Convection in stars

Abstract

Abstract. Convection is one of the most intricate processes studied in stellar astrophysics and has challenged both theorists and observers since the beginnings of astrophysics. But during the last two decades observational data of unprecedented resolution and accuracy have been collected in solar and stellar research which permit a new look at the field. An enormous increase of computer speed now permits solving more complete model equations with more accurate numerical approximations. Modelling and theoretical understanding of convection, however, are lagging behind observational progress and are still wanting.As a background to the contributions to this session on convection, I first provide an overview on its basic physics and its observational evidence. I point out why astrophysicists have a general interest in improvements of our understanding of stellar convection and then focus on convection in A-stars with their unique combination of convection zones. I summarise how this richness of different manifestations can arise in A-stars, such as convection zones near the surface and in the core, several on top of each other, or some of them depleted by diffusion processes, suppressed by or even creating magnetic fields, suspected to create a chromosphere in some of them, or influenced by binaries, to name just a few. In the last part I will present a few recent results on modelling of convection in A-stars.

Experimental study of low-frequency oscillations and large-scale circulations in turbulent mixed convection

“…Particularly turbulent MC has gained to a lot of interest during the last decades and is of utmost importance, e.g. in geophysics, astrophysics (Kupka, 2003), indoor climatisation (Costa et al, 2000;Linden, 1999) or in industrial processes and applications (Sillekens et al, 1998).…”

Analysing the Contributions in Moment Equations of Reynolds Stress Models of Convection with Numerical Simulations

“…Because of the non-linearity of the underlying hydrodynamical equations these equations are part of an unclosed hierarchy which requires additional assumptions (closure approximations) to formulate predictive models. In Kupka & Muthsam (2007a-c) and Kupka (2007) a study of a set of moment equations proposed by Canuto & Dubovikov (1998) (combined with results from Canuto 1992Canuto , 1993 was presented. They extended earlier tests by Kupka (1999), which had been based on 3D numerical simulations of compressible convection by Muthsam et al (1995Muthsam et al ( , 1999, to the case of deep convection zones.…”

Standard model atmospheres for A-type stars and non-LTE effects

“…This underlines the need for more intensive calculations of NLTE model atmospheres of A stars. Adding consistently new different physical processes to NLTE model atmospheres calculations, such as diffusion (see Michaud 2005), magnetic fields (see Moss 2005), and more accurate treatments of convection (see Kupka 2005), are necessary. This is a real challenging task for the near future.…”