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Post Info TOPIC: HD49933
Blobrana


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Posts: 131433
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HIP 32851
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Title: The CoRoT target HD49933: a possible seismic signature of heavy elements ionization in the deep convective zone
Author: Ana Brito, Ilídio Lopes

We use a seismic diagnostic, based on the derivative of the phase shift of the reflected by the surface acoustic waves, to probe the outer layers of the star HD 49933. This diagnostic is particularly sensitive to partial ionization processes occurring above the base of the convective zone. The regions of partial ionization of light elements, hydrogen and helium, have well known seismological signatures. In this work we detect a different seismic signature in the acoustic frequencies, that we showed to correspond to the location where the partial ionization of heavy elements occurs. The location of the corresponding acoustic glitch lies between the region of the second ionization of helium and the base of the convective zone, approximately 5% below the surface of the stars.

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Blobrana


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Posts: 131433
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HD 49933
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Title: The diameter of the CoRoT target HD 49933. Combining the 3D limb darkening, asteroseismology, and interferometry
Authors: L. Bigot, D. Mourard, P. Berio, F. Thévenin, R. Ligi, I. Tallon-Bosc, O. Chesneau, O. Delaa, N. Nardetto, K. Perraut, Ph. Stee, T. Boyajian, P. Morel, B. Pichon, P. Kervella, F. X. Schmider, H. McAlister, T. Ten Brummelaar, S. T. Ridgway, J. Sturmann, L. Sturmann, N. Turner, C. Farrington, P. J. Goldfinger

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) modelling, 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 {\theta}LD = 0.445 ±0.012 mas. With the Hipparcos parallax, we obtain a radius of R = 1.42 ±0.04 solar radii. The corresponding stellar evolution model that fits both large and small frequency separations has a mass of 1.20 ±0.08 solar masses and an age of 2.7 Gy. The atmospheric parameters are Teff = 6640 ±100 K, log g = 4.21 ±0.14, and [Fe/H] = -0.38.

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Blobrana


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Posts: 131433
Date:
RE: HD49933
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23 October 2008
Scientists measured vibrations from the Sun-like star HD49933.

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Blobrana


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Title: CoRoT reveals a magnetic activity cycle in a Sun-like star
Authors: Rafael A. Garcia, Savita Mathur, David Salabert, Jerome Ballot, Clara Regulo, Travis S. Metcalfe, Annie Baglin

The 11-year activity cycle of the Sun is a consequence of a dynamo process occurring beneath its surface. We analysed photometric data obtained by the CoRoT space mission, showing solar-like oscillations in the star HD49933, for signatures of stellar magnetic activity. Asteroseismic measurements of global changes in the oscillation frequencies and mode amplitudes reveal a modulation of at least 120 days, with the minimum frequency shift corresponding to maximum amplitude as in the Sun. These observations are evidence of a stellar magnetic activity cycle taking place beneath the surface of HD49933 and provide constraints for stellar dynamo models under conditions different from those of the Sun.

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Blobrana


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Posts: 131433
Date:
HD 49933
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Title: The CoRoT target HD 49933: 1- Role of the metal abundance
Authors: R. Samadi, H.-G. Ludwig, K. Belkacem, M.J. Goupil, M.-A. Dupret
(Version v2)

Solar-like oscillations are stochastically excited by turbulent convection at the surface layers of the stars. We study the role of the surface metal abundance on the efficiency of the stochastic driving in the case of the CoRoT target HD 49933. We compute two 3D hydrodynamical simulations representative -- in effective temperature and gravity -- of the surface layers of the CoRoT target HD 49933, a star that is rather metal poor and significantly hotter compared to the Sun. One 3D simulation has a solar metal abundance and the other has a surface iron-to-hydrogen, [Fe/H], abundance ten times smaller. For each 3D simulation we match an associated global 1D model and we compute the associated acoustic modes using a theoretical model of stochastic excitation validated in the case of the Sun and Alpha Cen A. The rate at which energy is supplied per unit time into the acoustic modes associated with the 3D simulation with [Fe/H]=-1 are found about three times smaller than those associated with the 3D simulation with [Fe/H]=0. As shown here, these differences are related to the fact that low metallicity implies surface layers with a higher mean density. In turn, a higher mean density favours smaller convective velocities and hence less efficient driving of the acoustic modes. Our result shows the importance of taking the surface metal abundance into account in the modelling of the mode driving by turbulent convection. A comparison with observational data is presented in a companion paper using seismic data obtained for the CoRoT target HD 49933.

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Blobrana


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Posts: 131433
Date:
HD49933
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Title: Impact of the physical processes in the modelling of HD49933
Authors: L. Piau, S. Turck-Chieze, V. Duez, R. F. Stein

Context : On its asteroseismic side, the initial run of CoRoT was partly devoted to the solar like star HD49933.The eigenmodes of this F dwarf have been observed with unprecedented accuracy.
Aims : We investigate quantitatively the impact of changes in the modelling parameters like mass and composition. More importantly we investigate how a sophisticated physics affects the seismological picture of HD49933. We consider the effects of diffusion, rotation and the changes in convection efficiency.
Methods : We use the CESAM stellar evolution code coupled to the ADIPLS adiabatic pulsation package to build secular models and their associated oscillation frequencies. We also exploited the hydrodynamical code STAGGER to perform surface convection calculations. The seismic variables used in this work are : the large frequency separation, the derivative of the surface phase shift,and the eigenfrequencies \nu_{\ell=0,n=14} and \nu_{\ell=0,n=27}.
Results : Mass and uncertainties on the composition have much larger impacts on the seismic variables we consider than the rotation. The derivative of the surface phase shift is a promising variable for the determination of the helium content. The seismological variables of HD49933 are sensitive to the assumed solar composition and also to the presence of diffusion in the models.

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