اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدSeismology of Altair with MOST
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Altair is the fastest rotating star at less than 10 parsecs from the Sun. Its precise modelling is a landmark for our understanding of stellar evolution with fast rotation, and all observational constraints are most welcome to better determine the fundamental parameters of this star. We wish to improve the seismic spectrum of Altair and confirm the $delta$-Scuti nature of this star. We used the photometric data collected by the Microvariability and Oscillations of STars (MOST) satellite in the form of a series of Fabry images to derive Altair light curves at four epochs, namely in 2007, 2011, 2012, and 2013. We first confirm the presence of $delta$-Scuti oscillations in the light curves of Altair. We extend the precision of some eigenfrequencies and add new ones to the spectrum of Altair, which now has 15 detected eigenmodes. The rotation period, which is expected at $sim$7h46min from models reproducing interferometric data, seems to appear in the 2012 data set, but it still needs confirmation. Finally, Altair modal oscillations show noticeable amplitude variations on a timescale of 10 to 15 days, which may be the signature of a coupling between oscillations and thermal convection in the layer where the kappa-mechanism is operating.The Altair oscillation spectrum does not contain a large number of excited eigenmodes, which is similar to the fast rotating star HD220811. This supports the idea that fast rotation hinders the excitation of eigenmodes as already pointed out by theoretical investigations.
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Fast rotation is responsible for important changes in the structure and evolution of stars. Optical long baseline interferometry now permits the study of its effects on the stellar surface, mainly gravity darkening and flattening. We aim to determine
the fundamental parameters of the fast-rotating star Altair, in particular its evolutionary stage, mass, and differential rotation, using state-of-the-art stellar interior and atmosphere models together with interferometric, spectroscopic, and asteroseismic observations. We use ESTER 2D stellar models to produce the relevant surface parameters needed to create intensity maps from atmosphere models. Interferometric and spectroscopic observables are computed from these intensity maps and several stellar parameters are then adjusted using the MCMC algorithm Emcee. We determined Altairs equatorial radius to be 2.008 +/- 0.006 Rsun, the position angle 301.1 +/- 0.3 degrees, the inclination 50.7 +/- 1.2 degrees, and the equatorial angular velocity 0.74 +/- 0.01 times the Keplerian angular velocity. This angular velocity leads to a flattening of 0.220 +/- 0.003. We also deduce from the spectroscopically derived vsini ~ 243 km/s, a true equatorial velocity of ~314 km/s corresponding to a rotation period of 7h46m (~3 c/d). The data also impose a strong correlation between mass, metallicity, hydrogen abundance, and core evolution. Thanks to asteroseismic data, we constrain the mass of Altair to 1.86 +/- 0.03 Msun and further deduce its metallicity Z = 0.019 and its core hydrogen mass fraction Xc = 0.71, assuming an initial solar hydrogen mass fraction X = 0.739. These values suggest that Altair is ~100 Myrs old. Finally, the 2D ESTER model also gives the internal differential rotation of Altair, showing that its core rotates approximately 50% faster than the envelope, while the surface differential rotation does not exceed 6%.
The space-borne missions CoRoT and Kepler are indiscreet. With their asteroseismic programs, they tell us what is hidden deep inside the stars. Waves excited just below the stellar surface travel throughout the stellar interior and unveil many secret
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