arXiv subjects

J. Montalban

Publications and source records attributed to J. Montalban.

83 records · Page 5Linked to original sources

arXiv2005

85Peg A: which age for a low metallicity solar like star?

We explore the possible evolutionary status of the primary component of the binary 85 Pegasi, listed as a target for asteroseismic observations by the MOST satellite. In spite of the assessed `subdwarf' status, and of the accurate distance determination from the Hipparcos data, the uncertainties in the metallicity and age, coupled with the uncertainty in the theoretical models, lead to a range of predictions on the oscillation frequency spectrum. Nevertheless, the determination of the ratio between the small separation in frequency modes, and the large separation as suggested by Roxburgh (2004), provides a very good measure of the star age, quite independent of the metallicity in the assumed uncertainty range. In this range, the constraint on the dynamical mass and the further constraint provided by the assumption that the maximum age is 14 Gyr limit the mass of 85PegA to the range from 0.75 to 0.82Msun. This difference of a few hundreths of solar masses leads to well detectable differences both in the evolutionary stage (age) and in the asteroseismic properties. We show that the age determination which will be possible through the asteroseismic measurements for this star is independent either from the convection model adopted or from the microscopic metal diffusion. The latter conclusion is strengthened by the fact that, although metal diffusion is still described in an approximate way, recent observations suggest that the real stars suffer a smaller metal sedimentation with respect to the models.

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arXiv2005

Constraining fundamental stellar parameters using seismology. Application to Alpha Centauri AB

We apply the Levenberg-Marquardt minimization algorithm to seismic and classical observables of the Alpha Cen binary system in order to derive the fundamental parameters of Alpha Cen A+B and to analyze the dependence of these parameters on the chosen observables, on their uncertainty and on the physics used in stellar modelling. The seismological data are those by Bouchy & Carrier (2002) for Alpha Cen A, and those by Carrier & Bourban (2003) for Alpha Cen B. We show that while the fundamental stellar parameters do not depend on the treatment of convection adopted (Mixing Length Theory -- MLT -- or ``Full Spectrum of Turbulence'' -- FST), the age of the system depends on the inclusion of gravitational settling, and is deeply biased by the small frequency separation of component B. We try to answer the question of the universality of the mixing length parameter, and we find a statistically reliable dependence of the alpha--parameter on the HR diagram location (with a trend similar to the one predicted by Ludwig et al.1999). We propose the frequency separation ratios introduced by Roxburgh & Voronstsov (2003) as better observables to determine the fundamental stellar parameters, and to use the large frequency separation and frequencies to extract information about the stellar structure. The effects of diffusion and equation of state on the oscillation frequencies are also studied, but present seismic data do not allow their detection.

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arXiv2004

A-type stars: evolution, rotation and binarity

We discuss the internal structure of stars in the mass range 1.5 to 4 M_sun from the PMS to the subgiant phase with a particular emphasis on the convective core and the convective superficial layers. Different physical aspects are considered such as overshooting, treatment of convection, microscopic diffusion and rotation. Their influence on the internal structure and on the photospheric chemical abundances is briefly described. The role of binarity in determining the observed properties and as a tool to constrain the internal structure is also introduced and the current limits of theories of orbital evolution and of available binary data--sets are discussed. keywords{stars: evolution, stars: binaries: general, stars: rotation}

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arXiv2004

Models of Stellar structure for asteroseismology

Among the problems still open in the study of stellar structure, we discuss in particular some issues related to the study of convection. We have recently built up complete stellar models, adopting a consistent formulation of convection both in the non gray atmosphere and in the interior, to be used for non adiabatic pulsational analysis, and discuss in some details two problems which have been clarified by these models: the physical interpretation of one of the main sequence "Boehm-Vitense gaps", and the necessity of parametrizing pre main sequence convection differently from the main sequence convection. We also report preliminary results of the application to the solar model of the non--local turbulence equations by Canuto and Dubovikov (1998) in the down-gradient approximation.

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arXiv2004

Solar model with CNO revised abundances

Recent three-dimensional, NLTE analyses of the solar spectrum have shown a significant reduction in the C, N, O and Ne abundances leading to a Z/X ratio of the order of 0.0177. We have computed solar models with this new mixture in the OPAL opacity tables. The present He abundance we find seems rather consistent with the helioseismic value. However, the convective envelope is too shallow, and diffusion, even if it reduces the discrepancy, is not able to give the current value. We present some numerical experiments consisting in changing the diffusion velocities and/or the value of opacity at the base of the convective envelope.

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arXiv2004

The shortest period M-dwarf eclipsing system BW3 V38, II: determination of absolute elements

The spectroscopic data for the short-period (0.1984 d)eclipsing binary V38, discovered by the OGLE micro-lensing team in Baade's Window field BW3, are analyzed. Radial velocity curves are derived from mid-resolution spectra obtained with EMMI-NTT at ESO - La Silla, and a simultaneous solution of the existing light curve by OGLE and of the new radial velocity curves is obtained. The system is formed by almost twin M3e dwarf components that are very close, but not yet in contact. The spectra of both dwarfs show signatures of the presence of strong chromospheres. Spectroscopy definitely confirms, therefore, what was suggested on the basis of photometry: BW3 V38 is indeed a unique system, as no other similar binary with M components and in such a tight orbit is known. Within the limits posed by the relatively large errors, due to the combined effect of system faintness and of the constraints on exposure time, the derived physical parameters seem to agree with the relations obtained from the other few known eclipsing binaries with late type components (which indicate a discrepancy between the available evolutionary models and the data at ~ 10% level). A possible explanation is the presence of strong magnetic fields and fast rotation (that applies to the BW3 V38 case as well). A simple computation of the system secular evolution by angular momentum loss and spin orbit synchronization shows that the evolution of a system with M dwarfs components is rather slow, and indicates as well a possible reason why systems similar to BW3 V38 are so rare.

