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Spectroscopic and interferometric approach for differential rotation in massive fast rotators
https://publi2-as.oma.be/record/2824
The coupling between the convective region in the envelope and rotation can produce a surface latitudinal differential rotation that may induce changes of the stellar geometry and on the spectral line profiles that it may be scrutinized spectroscopically and by interferometry. Zorec, J.Fri, 01 Jul 2016 15:17:11 GMThttps://publi2-as.oma.be/record/28242011Differential rotation in rapidly rotating early-type stars. I. Motivations for combined spectroscopic and interferometric studies
https://publi2-as.oma.be/record/2264
Context. Since the external regions of the envelopes of rapidly rotating early-type stars are unstable to convection, a coupling may exist between the convection and the internal rotation. Aims: We explore what can be learned from spectroscopic and interferometric observations about the properties of the rotation law in the external layers of these objects. Methods: Using simple relations between the entropy and specific rotational quantities, some of which are found to be efficient at accounting for the solar differential rotation in the convective region, we derived analytical solutions that represent possible differential rotations in the envelope of early-type stars. A surface latitudinal differential rotation may not only be an external imprint of the inner rotation, but induces changes in the stellar geometry, the gravitational darkening, the aspect of spectral line profiles, and the emitted spectral energy distribution. Results: By studying the equation of the surface of stars with non-conservative rotation laws, we conclude that objects undergo geometrical deformations that are a function of the latitudinal differential rotation able to be scrutinized both spectroscopically and by interferometry. The combination of Fourier analysis of spectral lines with model atmospheres provides independent estimates of the surface latitudinal differential rotation and the inclination angle. Models of stars at different evolutionary stages rotating with internal conservative rotation laws were calculated to show that the Roche approximation can be safely used to account for the gravitational potential. The surface temperature gradient in rapid rotators induce an acceleration to the surface angular velocity. Although a non-zero differential rotation parameter may indicate that the rotation is neither rigid nor shellular underneath the stellar surface, still further information, perhaps non-radial pulsations, is needed to determine its characteristics as a function of depth. Table 5 is only available in electronic form at http://aanda.orgZorec, J.Fri, 01 Jul 2016 15:17:02 GMThttps://publi2-as.oma.be/record/22642010Spectrally resolved interferometric observations of a Cephei and physical modeling of fast rotating stars
https://publi2-as.oma.be/record/2180
Context. When a given observational quantity depends on several stellar physical parameters, it is generally very difficult to obtain observational constraints for each of them individually. Therefore, we studied under which conditions constraints for some individual parameters can be achieved for fast rotators, knowing that their geometry is modified by the rapid rotation which causes a non-uniform surface brightness distribution. Aims: We aim to study the sensitivity of interferometric observables on the position angle of the rotation axis (PA) of a rapidly rotating star, and whether other physical parameters can influence the determination of PA, and also the influence of the surface differential rotation on the determination of the ß exponent in the gravity darkening law that enters the interpretation of interferometric observations, using a Cep as a test star. Methods: We used differential phases obtained from observations carried out in the Ha absorption line of a Cep with the VEGA/CHARA interferometer at high spectral resolution, R = 30 000 to study the kinematics in the atmosphere of the star. Results: We studied the influence of the gravity darkening effect (GDE) on the determination of the PA of the rotation axis of a Cep and determined its value, PA = -157-10°+17°. We conclude that the GDE has a weak influence on the dispersed phases. We showed that the surface differential rotation can have a rather strong influence on the determination of the gravity darkening exponent. A new method of determining the inclination angle of the stellar rotational axis is suggested. We conclude that differential phases obtained with spectro-interferometry carried out on the Ha line can in principle lead to an estimate of the stellar inclination angle i. However, to determine both i and the differential rotation parameter a, lines free from the Stark effect and that have collision-dominated source functions are to be preferred.Delaa, O.Fri, 01 Jul 2016 15:17:00 GMThttps://publi2-as.oma.be/record/21802013