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Stellar models with homogeneous abundances fail to reproduce the pulsation frequencies of early B-type stars. Their oscillations are excited by kappa-mechanism involving the Fe-peak elements where they are main contributors to the opacity (the "Z-bump") and a ad hoc increase of the opacity in these layers is necessary to match the observations. We test whether atomic diffusion can induce such an opacity increase through Fe and Ni accumulations in the Z-bump. With models computed using the Toulouse-Geneva Evolution Code, we show that atomic diffusion changes the abundance profiles inside the star, leading to an overabundance of the iron-peak elements in the upper envelope. The opacity may reach the amount required by seismic studies, provided that fingering mixing, which extends the size of the overabundance zone, is taken into account. Mass-loss is also required to evolve the model until the end of the main sequence., Comment: Proceedings of the PHOST "Physics of Oscillating Stars" conference, in honour of Prof. H. Shibahashi, 2-7 Sept. 2018, Banyuls-sur-mer (France), Edited by J. Ballot, S. Vauclair, and G. Vauclair

The 16 Cyg system is composed of two solar analogs with similar masses and ages. A red dwarf is in orbit around 16 Cyg A whereas 16 Cyg B hosts a giant planet. The abundances of heavy elements are similar in the two stars but lithium is much more depleted in 16 Cyg B that in 16 Cyg A, by a factor of at least 4.7. The interest of studying the 16 Cyg system is that the two star have the same age and the same initial composition. The presently observed differences must be due to their different evolution, related to the fact that one of them hosts a planet contrary to the other one. We computed models of the two stars which precisely fit the observed seismic frequencies. We used the Toulouse Geneva Evolution Code (TGEC) that includes complete atomic diffusion (including radiative accelerations). We compared the predicted surface abundances with the spectroscopic observations and confirmed that another mixing process is needed. We then included the effect of accretion-induced fingering convection. The accretion of planetary matter does not change the metal abundances but leads to lithium destruction which depends on the accreted mass. A fraction of earth mass is enough to explain the lithium surface abundances of 16 Cyg B. We also checked the beryllium abundances. In the case of accretion of heavy matter onto stellar surfaces, the accreted heavy elements do not remain in the outer convective zones but they are mixed downwards by fingering convection induced by the unstable $\mu$-gradient. Depending on the accreted mass, this mixing process may transport lithium down to its nuclear destruction layers and lead to an extra lithium depletion at the surface. A fraction of earth mass is enough to explain a lithium ratio of 4.7 in the 16 Cyg system. In this case beryllium is not destroyed. Such a process may be frequent in planet host stars and should be studied in other cases in the future., Comment: 8 pages, 10 figures, publication in A&A

Iron-rich layers are known to form in the stellar subsurface through a combination of gravitational settling and radiative levitation. Their presence, nature and detailed structure can affect the excitation process of various stellar pulsation modes, and must therefore be modeled carefully in order to better interpret Kepler asteroseismic data. In this paper, we study the interplay between atomic diffusion and fingering convection in A-type stars, and its role in the establishment and evolution of iron accumulation layers. To do so, we use a combination of three-dimensional idealized numerical simulations of fingering convection, and one-dimensional realistic stellar models. Using the three-dimensional simulations, we first validate the mixing prescription for fingering convection recently proposed by Brown et al. (2013), and identify what system parameters (total mass of iron, iron diffusivity, thermal diffusivity, etc.) play a role in the overall evolution of the layer. We then implement the Brown et al. (2013) prescription in the Toulouse-Geneva Evolution code to study the evolution of the iron abundance profile beneath the stellar surface. We find, as first discussed by Th\'eado et al. (2009), that when the concurrent settling of helium is ignored, this accumulation rapidly causes an inversion in the mean molecular weight profile, which then drives fingering convection. The latter mixes iron with the surrounding material very efficiently, and the resulting iron layer is very weak. However, taking helium settling into account partially stabilizes the iron profile against fingering convection, and a large iron overabundance can accumulate. The opacity also increases significantly as a result, and in some cases ultimately triggers dynamical convection., Comment: 38 pages, 16 figures, submitted to ApJ