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arXiv2003

Convection in the atmospheres and envelopes of Pre-Main Sequence stars

The Teff location of Pre-Main Sequence (PMS) evolutionary tracks depends on the treatment of over-adiabaticity. Since the convection penetrates into the stellar atmosphere, also the treatment of convection in the modeling of stellar atmospheres will affect the location of the Hayashi tracks. We present new non-grey PMS tracks for Teff>4000 K. We compute several grids of evolutionary tracks varying: 1. the treatment of convection: either the Mixing Length Theory (MLT) or Canuto et al. (1996, CGM) treatment; 2.the atmospheric boundary conditions: we use the new Vienna grids of ATLAS9 atmospheres (Heiter et al. 2002a), which were computed using either MLT (with alpha=Lambda/Hp=0.5) or CGM treatments. We compute as well grids of models with the NextGen (Allard & Hauschildt 1997) atmosphere models, and a 1 Msun grey MLT evolutionary track using the alpha calibration based on 2D-hydrodynamical models (Ludwig et al. 1999)}. These different grids of models allow us to analyze the effects of convection modeling on the non--grey PMS evolutionary tracks. We conclude that: 1. In spite of the solar calibration, if MLT convection is adopted a large uncertainty results in the shape and location of PMS tracks. 2. As long as the model of convection is not the same in the interior and in the atmosphere the optical depth at which we take the boundary conditions is an additional parameter of the models. 3. The comparison between NextGen and ATLAS9 based models shows that, in the Teff domain they have in common (4000--10000K), the PMS location is mainly determined by the treatment of the over-adiabatic convection. 4. Comparison with observations indicates that, for any convection model and for any of the atmosphere grids, convection in the PMS phase appears to be less efficient than what is necessary in order to fit the Sun.

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arXiv2003

Efficiency of convection and pre-mainsequence lithium depletion

We show by detailed model computation how much the Pre-Main Sequence (PMS) Lithium depletion depends on the treatment of over-adiabaticity, by taking advantage of the results of new models by Montalbán et al., which apply different treatments of convection to non-grey PMS models. In order to reproduce both the PMS lithium depletion (inferred from the lithium depletion patterns in young open clusters), and the location of PMS tracks in the HR diagram (inferred from the study of young PMS stars), convection both in the atmosphere and in a good fraction of the stellar envelope must be highly inefficient: e.g., in the Mixing Length Theory approximation, it must have a very low alpha=l/H_p. Unfortunately, the radii of these models are at variance with the solar radius, possibly indicating t hat there is some additional physical input, generally not taken into account in the stellar models, which affects the efficiency of convection in PMS stars, but probably not in the main sequence stars nor in evolved red giants. We stress the importance of determining precisely masses and lithium abundance in PMS binaries such as the important spectroscopic and eclipsing binary RXJ0529.4+0041.

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arXiv2003

A new seismic analysis of Alpha Centauri

Models of alpha Cen A & B have been computed using the masses determined by Pourbaix et al. (2002) and the data derived from the spectroscopic analysis of Neuforge and Magain (1997). The seismological data obtained by Bouchy and Carrier (2001, 2002) do help improve our knowledge of the evolutionary status of the system. All the constraints are satisfied with a model which gives an age of about 6 Gyr for the binary.

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arXiv2002

Helium variation due to self-pollution among Globular Cluster stars: consequences on the horizontal branch morphology

It is becoming clear that `self--pollution' by the ejecta of massive asymptotic giant branch stars has an important role in the early chemical evolution of globular cluster stars, producing CNO abundance spreads which are observed also at the surface of unevolved stars. Considering that the ejecta which are CNO processed must also be helium enriched, we have modelled stellar evolution of globular cluster stars by taking into account this possible helium enhancement with respect to the primordial value. We show that the differences between the main evolutionary phases (main sequence, turn--off and red giants) are small enough that it would be very difficult to detect them observationally. However, the difference in the evolving mass may play a role in the morphology of the horizontal branch, and in particular in the formation of blue tails, in those globular clusters which show strong CNO abundance variations, such as M13 and NGC 6752.

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arXiv2002

Planet accretion and the abundances of lithium isotopes

Planet accretion onto solar type stars may significantly change the stellar atmospheric abundances of 6Li and 7Li if it takes place after the star has arrived at the main sequence. Ingestion of planets at an earlier phase will not affect theses abundances because of extensive pre-main sequence mixing and burning. We present quantitative estimates of the main sequence evolution of stellar surface lithium abundances after planet ingestion. At solar metallicities, for stellar masses in the range ~ 1.3-1.1 M_sun, a large fraction of the ingested 6Li is likely to be preserved during the whole main sequence lifetime according to the standard model. Preservation of the less fragile 7Li isotope occurs in a larger mass range ~ 1.3-0.9 M_sun. At high metallicities typical of planet host stars, the ranges of masses are slightly reduced essentially due to the thicker convective zones. Further reduction is expected if non-standard processes cause extra-mixing of material below the base of the convective zone, but even in this case there are stellar masses where both isotopes are significantly preserved. We conclude that there is a range of stellar effective temperature where migration and accretion of planets and planetary material can be empirically tested using high-resolution spectroscopy of the lithium isotopes.

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