Accretion of planetary matter and the lithium problem in the 16 Cygni stellar system

Abstract

Context. The 16 Cygni system is composed of two solar analogues with similar masses and ages. A red dwarf is in orbit around 16 Cygni A, and 16 Cygni B hosts a giant planet. The abundances of heavy elements are similar in the two stars, but lithium is much more depleted in 16 Cygni B than in 16 Cygni A, by a factor of at least 4.7. Aims. The interest of studying the 16 Cygni system is that the two star have the same age and the same initial composition. The differences currently observed must be due to their different evolution, related to the fact that one of them hosts a planet while the other does not. Methods. We computed models of the two stars that precisely fit the observed seismic frequencies. We used the Toulouse Geneva Evolution Code (TGEC), which includes complete atomic diffusion (including radiative accelerations). We compared the predicted surface abundances with the spectroscopic observations and confirm that another mixing process is needed. We then included the effect of accretion-induced fingering convection. Results. The accretion of planetary matter does not change the metal abundances but leads to lithium destruction, which depends upon the accreted mass. A fraction of the Earth's mass is enough to explain the lithium surface abundances of 16 Cygni B. We also checked the beryllium abundances. Conclusions. In the case of accretion of heavy matter onto stellar surfaces, the accreted heavy elements do not remain in the outer convective zones, but are mixed downwards by fingering convection induced by the unstable μ-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 the Earth's mass is enough to explain a lithium ratio of 4.7 in the 16 Cygni system. In this case beryllium is not destroyed. Such a process may be frequent in planet-hosting stars and should be studied in other cases in the future.

Five transiting hot Jupiters discovered using WASP-South,Euler, and TRAPPIST: WASP-119 b, WASP-124 b, WASP-126 b, WASP-129 b, and WASP-133 b

“…The surface lithium abundance of WASP-133 A (A(Li) = 2.7) is significantly higher than other stars of similar age and effective temperature (A(Li) < 2.5, Sestito & Randich 2005). This is a counter-example of the general trend for planet host stars to be depleted in lithium compared to similar non-planet host stars (Maxted et al 2010;Gonzalez 2015;Deal et al 2015). Whether this anomaly is connected to the slow rotation of this star or some other factor is difficult to ascertain from the consideration of a single system.…”

In-depth study of 16CygB using inversion techniques

“…Therefore, a first step would be to perform a thorough study of the impact of extra mixing on lithium abundances and inversion results for 16CygA. The case of 16CygB should be re-analysed afterwards, since it is well-known that this star shows even lower lithium abundances and is believed to have triggered thermohaline diffusion by accreting planetary matter (See Deal et al 2015). As such, combining spectroscopic and seismic constraints in this binary system may provide new insights on stellar modelling of solar-like stars.…”

Lithium abundance and6Li/7Li ratio in the active giant HD 123351

“…It is even possible that the accretion mechanism would not lead to the desired Li enrichment, but to an additional destruction of lithium (see Deal et al 2015).…”