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

Abstract

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.

8 pages, 10 figures, publication in A&A

References in corpus (8)

• Correcting stellar oscillation frequencies for near-surface effects
• Asteroseismology of the solar analogs 16 Cyg A & B from Kepler observations
• Chemical Transport and Spontaneous Layer Formation in Fingering Convection in Astrophysics
• High precision abundances in the 16 Cyg binary system: a signature of the rocky core in the giant planet
• Asteroseismic modeling of 16 Cyg A & B using the complete Kepler data set
• The new Toulouse-Geneva Stellar Evolution Code including radiative accelerations of heavy elements
• Lithium abundances in exoplanet-host stars : modelling
• Fingering convection induced by atomic diffusion in stars: 3D numerical computations and applications to stellar models

Cited by in corpus (26)

• High-precision abundances of elements in Kepler LEGACY stars. Verification of trends with stellar age
• Search for giant planets in M67 IV: survey results
• Revisiting the 16 Cygni planet host at unprecedented precision and exploring automated tools for precise abundances
• Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits
• Revisiting the pre-main-sequence evolution of stars II. Consequences of planet formation on stellar surface composition
• 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
• In-depth study of 16CygB using inversion techniques
• Higher metal abundances do not solve the solar problem
• Importance of fingering convection for accreting white dwarfs in the framework of full evolutionary calculations: the case of the hydrogen-rich white dwarfs GD133 and G29-38
• Thorough characterisation of the 16 Cygni system Part I: Forward seismic modelling with WhoSGlAd
• The chemical composition of $\alpha$ Cen AB revisited
• Hot Jupiter and ultra-cold Saturn formation in dense star clusters
• Evidences of extragalactic origin and planet engulfment in the metal-poor twin pair HD 134439/HD 134440
• The evolution of lithium in FGK dwarf stars: The Li rotation connection and the Li desert
• Testing abundance-age relations beyond solar analogues with Kepler LEGACY stars
• Asteroseismology of the exoplanet-host F-type star 94 Ceti : impact of atomic diffusion on the stellar parameters
• Thorough characterisation of the 16 Cygni system. Part II. Seismic inversions of the internal structure
• Spinning up the Surface: Evidence for Planetary Engulfment or Unexpected Angular Momentum Transport?
• Sun-like stars unlike the Sun: Clues for chemical anomalies of cool stars
• Matter accretion in metal-poor stars down to extremely metal-poor stars and the lithium problem
• Assessing the processes behind planet engulfment and its imprints
• Sub-stellar engulfment by a main sequence star: where is the lithium?
• Detecting active latitudes of Sun-like stars using asteroseismic a-coefficients
• Accretion of Planetary Material onto Host Stars
• iota Horologii is unlikely to be an evaporated Hyades star
• Progress in global helioseismology: a new light on the solar modelling problem and its implications for solar-like stars