arXiv subjects

J. Provost

Publications and source records attributed to J. Provost.

31 records · Page 2Linked to original sources

arXiv2003

The diameters of Alpha Centauri A and B - A comparison of the asteroseismic and VINCI/VLTI views

We compare the first direct angular diameter measurements obtained on our closest stellar neighbour, Alpha Centauri, to recent model diameters constrained by asteroseismic observations. Using the VINCI instrument installed at ESO's VLT Interferometer (VLTI), the angular diameters of the two main components of the system, Alpha Cen A and B, were measured with a relative precision of 0.2% and 0.6%, respectively. Particular care has been taken in the calibration of these measurements, considering that VINCI is estimating the fringe visibility using a broadband K filter. We obtain uniform disk angular diameters for Alpha Cen A and B of UD[A] = 8.314 +/- 0.016 mas and UD[B] = 5.856 +/- 0.027 mas, and limb darkened angular diameters of LD[A] = 8.511 +/- 0.020 mas and LD[B] = 6.001 +/- 0.034 mas. Combining these values with the parallax from Soderhjelm (1999), we derive linear diameters of D[A] = 1.224 +/- 0.003 Dsun and D[B] = 0.863 +/- 0.005 Dsun. These values are compatible with the masses published by Thevenin et al.(2002) for both stars.

astro-ph↗

arXiv2002

Asteroseismology and calibration of alpha Cen binary system

Using the oscillation frequencies of alpha Cen A recently discovered by Bouchy & Carrier, the available astrometric, photometric and spectroscopic data, we tried to improve the calibration of the visual binary system alpha Cen. With the revisited masses of Pourbaix et al. (2002) we do not succeed to obtain a solution satisfying all the seismic observational constraints. Relaxing the constraints on the masses, we have found an age t_alpha Cen=4850+-500 Myr, an initial helium mass fraction Y_i = 0.300+-0.008, and an initial metallicity (Z/X)_i=0.0459+-0.0019, with M_A=1.100+-0.006M_o and M_B=0.907+-0.006M_o for alpha Cen A&B.

astro-ph↗

arXiv2001

The Zeta Herculis binary system revisited. Calibration and seismology

We have revisited the calibration of the visual binary system Zeta Herculis with the goal to give the seismological properties of the G0 IV sub-giant Zeta Her A. We have used the most recent physical and observational data. For the age we have obtained 3387 Myr, for the masses respectively 1.45 and 0.98 solar mass, for the initial helium mass fraction 0.243, for the initial mass ratio of heavy elements to hydrogen 0.0269 and for the mixing-length parameters respectively 0.92 and 0.90 using the Canuto & Mazitelli (1991, 1992) convection theory. Our results do not exclude that Zeta Her A is itself a binary sub-system; the mass of the hypothetical unseen companion would be smaller than 0.05 solar mass. The adiabatic oscillation spectrum of Zeta Her A is found to be a complicated superposition of acoustic and gravity modes; some of them have a dual character. This greatly complicates the classification of the non-radial modes. The echelle diagram used by the observers to extract the frequencies will work for ell=0, 2, 3. The large difference is found to be of the order of 42 mu Hz, in agreement with the Martic et al. (2001) seismic observations.

astro-ph↗

arXiv2000

Calibrations of alpha Cen A & B

Detailed evolutionary models of the visual binary alpha Centauri, including pre main-sequence evolution, have been performed using the masses recently determined by Pourbaix et al. (1999). Models have been constructed using the CEFF equation of state, OPAL opacities, NACRE thermonuclear reaction rates and microscopic diffusion. A chi^2-minimization is performed to derive the most reliable set of modeling parameters wp={t_alpha Cen, Yi, [Fe/H]i, alpha A, alpha B}, where t alpha Cen is the age of the system, Yi the initial helium content, [Fe/H]i the initial metallicity and, alpha A and alpha B the convection parameters of the two components. Using the basic Bohm-Vitense (1958) mixing-length theory of convection, we derive wp BV={2710 Myr, 0.284,0.257, 1.53, 1.57}. We obtain a noticeably smaller age than estimated previously, in agreement with Pourbaix et al. (1999), mainly because of the larger masses. If convective core overshoot is considered we get wp ov={3530 Myr, 0.279,0.264,1.64,1.66}. The use of Canuto & Mazitelli (1991, 1992) convection theory leads to the set wp CM={4086 Myr, 0.271, 0.264, 0.964, 0.986}. Using the observational constraints adopted by Guenther & Demarque (2000), and the basic mixing-length theory, we obtain wp GD={5640 Myr, 0.300, 0.296, 1.86, 1.97} and surface lithium depletions close to their observed values. A seismological analysis of our calibrated models has been performed. The determination of large and small spacings between the frequencies of acoustic oscillations from seismic observations would help to discriminate between the models of alpha Cen computed with different masses and to confirm or rules out the new determination of masses.

