arXiv subjects

P. Morel

Publications and source records attributed to P. Morel.

50 records · Page 3Linked to original sources

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.

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arXiv2000

Shear turbulence beneath the solar tachocline

Helioseismic inversions of the Sun's internal angular velocity profile show that the rotation changes from differential in latitude in the convection zone to almost uniform in the radiative region below. The transition occurs in a thin layer, the tachocline, which is the seat of strong shear in the vertical direction. In this Note we examine whether this rotation profile can lead to shear turbulence at the top of the radiation zone. By using the standard solar model, we show that such turbulence can be generated only in a narrow region 0.695R_sun < r < 0.713R_sun at the equator and even in narrower layers at higher latitudes. We conclude that the turbulence generated by this vertical shear is unlikely to play a significant role in the transport of matter and angular momentum, and that other mechanisms must be invoked to achieve this.

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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.

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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.

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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}.

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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.

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arXiv1999

Microscopic diffusion and subdwarfs

The recent distance determinations by HIPPARCOS have allowed to locate very precisely a reasonable set of subdwarfs in the HR diagram. A detailed comparison with evolutionary models is now possible and it has been recently claimed that the observed lower part of the main-sequence is cooler than the computed one. In this paper we show that microscopic diffusion could be the physical explanation of such a systematic shift. We emphazise the fact that, as diffusion alters surface abundances, the initial metal content of the stellar material is higher than the present one. A "calibration" procedure is necessary when comparing observed and theoretical HR diagram, to take into account this effect.

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arXiv1999

Survival of 6li and 7li in Metal-Poor Stars

The relationship between the respective depletion factors for 6Li and 7Li in metal-poor stars is studied for two simple models of depletion by nuclear burning by mixing with deeper layers, below the bottom of the convective zone. The depletion of 6Li is usually much larger than the depletion of 7Li because it occurs during the pre-mainsequence already.

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arXiv1998

Standard Solar Models in the Light of New Helioseismic Constraints: I The Solar Core

In this paper, we examine a new updated solar model taking advantage of recent reexamination of the nuclear reactions rates and of the microscopic diffusion of helium and heavy elements. Our best model fits the helioseismic data reasonably well: the base of the convective zone is at r=0.715 Ro, the photospheric helium in mass fraction is 0.243 and the sound speed square difference between the Sun and the model is < 1%. This model leads to a reestimate of the neutrinos fluxes : 7.18 SNU for the chlorine experiment, 127.2 SNU for the gallium detector and 4.82 10^6/cm2/s for the B8 neutrino flux. Acoustic and gravity mode predictions are also estimated. We present 3 models which respect the helioseismic constraints and fit the seismic observations better by taking advantage of the known physical uncertainties: nuclear reaction rates, CNO abundances and microscopic diffusion. We conclude that the standard model in side its inherent uncertainties, is robust in the light of the present acoustic mode detection and that the neutrino fluxes prediction is not well constrained.

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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)

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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.

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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.

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arXiv1997

Updated Solar Models

Solar models computed with mass loss, microscopic diffusion of helium and heavy elements and with updated physics have been evolved from the pre-main sequence to present days; they are compared to the observational constraints including lithium depletion and to the Solar Reference Model (SRM) of Basu et al. (1996), derived by inversion. Microscopic diffusion significantly improves the agreement with the observed solar frequencies and agree with the SRM within +- 0.2% for the sound velocity and +- 1% for the density, but slightly worsens the neutrino problem. Neither microscopic diffusion nor overshooting explain the observed lithium depletion consistently with helioseismological constraints, while a mass loss process does it. Models computed with OPAL equation of state and opacities are in better agreement with the seimic sound speed. To reach the level of precision of helioseismological observations the accuracy of solar models still needs to be improved by one magnitude; any such improvement will necessitate equation of state and opacity data taking into account of detailed changes in the mixture.

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arXiv1997

A Reestimate of the Protosolar 2H/1H ratio from 3He/4He solar wind measurements

We reanalyze the inference of the protosolar abundance of deuterium made by Geiss (1993) from measurements of 3He/4He in the solar wind. We use an evolutionary solar model with microscopic diffusion, constrained to fit the present age, radius and luminosity, as well as the observed ratio of heavy elements to hydrogen. The protosolar 2H/1H is obtained from the best fit of 3He/4He in the solar wind. Taking for the protosolar 3He/4He the value measured in Jupiter by the Galileo probe (Niemann et al. 1996), we derive the protosolar 2H/1H=(3.01 +- 0.17) 10^{-5} ratio. Compared to the present interstellar medium value (Linsky et al. 1993), this result is compatible with models of the chemical evolution of the Galaxy in the solar neighborhood; it is also marginally compatible with the Jovian 2H/1H=(5 +- 2) 10^{-5} ratio measured by Galileo.

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