Lithium Depletion in Solar-like Stars: Effect of Overshooting Based on Realistic Multi-dimensional Simulations

Abstract

We study lithium depletion in low-mass and solar-like stars as a function of time, using a new diffusion coefficient describing extra-mixing taking place at the bottom of a convective envelope. This new form is motivated by multidimensional fully compressible, time-implicit hydrodynamic simulations performed with the MUSIC code. Intermittent convective mixing at the convective boundary in a star can be modeled using extreme value theory, a statistical analysis frequently used for finance, meteorology, and environmental science. In this Letter, we implement this statistical diffusion coefficient in a one-dimensional stellar evolution code, using parameters calibrated from multi-dimensional hydrodynamic simulations of a young low-mass star. We propose a new scenario that can explain observations of the surface abundance of lithium in the Sun and in clusters covering a wide range of ages, from ∼50 Myr to ∼4 Gyr. Because it relies on our physical model of convective penetration, this scenario has a limited number of assumptions. It can explain the observed trend between rotation and depletion, based on a single additional assumption, namely, that rotation affects the mixing efficiency at the convective boundary. We suggest the existence of a threshold in stellar rotation rate above which rotation strongly prevents the vertical penetration of plumes and below which rotation has small effects. In addition to providing a possible explanation for the long-standing problem of lithium depletion in pre-main-sequence and main-sequence stars, the strength of our scenario is that its basic assumptions can be tested by future hydrodynamic simulations.

Lithium-rotation connection in the newly discovered young stellar stream Psc–Eri (Meingast 1)

“…Assuming a relationship between rotation and magnetic activity during the PMS, the slow, more moderately active stars would not experience radius inflation, and would thus start to deplete lithium earlier. Finally, Baraffe et al (2017) argued that stellar rotation has a direct impact on the efficiency of internal transport processes, and proposed that fast rotation reduces the penetration of convective plumes into the radiative core, thus leading to a lower depletion rate in fast rotators. Any of these models, or combination of them, has the potential to explain the lithium-rotation connection reported here and in previous studies.…”

Kronos and Krios: Evidence for Accretion of a Massive, Rocky Planetary System in a Comoving Pair of Solar-type Stars

“…The element Li is worth special attention in the context of the accretion scenario. Because Li is present in either carbonaceous chondrites or bulk Earth with a concentration of 1 − 1.5 ppm in mass (McDonough 2003), but is depleted quickly within the first Gyr on the surface of a Sun-like star (Thévenin et al 2017;Baraffe et al 2017), accretion of either material at later times will significantly replenish the lithium on the star's surface. For the present-day Sun (A(Li) = 1.05), the accretion of 15 M ⊕ of bulk Earth-like material would result in ∆[Li/H] ≈ 1.65 dex (see the inset of Figure 9).…”

Impact of convective boundary mixing on the TP-AGB

“…The treatment of mixing at convective boundaries presents a challenge which, despite the advent of 3D hydrodynamical simulations (Hurlburt et al 1994;Freytag et al 1996;Herwig et al 2007; Baraffe et al 2017), has proven to be a persistent source of uncertainties in 1D stellar evolution models. This is a fact which can become especially relevant when considering the evolution during the thermally pulsing Asymptotic Giant Branch (TP-AGB), where boundaries at the edge of convective zones are known to play an important E-mail: wagstaff@mpa-garching.mpg.de † E-mail: marcelo@mpa-garching.mpg.de part in governing many important properties and observables of these stars (Herwig et al 1997;Herwig 2005), such as third dredge-up (TDU) and the Initial-Final Mass Relation (IFMR).…”

“…This is a fact which can become especially relevant when considering the evolution during the thermally pulsing Asymptotic Giant Branch (TP-AGB), where boundaries at the edge of convective zones are known to play an important E-mail: wagstaff@mpa-garching.mpg.de † E-mail: marcelo@mpa-garching.mpg.de part in governing many important properties and observables of these stars (Herwig et al 1997;Herwig 2005), such as third dredge-up (TDU) and the Initial-Final Mass Relation (IFMR). Certainly, it has been seen in hydrodynamic simulations (Freytag et al 1996;Herwig et al 2007;Meakin & Arnett 2007;Mocák et al 2009;Baraffe et al 2017;Pratt et al 2017) that the strict Schwarzschild boundary, as implemented in 1D stellar evolution codes, simply does not appear in the form of a composition discontinuity in a spherically symmetric manner when it comes to multi-dimensional models.…”