Rotation is thought to drive cyclic magnetic activity in the Sun and Sun-like stars. Stellar dynamos, however, are poorly understood owing to the scarcity of observations of rotation and magnetic fields in stars. Here, inferences are drawn on the internal rotation of a distant Sun-like star by studying its global modes of oscillation. We report asteroseismic constraints imposed on the rotation rate and the inclination of the spin axis of the Sun-like star HD 52265, a principal target observed by the CoRoT satellite that is known to host a planetary companion. These seismic inferences are remarkably consistent with an independent spectroscopic observation (rotational line broadening) and with the observed rotation period of star spots. Furthermore, asteroseismology constrains the mass of exoplanet HD 52265b. Under the standard assumption that the stellar spin axis and the axis of the planetary orbit coincide, the minimum spectroscopic mass of the planet can be converted into a true mass of 1.85 (+0.52,-0.42) M_Jupiter, which implies that it is a planet, not a brown dwarf., Comment: Published in Proceedings of the National Academy of Sciences (5 pages, 5 figures, 3 tables). Available at http://www.pnas.org/cgi/doi/10.1073/pnas.1303291110

We have derived a new expression for the thermohaline mixing coefficient in stars, including the effects of radiative levitation and external turbulence, by solving Boussinesq equations in a quasi-incompressible fluid with a linear approximation. It is well known that radiative levitation of individual elements can lead to their accumulation in specific stellar layers. In some cases, it can induce important effects on the stellar structure. Here we confirm that this accumulation is moderated by thermohaline convection due to the resulting inverse $\mu$-gradient. The new coefficient that we have derived shows that the effect of radiative accelerations on the thermohaline instability itself is small. This effect must however be checked in all computations. We also confirm that the presence of large horizontal turbulence can reduce or even suppress the thermohaline convection. These results are important as they concern all the cases of heavy element accumulation in stars. The computations of radiative diffusion have to be revisited including thermohaline convection and its consequences. It may be one of the basic reasons for the fact that the observed abundances are always smaller than those predicted by pure atomic diffusion. In any case, these processes have to compete with rotation-induced mixing, but this competition is more complex than previously thought due to their mutual interaction., Comment: 15 pages, 3 figures, to be published in Ap.J

The early evolution of planetary systems is expected to depend on various periods of disk matter accretion onto the central star, which may include the accretion of metal-rich matter after the star settles on the main sequence. When this happens, the accreted material is rapidly mixed within the surface convective zone and induces an inverse mean-molecular-weight gradient, unstable for thermohaline convection. The induced mixing, which dilutes the metal excess, may also have important consequences on the light elements abundances. We model and analyze this process, and present the results according to various possible accretion scenarios. We give a detailed discussion of the different ways of treating thermohaline mixing, as proposed by previous authors, and we converge on a consistent view, including the most recent numerical simulations. We show how the observations of light elements in stars can be used as tracers of such events., Comment: 26 pages, 15 figures, accepted for publication in ApJ

The abundances of the chemical elements observed at the surface of metal-poor stars are not always representative of their initial values. During stellar evolution, various physical processes modify their internal composition. In this short paper, in honor of George W. Preston, I remind the importance of atomic diffusion, and the possible effects of accretion processes which may lead to double-diffusive (thermohaline) instabilities. I discuss the consequences of these processes and compare them to current observations., Comment: To appear in the Carnegie Observatories Astrophysics Series, volume 5

Atomic diffusion may lead to heavy element accumulation inside stars in certain specific layers. Iron accumulation in the Z-bump opacity region has been invoked by several authors to quantitatively account for abundance anomalies observed in some stars, or to account for stellar oscillations through the induced $\kappa$-mechanism. These authors however never took into account the fact that such an accumulation creates an inverse $\mu$-gradient, unstable for thermohaline convection. Here, we present results for A-F stars, where abundance variations are computed with and without this process. We show that iron accumulation is still present when thermohaline convection is taken into account, but much reduced compared to when this physical process is neglected. The consequences of thermohaline convection for A-type stars as well as for other types of stars are presented, Comment: 19 pages, 8 figures Accepted for publication in Astrophysical Journal, August 4, 2009

