars, even those that rotate at the critical limit and experience episodes of mechanical mass loss Granada et al 2013).…”