L. Bigot

The oblique pulsator model revisited

The oblique pulsator model accounts for most of the pulsation properties of the rapidly oscillating Ap (roAp) stars. The model predicts that modes are seen as equidistant multiplets separated by the angular frequency of rotation. The relative amplitudes of the components may be calculated and directly compared with observations. The eects of rotation introduce amplitude asymmetry, that is peaks corresponding to azimuthal numbers m and m are unequal. In this paper we propose improvements to the model that consist of including eects of the centrifugal force and in using a non-perturbative treatment of the magnetic field influence. We show that in roAp stars the centrifugal force is the primary source of the rotational frequency shift. Although the amplitude asymmetry arises from the Coriolis force, its size is strongly aected by the centrifugal force. For dipole modes (= 1) we develop a simple geometrical picture of pulsation in the presence of rotation and a magnetic field. We provide some numerical results for a representative model of roAp stars which is applied to the case of HR 3831. We find that the mode that agrees with the observed amplitude ratios in this star significantly departs from alignment with the magnetic axis. We discuss problems posed by the observational data of HR 3831, emphasizing diculties of the standard oblique pulsator model which assumes that the excited mode is nearly aligned with the magnetic field. The rapidly oscillating Ap (roAp) stars are high order p-mode pulsators. The pulsation periods are in the 5-15 min range which is nearly the same as that of the solar oscillations. However, the mode amplitudes in roAp stars, with typical val- ues of one millimagnitude, are higher by three orders of mag- nitude than those in the Sun. Kurtz (1982) argued that the mag- netic field must play an essential role in roAp oscillations as the maxima of the oscillation amplitudes coincide with the max- ima of the longitudinal field. The properties of the magnetic field in roAp stars are similar to that in the whole group of Ap stars. The observed field is predominantly dipolar and has a kiloGauss strength. Since their discovery two decades ago (Kurtz 1978), the number of roAp stars has grown to 32. Most of the pulsation data in roAp stars may be interpreted in terms of rotating dipole modes that Kurtz assumed to be symmetric around the magnetic axis. The model provides a natural explanation of the observed multiplets in the spectrum of the oscillations in which the components are split by ex- actly the frequency of rotation. Dziembowski & Goode (1985) generalized the oblique pulsator model bytaking into account

The limb darkening of

For the nearby dwarf star α u2009Cenxa0B (K1u2009V), we present limb-darkening predictions from a 3D hydrodynamical radiative transfer model of its atmosphere. We first compared the results of this model to a standard Kuruczs atmosphere. Then we used both predictions to fit the new interferometric visibility measurements of α u2009Cenxa0B obtained with the VINCI instrument of the VLT Interferometer. Part of these new visibility measurements were obtained in the second lobe of the visibility function, which is sensitive to stellar limb-darkening. The best agreement is found for the 3D atmosphere limb-darkening model and a limb-darkened angular diameter of

\mathsf{\alpha}

theta_{rm 3D} = 6.000pm 0.021

Centauri B - Matching 3D hydrodynamical models with interferometric measurements

u2009mas, corresponding to a linear radius of

The diameter of the CoRoT target HD 49933 - Combining the 3D limb darkening, asteroseismology, and interferometry

0.863 pm 0.003,R_odot

Magnetic variability in the young solar analog KIC 10644253 - Observations from the Kepler satellite and the HERMES spectrograph

(assuming

Theoretical light curves of dipole oscillations in roAp stars

pi = 747.1 pm 1.2

The radii and limb darkenings of α Centauri A and B - Interferometric measurements with VLTI/PIONIER

u2009mas). Our new linear radius agrees well with the asteroseismic value predicted by Thevenin etxa0al. (2002, A&A, 392, L9). In view of future observations of this star with the VLTI/AMBER instrument, we also present limb-darkening predictions in the J , H , and K bands.

The fundamental parameters of the roAp star 10 Aquilae

Context. The interpretation of stellar pulsations in terms of internal structure depends on the knowledge of the fundamental stellar parameters. Long-base interferometers permit us to determine very accurate stellar radii, which are independent constraints for stellar models that help us to locate the star in the HR diagram. Aims. Using a direct interferometric determination of the angular diameter and advanced three-dimensional (3D) modeling, we derive the radius of the CoRoT target HD 49933 and reduce the global stellar parameter space compatible with seismic data. Methods. The VEGA/CHARA spectro-interferometer is used to measure the angular diameter of the star. A 3D radiative hydrodynamical simulation of the surface is performed to compute the limb darkening and derive a reliable diameter from visibility curves. The other fundamental stellar parameters (mass, age, and Teff) are found by fitting the large and small p-mode frequency separations using a stellar evolution model that includes microscopic diffusion. Results. We obtain a limb-darkened angular diameter of θLD = 0.445 ± 0.012 mas. With the Hipparcos parallax, we obtain a radius of R = 1.42 ± 0.04 R� . The corresponding stellar evolution model that fits both large and small frequency separations has a mass of 1

Using hydrodynamical simulations of stellar atmospheres for periodogram standardization: Application to exoplanet detection

The continuous photometric observations collected by the Kepler satellite over 4 yr provide a wealth of data with an unequalled quantity and quality for the study of stellar evolution of more than 200u2009000 stars. Moreover, the length of the dataset provides a unique source of information for detecting magnetic activity and associated temporal variability in the acoustic oscillations. In this regards, the Kepler mission was awaited with great expectations. The search for the signature of magnetic activity variability in solar-like pulsations still remained unfruitful more than 2 yr after the end of the nominal mission. Here, however, we report the discovery of temporal variability in the low-degree acoustic frequencies of the young (1 Gyr-old) solar analog KICu200910644253 with a modulation of about 1.5 yr with significant temporal variations for the duration of the Kepler observations. The variations agree with the derived photometric activity. The frequency shifts extracted for KICu200910644253 are shown to result from the same physical mechanisms involved in the inner subsurface layers as in the Sun. In parallel, a detailed spectroscopic analysis of KICu200910644253 is performed based on complementary ground-based, high-resolution observations collected by the HERMES instrument mounted on the Mercator telescope. Its lithium abundance and chromospheric activity 𝒮 index confirm that KICu200910644253 is a young and more active star than the Sun.