astro-ph↗

arXiv2000

Structure of the solar core: Effect of asymmetry of peak profiles

Recent studies have established that peaks in solar oscillation power spectra are not Lorentzian in shape, but have a distinct asymmetry. Fitting a symmetric Lorentzian profile to the peaks therefore produces a shift in frequency of the modes. Accurate determination of low-frequency modes is essential to infer the structure of the solar core by inversion of the mode frequencies. In this paper we investigate how the changes in frequencies of low-degree modes obtained by fitting symmetric and asymmetric peak profiles change the inferred properties of the solar core. We use data obtained by the Global Oscillations at Low Frequencies (GOLF) project on board the SoHO spacecraft. Two different solar models and inversion procedures are used to invert the data to determine the sound speed in the solar core. We find that for a given set of modes no significant difference in the inferred sound-speed results from taking asymmetry into account when fitting the low-degree modes.

astro-ph↗

arXiv1999

A calibration of iota Pegasi system

Recent observations provide determinations of individual masses, chemical composition and metallicity of the components of the spectroscopic and interferometric binary ιPeg (Boden et al. 1998). Using updated physics, to calibrate the system, we have computed using the stellar evolutionary code CESAM, evolutionary sequences of stellar models with the masses of ιPeg A 1.326 Msol and iota Peg B 0.819 Msol (Boden et al. loc.cit) and with different values of the mixing-length parameter α, the helium Y and the heavy element Z initial mass fraction with the constraint of the observed metallicity. Adopting effective temperatures and luminosities, as derived from observations with the bolometric corrections, and the empirical scale of temperatures of Alonso et al. (1995, 1996), we find α_A=1.46, α_B=1.36, Y=0.278, Z=0.017. The evolution time, including pre-main sequence, is found within 40 My < t_ev < 0.5Gy. The calibrated models of ιPeg. A and B are non homogeneous zero age main sequence stars with the evolutionary time t_ev=56 My. Due to the large uncertainties of their determinations, the values derived for the mixing-length parameters are smaller than the solar one but however marginally compatible with it. Our results ought to be improved as soon as a more accurate value of the magnitude difference in the V filter will be available. Detailed spectroscopic analysis for both components looks practicable, so it is urgently needed.

astro-ph↗

arXiv1999

About the time of evolution of a Solar Model

The evolution of a solar model is initialized with homogeneous models of either, pre-main sequence (P-models) or, zero-age main sequence (Z-models). The zero-age of a solar model is conventionally referenced as the time where the nuclear reactions just begin to dominate gravitation as the primary source of energy. Fixing the physics, we found that the structure of P- and Z-models computed with the same physics are almost similar soon after the exhaustion of their convective core. This similarity gives a connection between the age of the Sun t_sun and the time t_cal elapsed in the calculation of calibrated solar models. We found that a Z-model calibrated with t_cal=t_sun and a P-model calibrated with t_cal=t_sun+25My, are indistinguishable at the relative accuracy level of a few 10^{-4}.

astro-ph↗

arXiv1999

Solar Models and NACRE thermonuclear reaction rates

Using the most recent updated physics, calibrated solar models have been computed with the new thermonuclear reaction rates of NACRE, the recently available European compilation. Comparisons with models computed with the reaction rates of Caughlan & Fowler (\cite{cf88}) and of Adelberger et al. (\cite{a98}) are made for global structure, expected neutrinos fluxes, chemical composition and sound speed profiles, helioseismological properties of p-modes and g-modes.

astro-ph↗

arXiv1999

Non linear regularization for helioseismic inversions. Application for the study of the solar tachocline

Inversions of rotational splittings have shown that there exists at the base of the solar convection zone a region called the tachocline in which high radial gradients of the rotation rate occur. The usual linear regularization methods tend to smooth out any high gradients in the solution, and may not be appropriate for the study of this zone. In this paper we use, in the helioseismic context of rotation inversions, regularization methods that have been developed for edge-preserving regularization in computed imaging. It is shown from Monte-Carlo simulations that this approach can lead directly to results similar to those reached by linear inversions which however required some assumptions on the shape of the transition in order to be deconvolved. The application of this method to LOWL data leads to a very thin tachocline. From the discussions on the parameters entering the inversion and the Monte-Carlo simulations, our conclusion is that the tachocline width is very likely below 0.05R_sun which lowers our previous estimate of 0.05+/- 0.03R_sun obtained from the same dataset (Corbard et al. 1998).