Studying the internal structure of exoplanet-host stars compared to that of similar stars without detected planets is particularly important for the understanding of planetary formation. The observed average overmetallicity of stars with planets is an interesting point in that respect. In this framework, asteroseismic studies represent an excellent tool to determine the structural differences between stars with and without detected planets. It also leads to more precise values of the stellar parameters like mass, gravity, effective temperature, than those obtained from spectroscopy alone. Interestingly enough, the detection of stellar oscillations is obtained with the same instruments as used for the discovery of exoplanets, both from the ground and from space. The time scales however are very different, as the oscillations of solar type stars have periods around five to ten minutes, while the exoplanets orbits may go from a few days up to many years. Here I discuss the asteroseismology of exoplanet-host stars, with a few examples., Comment: to appear in the proceedings of the Les Houches Winter School "Physics and Astrophysics of Planetary Systems", (EDP Sciences: EAS Publications Series)

A detailed high-resolution spectroscopic analysis is presented for the carbon-rich low metallicity Galactic halo object CS 22964-161. We have discovered that CS 22964-161 is a double-lined spectroscopic binary, and have derived accurate orbital components for the system. From a model atmosphere analysis we show that both components are near the metal-poor main-sequence turnoff. Both stars are very enriched in carbon and in neutron-capture elements that can be created in the s-process, including lead. The primary star also possesses an abundance of lithium close to the value of the ``Spite-Plateau''. The simplest interpretation is that the binary members seen today were the recipients of these anomalous abundances from a third star that was losing mass as part of its AGB evolution. We compare the observed CS 22964-161 abundance set with nucleosynthesis predictions of AGB stars, and discuss issues of envelope stability in the observed stars under mass transfer conditions, and consider the dynamical stability of the alleged original triple star. Finally, we consider the circumstances that permit survival of lithium, whatever its origin, in the spectrum of this extraordinary system., Comment: manuscript, 7 tables, 13 figures. ApJ, in press

Studying the internal structure of exoplanets-host stars compared to that of similar stars without detected planets is particularly important for the understanding of planetary formation. The observed overmetallicity of stars around which planets have been detected may be a hint in that respect. In this framework, asteroseismic studies represent an excellent tool to determine the structural differences between stars with and without detected planets. After a general discussion on this subject, I present the special cases of three different stars: $\mu$ Arae which has been observed with the HARPS spectrograph in June 2004, $\iota$ Horologii, that we have studied in detail and will be observed with HARPS in November 2006, and finally HD 52265, one of the main targets of the COROT mission, an exoplanets-host star which will be observed with the COROT satellite during five consecutive months., Comment: To appear in "Beyond the Spherical Sun, a new era for helio- and asteroseismology", SOHO-GONG-HELAS, Sheffield, UK, 7-11 October 2006

Studying the internal structure of exoplanets-host stars compared to that of similar stars without detected planets is particularly important for the understanding of planetary formation. The observed overmetallicity of stars with planets may be a hint in that respect. Although it is obviously related to the physical processes which occur during the early phases of planetary formation, the origin of this overmetallicity is unclear. It may be either primordial or related to accretion processes or both. In this framework, asteroseismic studies represent an excellent tool to determine the structural differences between stars with and without detected planets. The two different missions of COROT are linked in this programme: the detection of new planets and the seismic studies of their host stars share the same goal of a better understanding of planetary formation and evolution. The COROT main target HD52265, which is known to host at least one giant planet, will be observed continuously during five months: many interesting results are expected from this long run. Meanwhile, stellar oscillations will be searched for in all stars around which new planets will be discovered., Comment: To appear in: "The CoRoT Book", (Eds) F. Favata, A. Baglin & J. Lochard, ESA Publications Division, ESA SP

We present HARPS spectroscopy of mu Arae (HD160691) performed for studying the origin of the metallicity excess in this planet-hosting stars. The asteroseismologic campaign led to the previously reported discovery of a 14 earth mass planetary companion (Santos et al. 2004). The present analysis reinforces this interpretation by excluding other possible processes for explaining the observed Doppler variation and leads to the identification of up to 43 p-mode oscillations with l=0-3, frequencies in the range 1.3-2.5 mHz and amplitudes in the range 10-40 cm/s.

