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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/28242011Achernar can be a differential rotator
https://publi2-as.oma.be/record/2658
We take advantage of interferometric measurements of Achernar to inquire on its internal rotational law. The reinterpretation of interferometric data and the use of fundamental parameters corrected for gravitational darkening effects and 2D-models of internal stellar structures, lead us to the conclusion that the star could not be a rigid, near critical, rotator but a differential rotator with the core rotating ˜3 times faster than the surface. Zorec, J.Fri, 01 Jul 2016 15:17:08 GMThttps://publi2-as.oma.be/record/26582005Differential Rotation in Early-Type Stars
https://publi2-as.oma.be/record/2619
Using 2D models of rotating stars, the interferometric measurements of aEri and its fundamental parameters corrected for gravitational darkening effects we infer that the star might have a core rotating 2.7 times faster than the surface. We explore the consequences on spectral lines produced by surface differential rotation combined with the effects due to a kind of internal differential rotation with rotational energies higher than allowed for rigid rotation which induce geometrical deformations that do not distinguish strongly from those carried by the rigid rotation. Zorec, J.Fri, 01 Jul 2016 15:17:08 GMThttps://publi2-as.oma.be/record/26192007Differential Rotation and the υ sin i Parameter
https://publi2-as.oma.be/record/2538
Zorec, J.Fri, 01 Jul 2016 15:17:07 GMThttps://publi2-as.oma.be/record/25382004Time Variability of NRP Modes along Orbital Phase in Per
https://publi2-as.oma.be/record/2467
Jankov, S.Fri, 01 Jul 2016 15:17:06 GMThttps://publi2-as.oma.be/record/24672003Differential 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/21802013Disk and wind evolution of Achernar: the breaking of the fellowship
https://publi2-as.oma.be/record/1982
Aims: We use spectral energy distributions (SEDs), Ha line profiles, and visibilities available in the literature to study Achernar's envelope geometry and to propose a possible scenario for its circumstellar disk formation and dissipation. Methods: We use the SIMECA code to investigate possible geometries of the circumstellar environment by comparing our synthetic results with spectroscopic and high angular resolution data from the VLTI/VINCI instrument. We compute three different kinds of models: an equatorial disk, a polar wind, and a disk+wind model. Results: We develop a 2D axial symmetric kinematic model to study the variation of the observed Ha line profiles, which provides clear evidence of Achernar's equatorial disk formation and dissipation between 1991 and 2002. Our model can reproduce the polar-wind extension greater than 10 R_star and a possible equatorial disk (=5 R_star) but we were unable to estimate the wind opening angle. We reproduce the Ha line-profile variations using an outburst scenario, but the disk final contraction requieres an additional physical effect to be taken into account. The polar stellar wind does not appear to be linked to the presence of a disk or a ring around the star. We test the possibility of a binary companion to Achernar, as found by Kervella Domiciano de Souza (2007, A, 474, L49), but conclude that the VLTI/VINCI visibilities cannot be explained by a rotationally-distorted Be star and a companion alone. The presence of a polar jet provides an important component to reproduce the observations even if it is not excluded that the companion could partially influence the observations. New interferometric observations at short baselines (5 = B = 20 m) are mandatory to constrain Achernar's circumstellar envelope, as well as spectroscopic long-term follow-up observational programs to link Achernar's mass-loss episodes with its circumstellar disk formation. Kanaan, S.Fri, 01 Jul 2016 15:16:56 GMThttps://publi2-as.oma.be/record/19822008