Stellar Hydrodynamics in Radiative Regions

Abstract

We present an analysis of the response of a radiative region to waves generated by a convective region of the star; this wave treatment of the classical problem of ``overshooting'' gives extra mixing relative to the treatment traditionally used in stellar evolutionary codes. The interface between convectively stable and unstable regions is dynamic and nonspherical, so that the nonturbulent material is driven into motion, even in the absence of ``penetrative overshoot.'' These motions may be described by the theory of nonspherical stellar pulsations, and are related to motion measured by helioseismology. Multi-dimensional numerical simulations of convective flow show puzzling features which we explain by this simplified physical model. Gravity waves generated at the interface are dissipated, resulting in slow circulation and mixing seen outside the formal convection zone. The approach may be extended to deal with rotation and composition gradients. Tests of this description in the stellar evolution code TYCHO produce carbon stars on the asymptotic giant branch (AGB), an isochrone age for the Hyades and three young clusters with lithium depletion ages from brown dwarfs, and lithium and beryllium depletion consistent with observations of the Hyades and Pleiades, all without tuning parameters. The insight into the different contributions of rotational and hydrodynamic mixing processes could have important implications for realistic simulation of supernovae and other questions in stellar evolution.Comment: 27 pages, 5 figures, accepted to the Astrophysical Journa

Dust in brown dwarfs and extra-solar planets

“…The grains shrink in size and dissolve into the surrounding hot and convective gas (the Schwarzschild pressure, where the atmosphere becomes convectively unstable, is 3.54 bar in this model). These results reflect the stationary character of the dust component in substellar atmospheres, where dusty material constantly forms at high altitudes and settles downward, and simultaneously, fresh uncondensed material is mixed up by convective motion and overshoot (see Ludwig et al 2006Ludwig et al , 2006Young et al 2003). These general results resemble well the results of Paper III, where only one sample dust species, TiO 2 [s], was considered.…”

Deep inside low-mass stars

“…In astrophysics, IGWs have initially been invoked as a source of mixing for chemicals (Press 1981;Garcia Lopez & Spruit 1991;Schatzman 1993;Montalban 1994;Montalban & Schatzman 1996Young et al 2003). Ando (1986) studied the transport of angular momentum associated with standing gravity waves in Be stars.…”

Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 – A new test for hydrodynamical stellar models

“…Many different physical mechanisms have been proposed to explain these observations: atomic diffusion (Michaud 1986;Michaud et al 2004), mass loss (Hobbs et al 1989;Swenson & Faulkner 1992), rotation-induced mixing (Charbonnel et al 1992;Talon & Charbonnel 1998;Palacios et al 2003;Théado & Vauclair 2003), mixing by internal gravity waves (Garcia Lopez & Spruit 1991;Montalbán & Schatzman 2000;Young et al 2003), or a combination of some of these processes. So far, only the hydrodynamic stellar models that combine the effects of meridional circulation, shear turbulence, internal gravity waves, and atomic diffusion have been able to account for Li observations over a broad range of stellar masses, ages, and evolutionary status .…”