Asteroseismology is found to be a excellent tool for detecting diffusion-induced helium gradients inside main-sequence A stars. Models have been computed for 1.6 and 2.0 M$_{\odot}$ stars with pure helium diffusion, at different ages, so that the helium gradient lies at different depths inside the star. The adiabatic oscillation frequencies have been analysed and compared with those of a model without diffusion. Clear signatures of the diffusion-induced helium gradient are found in the so-called ``second differences" : these frequency differences present modulations due to the partial reflexion of the sound waves on the layer where the helium gradient takes place. A tentative application to the roAp star HD60435, which presents enough detected oscillation frequencies for the test to be possible, is very encouraging. The results suggest the presence of a helium gradient inside the star, which is consistent with the idea that the triggering of the oscillations is due to the hydrogen $\kappa$-mechanism., Comment: to be published in A&A ; 17 pages ; 8 figures

Central stars of extra-solar planetary systems are metal-rich. Planet accretion or initial surmetallicity can explain this observationnal fact. These scenarios can be tested with asteroseismology. We calibrate two stellar models, one with accretion and one with high initial metallicity, in order to obtain the same external parameters for both of them. We then compare their internal structures and their oscillation frequencies., Comment: 4 pages, 2 figures, ASP Conferences Series, D.W. Kurtz & Karen Pollarrd, Eds

The element settling due to the combined effects of gravity, thermal gradient, radiative acceleration and concentration gradient may lead to important abundance variations inside the stars, which cannot be neglected in the computations of stellar structure. These processes where first introduced to account for abundance anomalies in "peculiar stars", but their importance in the so-called "normal" stars is now fully acknowledged, specially after the evidence of helium settling in the Sun from helioseismology. These microscopic processes work in competition with macroscopic motions, like rotation-induced mixing or mass loss, which increase the settling time scales. We have recently obtained clear evidences that asteroseismology of main sequence solar type stars can give signatures of the chemical variations inside the stars and help for a better understanding of these processes., Comment: 10 pages, 2 figures, proceedings of the IAU colloquium 193 "variable stars in the local group", ed. D.W. Kurtz and K. Pollard, in press

While the metallicity excess observed in the central stars of planetary systems is confirmed by all recent observations, the reason for this excess is still a subject of debate : is it primordial, is it the result of accretion or both? The basic argument against an accretion origin is related to the masses of the outer convective zones which vary by more than one order of magnitude among the considered stars while the observed overabundances of metals are similar. We show here that in previous discussions a fundamental process was forgotten : thermohaline convection induced by the inverse mu-gradient. "Metallic fingers" may be created which dilute the accreted matter inside the star. Introducing this effect may reconcile the overabundances expected in case of accretion with the observations in stars of different masses., Comment: 15 pages, 3 figures. to be published in Ap.J, April 20, 2004

In the first paper of this series, we discussed analytically, in an approximate way, the mixing processes which occur in slowly rotating stars when the feed-back effects due to the diffusion-induced $\mu$-gradients are introduced in the computations. We found that the classical scheme of meridional circulation was dramatically modified, with important possible consequences. Here we present a complete 2D numerical simulation of these processes in a static stellar model. The helium microscopic diffusion and the meridional advection are computed simultaneously, so that we can follow the evolution of the abundances and mixing processes in the radiative zone. Most of the effects discussed in paper I are confirmed. Some consequences will be discussed in a forthcoming paper (Th\'eado and Vauclair 2002, paper III)., Comment: 11 printed pages, 9 figures. ApJ, in press, April 20, 2003

In the two previous papers of this series, we have discussed the importance of t he $\mu$-gradients due to helium settling on rotation-induced mixing, first in a n approximate analytical way, second in a 2D numerical simulation. We have found that, for slowly rotating low mass stars, a process of ``creeping paralysis" in which the circulation and the diffusion are nearly frozen may take place below the convective zone. Here we apply this theory to the case of lithium and beryll ium in galactic clusters and specially the Hyades. We take into account the rota tional braking with rotation velocities adjusted to the present observations. We find that two different cells of meridional circulation appear on the hot side of the "lithium dip" and that the "creeping paralysis" process occurs, not dir ectly below the convective zone, but deeper inside the radiative zone, at the to p of the second cell. As a consequence, the two cells are disconnected, which ma y be the basic reason for the lithium increase with effective temperature on thi s side of the dip. On the cool side, there is just one cell of circulation and t he paralysis has not yet set down at the age of the Hyades; the same modelisatio n accounts nicely for the beryllium observations as well as for the lithium ones ., Comment: 13 printed pages, 10 figures. ApJ, in press (April 20, 2003)

