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Aims. We report the detection of an unusually high lithium content in HD 232 862, a field giant classified as a G8II star, and hosting a magnetic field.

Precise Li abundances are determined for 54 giant stars mostly evolving across the Hertzsprung gap. We combine these data with rotational velocity and with information related to the deepening of the convective zone of the stars to analyse their link to Li dilution in the referred spectral region. A sudden decline in Li abundance paralleling the one already established in rotation is quite clear. Following similar results for other stellar luminosity classes and spectral regions, there is no linear relation between Li abundance and rotation, in spite of the fact that most of the fast rotators present high Li content. The effects of convection in driving the Li dilution is also quite clear. Stars with high Li content are mostly those with an undeveloped convective zone, whereas stars with a developed convective zone present clear sign of Li dilution.

The connection rotation–CaII emission flux–lithium abundance is analyzed for a sample of bona fide subgiant stars, with evolutionary status determined from HIPPARCOS trigonometric parallax measurements and from the Toulouse–Geneva code. The distribution of rotation and CaII emission flux as a function of effective temperature shows a discontinuity located around the same spectral type, F8IV. Blueward of this spectral type, subgiants have a large spread of values of rotation and CaII flux, whereas stars redward of F8IV show essentially low rotation and low CaII flux. The strength of these declines depends on stellar mass. The abundance of lithium also shows a sudden decrease. For subgiants with mass lower than about 1.2 $M_{\odot}$the decrease is located later than that in rotation and CaII flux, whereas for masses higher than 1.2 $M_{\odot}$the decrease in lithium abundance is located around the spectral type F8IV. The discrepancy between the location of the discontinuities of rotation and CaII emission flux and $\log~n$(Li) for stars with masses lower than 1.2 $M_{\odot}$seems to reflect the sensitivity of these phenomena to the mass of the convective envelope. The drop in rotation, which results mostly from a magnetic braking, requires an increase in the mass of the convective envelope less than that required for the decrease in $\log~n$(Li). The location of the discontinuity in $\log~n$(Li) for stars with masses higher than 1.2 $M_{\odot}$, in the same region of the discontinuities in rotation and CaII emission flux, may also be explained by the behavior of the deepening of the convective envelope. The more massive the star is, the earlier is the increase of the convective envelope. In contrast to the relationship between rotation and CaII flux, which is fairly linear, the relationship between lithium abundance and rotation shows no clear tendency toward linear behavior. Similarly, no clear linear trend is observed in the relationship between lithium abundance and CaII flux. In spite of these facts, subgiants with high lithium content also have high rotation and high CaII emission flux.

We analyse the behavior of lithium abundance as a function of effective temperature, projected rotational velocity, orbital period and eccentricity for a sample of 68 binary systems with giant component and orbital period ranging from about 10 to 6400 days. For these binary systems the Li abundances show a gradual decrease with temperature, paralleling the well established result for single giants. We have also observed a dependence of lithium content on rotation. Binary systems with moderate to high rotation present also moderate to high Li content. This study shows also that synchronized binary systems with giant component seem to retain more of their original lithium than the unsynchronized systems. For orbital periods lower than 100 to 250 days, typically the period of synchronization for this kind of binary systems, lithium depleted stars seems to be unusual. The suggestion is made that there is an “inhibited zone" in which synchronized binary systems with giant component having lithium abundance lower than a threshold level should be unusual.

The growing interest in solar twins is motivated by the possibility of comparing them directly to the Sun. To carry on this kind of analysis, we need to know their physical characteristics with precision. Our first objective is to use asteroseismology and interferometry on the brightest of them: 18 Sco. We observed the star during 12 nights with HARPS for seismology and used the PAVO beam-combiner at CHARA for interferometry. An average large frequency separation 134.4 ± 0.3 μHz and angular and linear radiuses of 0.6759 ± 0.0062 mas and 1.010 ± 0.009 R⊙were estimated. We used these values to derive the mass of the star, 1.02 ± 0.03 M⊙.

Context. Rotational velocity, lithium abundance, and the mass depth of the outer convective zone are key parameters in the study of the processes at work in the stellar interior, in particular when examining the poorly understood processes operating in the interior of solar–analog stars.Aims. We investigate whether the large dispersion in the observed lithium abundances of solar-analog stars can be explained by the depth behavior of the outer convective zone masses, within the framework of the standard convection model based on the local mixing-length theory. We also analyze the link between rotation and lithium abundance in solar-analog stars.Methods. We computed a new extensive grid of stellar evolutionary models, applicable to solar-analog stars, for a finely discretized set of mass and metallicity. From these models, the stellar mass, age, and mass depth of the outer convective zone were estimated for 117 solar-analog stars, using Teff and [Fe/H] available in the literature, and the new HIPPARCOS trigonometric parallax measurements.Results. We determine the age and mass of the outer convective zone for a bona fide sample of 117 solar-analog stars. No significant one-to-one correlation is found between the computed convection zone mass and published lithium abundance, indicating that the large A(Li) dispersion in solar analogs cannot be explained by the classical framework of envelope convective mixing coupled with lithium depletion at the bottom of the convection zone.Conclusions. These results illustrate the need for an extra-mixing process to explain lithium behavior in solar-analog stars, such as, shear mixing caused by differential rotation. To derive a more realistic definition of solar-analog stars, as well as solar-twin stars, it seems important to consider the inner physical properties of stars, such as convection, hence rotation and magnetic properties.

Context. Rotational velocity, lithium abundance, and the mass depth of the outer convective zone are key parameters in the study of the processes at work in the stellar interior, in particular when examining the poorly understood processes operating in the interior of solar–analog stars.