CAUP Researchers: Nuno C. Santos, Sérgio A. G. Sousa
Enhanced lithium depletion in Sun-like stars with orbiting planets,
The surface abundance of lithium on the Sun is 140 times less than the protosolar value, yet the temperature at the base of the surface convective zone is not hot enough to burn-and hence deplete-Li. A large range of Li abundances is observed in solar-type stars of the same age, mass and metallicity as the Sun, but such a range is theoretically difficult to understand. An earlier suggestion that Li is more depleted in stars with planets was weakened by the lack of a proper comparison sample of stars without detected planets. Here we report Li abundances for an unbiased sample of solar-analogue stars with and without detected planets. We find that the planet-bearing stars have less than one per cent of the primordial Li abundance, while about 50 per cent of the solar analogues without detected planets have on average ten times more Li. The presence of planets may increase the amount of mixing and deepen the convective zone to such an extent that the Li can be burned.
Some recent studies have suggested a connection between lithium depletion in solar-like stars and the presence of planets around them but these attempts were hindered by the lack of a reliable sample of planet-free stars. In this work, we use an unbiased sample of solar-like stars to improve results obtained by previous attempts to establish such relation. The sample used consists of 451 stars in the HARPS high-precision radial velocity exoplanet survey, for which high-resolution and high signal-to-noise spectra were available. These stars are non-evolved, slowly rotating non-active stars with effective temperature (Teff) ranging from 4,900 K to 6,500 K. Of these 451 stars, 70 are reported to host planets and the rest have no detected planets, so far, in spite of having been monitored with high-precision spectroscopic observations for years. We use this comparison sample to show that the reason for this extra Li depletion is not related to high metallicity (characteristic of planet-hosting stars) or to age (old stars are more Li-depleted).
Standard prescriptions for stellar models, spectral synthesis code and stellar parameter determination were followed by the researchers in CAUP, which obtained the required parameters, such as Teff, surface gravity or metallicity. These parameters allowed the selection of the stars which most resemble the Sun, as well as the study of their Li abundances.
The results show that, when considering only the 133 stars - of which 30 are known to host planets - with Teff in the range 5,600 - 5,900 K, the majority of planet-hosting stars have severely depleted Li, whereas in the comparison sample a large fraction has only partially inhibited depletion. In order to improve the statistical significance of our results, we added 16 planet-hosting and 13 comparison-sample stars, for which we have obtained new Li abundances from high-quality spectroscopic observations. At higher and lower temperatures, planet-hosting stars do not appear to show any peculiar behavior in their Li abundance. The explanation of Li survival at Teff > 5,850 K is that the convective layers of stars more massive than the Sun are shallow and too remote to reach the Li-burning layers. For stars with Teff < 5,700 K, we note that they have deeper convective layers that transport surface material to high-temperature regions in their interiors, where Li can be destroyed more efficiently.
The Li abundance of some 20% of stars with exoplanets in the temperature range 5,600-5,900 K is log[N(Li)]≥1.5, while for the 116 comparison stars the Li abundance shows a rather high dispersion, with some 43% of the stars displaying Li abundances of log[N(Li)]≥1.5. This result becomes more obvious in solar-analogue stars, for which some 50% of the 60 single stars in the narrow window of TSun±80 K (with TSun=5,777 K) have log[N(Li)]≥1.5, while only two planet-hosting stars out of 24 have log[N(Li)]≥1.5 (Fig. 1).
We also investigated the impact of the age of the stars on our sample, using chromospheric activity indices and stellar rotation but found no correlation. As most of the planet-hosting stars discovered so far are metal-rich, we also determined if high metallicity could be responsible for enhanced Li depletion in these stars. Our data show that the fraction of single stars with log[N(Li)]>1.5 is 50% at both [Fe/H]<0 and [Fe/H]>0. This suggests that the Li-depletion mechanism does not depend on the metallicity in the range -0.5< [Fe/H] >+0.5. Comparison with field stars then leads us to the conclusion that neither age nor metallicity is responsible for the excess Li depletion. This is reinforced by observations of Li in solar-type stars in old clusters, which indeed show a wide dispersion of Li abundances with values ranging from log[N(Li)]=2.5 down to 1.0 and lower.
The discussion of the results, which also received an important contribution from the CAUP team, led to the proposal that the low Li abundance of planet-hosting solar-analogue stars is directly associated with the presence of planets through the effect these have in the angular momentum of the star, which may indirectly cause a violent diffusion mixing. Alternatively, a long-lasting star-disk interaction during the pre-main sequence may cause a high degree of differential rotation between the radiative core and the convective envelope, also leading to enhanced Li depletion.