Critical study of the distribution of rotational velocities of Be stars. II: Differential rotation and some hidden effects interfering with the interpretation of the V sin I parameter
Abstract
Aims: We assume that stars may undergo surface differential rotation to study its impact on the interpretation of Vsini and on the observed distribution Φ(u) of ratios of true rotational velocities u = V/V_{c} (V_{c} is the equatorial critical velocity). We discuss some phenomena affecting the formation of spectral lines and their broadening, which can obliterate the information carried by Vsini concerning the actual stellar rotation.
Methods: We studied the line broadening produced by several differential rotational laws, but adopted Maunder's expression Ω(θ) = Ω_{0}(1 + αcos^{2}θ) as an attempt to account for all of these laws with the lowest possible number of free parameters. We studied the effect of the differential rotation parameter α on the measured Vsini parameter and on the distribution Φ(u) of ratios u = V/V_{c}.
Results: We conclude that the inferred Vsini is smaller than implied by the actual equatorial linear rotation velocity V_{eq} if the stars rotate with α < 0, but is larger if the stars have α > 0. For a given  α  the deviations of Vsini are larger when α < 0. If the studied Be stars have on average α < 0, the number of rotators with V_{eq} ≃ 0.9V_{c} is larger than expected from the observed distribution Φ(u); if these stars have on average α > 0, this number is lower than expected. We discuss seven phenomena that contribute either to narrow or broaden spectral lines, which blur the information on the rotation carried by Vsini and, in particular, to decide whether the Be phenomenon mostly rely on the critical rotation. We show that twodimensional radiation transfer calculations are needed in rapid rotators to diagnose the stellar rotation more reliably.
 Publication:
 Astronomy and Astrophysics
 Pub Date:
 June 2017
 DOI:
 10.1051/00046361/201628761
 arXiv:
 arXiv:1702.07684
 Bibcode:
 2017A&A...602A..83Z
 Keywords:

 stars: emissionline;
 Be;
 stars: rotation;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 To appear in A&