In the presence of rotation-induced mixing, element diffusion still occurs in stellar radiative zones, although at a slower rate than in the case of a complete stability of the stellar gas. As a consequence, helium settling leads to vertical $\displaystyle \mu $-gradients which, due to the meridional circulation, turn into horizontal fluctuations. Up to now, the feed-back effect of this process on the rotation-induced mixing was currently neglected in the computations of abundance variations in stellar surfaces, or artificially reduced. Here we analyse its consequences and derive an approximate analytical solution in a quasi-stationary case. We also discuss the relative importance of the various physical effects which influence the meridional circulation velocity. In a second paper (Th\'eado and Vauclair 2002a), we will present a complete 2D numerical simulation of this process while a third paper (Th\'eado and Vauclair 2002b) will be devoted to special applications to Pop I stars., Comment: 7 printed pages, 1 figure. ApJ, in press (April 20, 2003)

The lithium plateau observed in halo stars has long appeared as a paradox in the general context of the lithium abundance behavior in stellar outer layers. First, the plateau is flat, second, the lithium abundance dispersion is extremely small. This seems in contradiction with the large lithium variations observed in younger stars. It is also difficult to understand theoretically as lithium nuclei are easily destroyed by nuclear reactions, and subject to microscopic diffusion which, in the case of halo stars, should also lead to depletion. Several ideas have been proposed to account for the lithium behavior in halo stars. The most promising possibilities were rotational-induced mixing, which could reduce lithium in the same way for all the stars, and mass-loss, which could oppose the lithium settling. In both cases however, the parameters should be tightly adjusted to prevent any dispersion in the final results. Vauclair 1999 (paper I) pointed out that the mu-gradient terms which appear in the computations of the meridional circulation velocity were not introduced in previous computations of rotationally-induced mixing. This can lead to a self-regulating process which reduces the efficiency of the meridional circulation as well as the microscopic diffusion. Here we present numerical computations of this process and its influence on the lithium abundance variations in halo stars. We show that in slowly rotating stars, under some conditions, lithium can be depleted by a factor of up to two with a dispersion smaller than 0.1 dex in the middle part of the lithium plateau. We derive a primordial lithium abundance of 2.5 +/- 0.1, consistent with the recent determinations of D/H and 4He/H., Comment: 15 pages, 10 figures. to be published in A&A

The application to main-sequence stars of the rotation-induced mixing theory in the presence of mu-gradients leads to partial mixing in the lithium destruction region, not visible in the atmosphere. The induced lithium depletion becomes visible in the sub-giant phase as soon as the convective zone deepens enough. This may explain why the observed " lithium dilution " is smoother and the final dilution factor larger than obtained in standard models, while the lithium abundance variations are very small on the main sequence., Comment: to appear in "The Light Elements and their Evolution", ASP conf. Series

The observations of light elements in the Sun and Solar type stars give special clues for understanding the hydrodynamical processes at work in stellar interiors. In the Sun 7Li is depleted by 140 while 3He has not increased by more than 10% in 3 Gyrs. Meanwhile the inversion of helioseismic modes lead to a precision on the sound velocity of about .1%. The mixing processes below the solar convection zone are constrained by these observations. Lithium is depleted in most Pop I solar type stars. In halo stars however, the lithium abundance seems constant in the "spite plateau" with no observed dispersion, which is difficult to reconcile with the theory of diffusion processes. In the present paper, the various relevant observations will be discussed. It will be shown that the mu-gradients induced by element settling may help solving the "lithium paradox"., Comment: to appear in "The Light Elements and their Evolution", ASP conf. Series

It has been known for a long time (Mestel~1953) that the meridional circulation velocity in stars, in the presence of mu-gradients, is the sum of two terms, one due to the classical thermal imbalance (Omega-currents) and the other one due to the induced horizontal mu-gradients (mu-induced currents, or mu-currents in short). In the most general cases, mu-currents are opposite to Omega-currents. Vauclair (1999) has shown that such processes can, in specific cases, lead to a quasi-equilibrium stage in which both the circulation and the helium settling is frozen. Here we present computations of the circulation currents in halo star models, along the whole evolutionary sequences for four stellar masses with a metallicity of [Fe/H] = -2. We show that such a self-regulated process can account for the constancy of the lithium abundances and the small dispersion in the Spite plateau., Comment: to appear in "The Light Elements and their Evolution", ASP conf. Series

