Rotational velocities of A-type stars

Abstract

Context. In previous works of this series, we have shown that late B-and early A-type stars have genuine bimodal distributions of rotational velocities and that late A-type stars lack slow rotators. The distributions of the surface angular velocity ratio Ω/Ω crit (Ω crit is the critical angular velocity) have peculiar shapes according to spectral type groups, which can be caused by evolutionary properties. Aims. We aim to review the properties of these rotational velocity distributions in some detail as a function of stellar mass and age. Methods. We have gathered v sin i for a sample of 2014 B6-to F2-type stars. We have determined the masses and ages for these objects with stellar evolution models. The (T eff , log L/L )-parameters were determined from the uvby-β photometry and the HIPPARCOS parallaxes.Results. The velocity distributions show two regimes that depend on the stellar mass. Stars less massive than 2.5 M have a unimodal equatorial velocity distribution and show a monotonical acceleration with age on the main sequence (MS). Stars more massive have a bimodal equatorial velocity distribution. Contrarily to theoretical predictions, the equatorial velocities of stars from about 1.7 M to 3.2 M undergo a strong acceleration in the first third of the MS evolutionary phase, while in the last third of the MS they evolve roughly as if there were no angular momentum redistribution in the external stellar layers. The studied stars might start in the ZAMS not necessarily as rigid rotators, but with a total angular momentum lower than the critical one of rigid rotators. The stars seem to evolve as differential rotators all the way of their MS life span and the variation of the observed rotational velocities proceeds with characteristic time scales δt ≈ 0.2 t MS , where t MS is the time spent by a star in the MS.

Near-uniform internal rotation of the main-sequence γ Doradus pulsator KIC 7661054

“…There is some evidence that the surface rotation rates of main-sequence A stars increase during the first third of their main-sequence lifetimes (Zorec & Royer 2012). found this counter-intuitive result to be corroborated by literature data (Wolff & Simon 1997;Erspamer & North 2003).…”

Asteroseismic Signatures of Evolving Internal Stellar Magnetic Fields

“…In this case, the field is expected to be large scale and with an amplitude corresponding to equipartition between the magnetic energy density ( p B 8 2 ) and the kinetic energy density in the flow (rv 2 con 2 ), where v con is an rms convective velocity. Typical values for the surface rotation periods of A stars are short (about 1 day; see, e.g., Zorec & Royer 2012). Moreover, asteroseismic observations of slowly rotating A stars suggest that these stars are nearly rigidly rotating (Kurtz et al 2014 Brun et al (2005) show dynamo action with magnetic fields reaching a considerable fraction of equipartition (B ≈ 10 4 -10 5 G).…”

Critical study of the distribution of rotational velocities of Be stars

“…2.2.1 and 2.2.2. They also determine the pnrc stellar radius R o (M, t)/R used to obtain R c (M, t)/R , where the ratio R c (M, t)/R o (M, t) comes from two-dimensional (2D) models of rotating stars (Zorec et al 2011;Zorec & Royer 2012). The mass and radius of the parent nonrotating object needed to estimate V c cannot be estimated in a simple way from the apparent effective temperature and apparent bolometric luminosity because in rapidly rotating stars both are deeply marred by rotational effects.…”

“…where R e (M, t, η) and R c (M, t) are the actual and critical stellar equatorial radii, which are determined using our 2D models of rigidly rotating stars (Zorec et al 2011;Zorec & Royer 2012). The left side of Eq.…”

“…This is because of two important simplifications introduced in their calculations: (1) the "masslowering effect" was neglected; and (2) the polar radius R p is maintained unchanged, even though its variation with η introduces non-negligible effects on the magnitudes as i 50 • . The changes of the bolometric luminosity used in the present work are detailed in Frémat et al (2005), while the variation of the R e and R p radii are given in Zorec et al (2011) and Zorec & Royer (2012). We recall that for M/M = 3 the lowering in the bolometric magnitude ranges from ∆M bol = +0.12 to +0.22 mag as η changes from 0.3 to 1.0, while for M/M = 20 we have ∆M bol = +0.08 mag at η = 0.3 and ∆M bol = +0.12 mag at η = 1.0.…”