astro-ph↗

arXiv1998

On the use of nonlinear regularization in inverse method for the tachocline profile determination

Inversions of rotational splittings have shown that the surface layers and the so-called solar tachocline at the base of the convection zone are regions in which high radial gradients of the rotation rate occur. The usual regularization methods tend to smooth out every high gradients in the solution and may not be appropriate for the study of a zone like the tachocline. In this paper we use nonlinear regularization methods that are developed for edge-preserving regularization in computed imaging (e.g. Blanc-Féraud et al. 1995) and we apply them in the helioseismic context of rotational inversions.

astro-ph↗

arXiv1998

Atmospheric structure and acoustic cut-off frequency of roAp stars

Some of the rapidly oscillating (CP2) stars, have frequencies which are larger than the theoretical acoustic cut-off frequency. As the cut-off frequency depends on the T(tau) relation in the atmosphere, we have computed models and adiabatic frequencies for pulsating Ap stars with T(tau) laws based on Kurucz model atmospheres and on Hopf's purely radiative relation. The frequency-dependent treatment of radiative transfer as well as an improved calculation of the radiative pressure in Kurucz model atmospheres increase the theoretical acoustic cut-off frequency by about 200 microHz, which is closer to the observations. For alpha Cir we find models with Kurucz atmospheres which have indeed a cut-off frequency beyond the largest observed frequency and which are well within the Teff - L error box. For HD 24712 only models which are hotter by about 100 K and less luminous by nearly 10% than what is actually the most probable value would have an acoustic cut-off frequency large enough. One may thus speculate that the old controversy about a mismatch between observed largest frequencies and theoretical cut-off frequencies of roAp star models is resolved. However, the observational errors for the astrophysical fundamental parameters have to be reduced further and the model atmospheres refined. Further details can be found in Audard et al. (1997)

astro-ph↗

arXiv1997

The acoustic cut-off frequency of roAp stars

Some of the rapidly oscillating (roAp) stars, have frequencies which are larger than the theoretical acoustic cut-off frequency. We have computed models and adiabatic frequencies for pulsating Ap stars with T(tau) laws based on Kurucz model atmospheres and on the Hopf's purely radiative relation. We compare the values of the cut-off frequency derived from expressions for the potential from Vorontsov & Zarkhov (1989), from Gough (1986), and from the approximation of an isothermal atmosphere. Kurucz model atmospheres increase the theoretical acoustic cut-off frequency for roAp stars by about 200 mu$Hz, which is closer to the observations. We compare our computations with observations of the 3 roAp stars HD 24712, alphaCir, and HD 134214. For alpha Cir we find models with Kurucz atmospheres which have indeed a cut-off frequency beyond the largest observed frequency. For HD 24712 only models which are hotter by about 100 K and less luminous by nearly 10% than what is actually the most probable value would have an acoustic cut-off frequency large enough. One may thus speculate that the old controversy about a mismatch between observed largest frequencies and theoretical cut-off frequencies of roAp star models is resolved. However, the model atmospheres have to be refined by investigating NLTE effects, among others, before a definite conclusion can be drawn.

astro-ph↗

arXiv1997

Inferring the equatorial solar tachocline from frequency splittings

Helioseismic inversions, carried out for several years on various ground-based and spatial observations, have shown that the solar rotation rate presents two principal regimes: a quasi-rigid rotation in the radiative interior and a latitude-dependent rotation in the whole convection zone. The thin layer, named solar tachocline, between these two regimes is difficult to infer through inverse techniques because of the ill-posed nature of the problem that requires regularization techniques which, in their global form, tend to smooth out any high gradient in the solution. Thus, most of the previous attempts to study the rotation profile of the solar tachocline have been carried out through forward modeling. In this work we show that some appropriate inverse techniques can also be used and we compare the ability of three 1D inverse techniques combined with two automatic strategies for the choice of the regularization parameter, to infer the solar tachocline profile in the equatorial plane. Our work, applied on LOWL (LOWL is an abbreviation for low degree denoted by L) two years dataset, argue in favor of a very sharp (0.05+/-0.03R_sun) transition zone located at 0.695+/-0.005R_sun which is in good agreement with the previous forward analysis carried out on Global Oscillations Network Group (GONG), Big Bear Solar Observatory (BBSO) and LOWL datasets.

astro-ph↗