It has been known for a long time (Mestel 1953) that the meridional circulation velocity in stars, in the presence of mu-gradients, is the sum of two terms, one due to the classical thermal imbalance (omega-currents) and the other one due to the induced horizontal mu-gradients (mu-induced currents, of mu-currents in short). In the most general cases, mu-currents are opposite to omega-currents. Simple expressions for these currents are derived under some simplifying physical assumptions presented in the text, and their physical interpretations are discussed. Computations of the omega and mu-currents in a 0.8 Mo halo stellar model including classical element settling show that the mu-currents are larger (in absolute values) than the omega-currents in all the star : some new physics has thus to be invoked in this case. We show here how such processes could possibly lead to a quasi-equilibrium stage in which both the circulation and the helium settling could be cancelled out. As lithium diffuses in the same way as helium, we expect a very small lithium concentration gradient below the convective zone in "plateau stars" (main sequence PopII stars), much smaller than the one expected for pure element settling. This could possibly account for the very small dispersion observed for the lithium abundances at the surface of these stars. This should also have important consequences in other contexts which will be discussed in forthcoming papers. The present computations show that element settling in slowly rotating stars leads to surface abundances which depend on the competition between mu-currents and omega-currents, in a way which had not been taken into account in previous computations. This may change our general understanding of the diffusion processes of chemical species in rotating stars., Comment: 9 pages, 7 figures, to be published in A&A

While theoretical computations of the lithium abundances in Pop II stars predict variations from star to star, the most recent observations confirm the existence of a "lithium plateau" with no abundance dispersion. This apparent paradox has to be faced and a solution has to be found before deriving the primordial lithium abundance. In any case lithium 7 has been depleted in these stars by at least 20 percent and lithium 6 by at least a factor 2, even in the hottest stars of the "plateau". Consequences for other elements are also discussed., Comment: 7pages, 5figures, paper given at the LiBeB workshop, IAP, Paris, Dec'98

Element settling inside the Sun now becomes detectable from the comparison of the observed oscillation modes with the results of the theoretical models. This settling is due, not only to gravitation, but also to thermal diffusion and radiative acceleration (although this last effect is small compared to the two others). It leads to abundance variations of helium and heavy elements of $\cong 10%$ below the convective zone. Although not observable from spectroscopy, such variations lead to non-negligible modifications of the solar internal structure and evolution. Helioseismology is a powerful tool to detect such effects, and its positive results represent a great success for the theory of stellar evolution. Meanwhile, evidences are obtained that the element settling is slightly smoothed down, probably due to mild macroscopic motions below the convective zone. Additional observations of the abundances of both Solar $^7$Li and $^3$He lead to specific constraints on these particular motions., Comment: 9 pages, 2 figures, paper presented at the workshop on Solar Composition and its Evolution - From Core to Corona, Berne, 26-30 January 98

Improvements induced by helioseismology show that microscopic diffusion cannot be neglected in solar models. Helium and metal diffusion is now generally introduced in the computations. They cannot however explain the observed lithium depletion. Macroscopic motions are needed to bring up to the convection zone the nuclearly depleted matter. Such motions also lead to a better agreement between the sound velocity in the model and that deduced from the inversion of seismic modes. On the other hand, they must not bring up too much $^3$He, as we now know that the $^3$He/$^4$He ratio cannot have changed by more than 10 percent during these last 3 Gyr. Here we present new consistent solar models in which the $^3$He constraint is satisfied. The fact that $^3$He must remain constant while $^7$Li must be depleted leads to strong constraints on the effective macroscopic diffusion coefficient allowed below the solar convection zone., Comment: 3 pages with 4 figures included, To appear in SOHO6/GONG98 Proceedings "Structure and Dynamics of the Interior of the Sun and Sun-like Stars", ESA Publications Division, ESA SP-418

Observational constraints on the primordial lithium abundance are important for the evaluation of the baryonic density of the universe. Its precise determination however suffers from uncertainties due to the possible depletion of this element inside the stars. Here we present and discuss new results on the lithium abundances in pop II stars obtained with most recent stellar models including the best available physics. We show that it is possible to account for the general behavior of lithium observed in pop II stars without any free parameters. Macroscopic motions inside the stars are needed but it is not necessary to specify their exact nature to interpret the observational data. This constatation allows to derive a parameter-free value for the lithium primordial abundance : log(Lip) = 2.35 +/- .10 in the log(H) = 12 scale., Comment: 5 pages, 3 figures, to be published in Ap.J