Rapidly Rotating Suns and Active Nests of Convection

Abstract

In the solar convection zone, rotation couples with intensely turbulent convection to drive a strong differential rotation and achieve complex magnetic dynamo action. Our Sun must have rotated more rapidly in its past, as is suggested by observations of many rapidly rotating young solar-type stars. Here we explore the effects of more rapid rotation on the global-scale patterns of convection in such stars and the flows of differential rotation and meridional circulation, which are self-consistently established. The convection in these systems is richly time-dependent, and in our most rapidly rotating suns a striking pattern of localized convection emerges. Convection near the equator in these systems is dominated by one or two nests in longitude of locally enhanced convection, with quiescent streaming flow in between them at the highest rotation rates. These active nests of convection maintain a strong differential rotation despite their small size. The structure of differential rotation is similar in all of our more rapidly rotating suns, with fast equators and slower poles. We find that the total shear in differential rotation Á grows with more rapid rotation, while the relative shear Á/ 0 decreases. In contrast, at more rapid rotation, the meridional circulations decrease in energy and peak velocities and break into multiple cells of circulation in both radius and latitude. Subject headingg s: convection -stars: interiors -stars: rotation -Sun: interior -Sun: rotation CONVECTION, ROTATION, AND MAGNETISMOur Sun is a magnetic star whose cycles of magnetic activity must arise from organized dynamo action in its interior. This dynamo action is achieved by turbulent plasma motions in the solar convection zone, which spans the outer 29% of the Sun in radius. Here vigorous convective motions and rotation couple to drive the differential rotation and meridional circulation. These globalscale flows are important ingredients in stellar dynamo theory, providing shear, which may build and organize fields on global scales. When our Sun was younger, it must have rotated much more rapidly, as is suggested both by the solar wind, which continually removes angular momentum from the Sun, and by many observations of rapidly rotating solar-like stars. In more rapidly rotating suns the coupling between rotation and convection is strong and must continue to drive global scales of flow. Understanding the nature of convection, differential rotation, and meridional circulation in more rapidly rotating stars is a crucial step toward understanding stellar dynamos.The manner in which the Sun achieves global-scale dynamo action is gradually being sorted out. Helioseismology, which uses acoustic oscillations to probe the radial structure of the star, as well as the convective flows beneath the surface, has revealed that the solar differential rotation profile observed at the surface prints throughout the convection zone, with two prominent regions of radial shear. The near-surface shear layer occupies the outer 5% of the Sun, where...

The Solar-Stellar Connection

“…These profiles can be understood by considering the transport of angular momentum in a spherical convective shell. When performing this analysis as in (Brun and Toomre 2002;Brown et al 2008;Augustson et al 2012), we find that the Reynolds stresses are at the origin of the prograde equator for models with a low Rossby number. Through gyroscopic pumping the meridional circulation transport angular momentum to counterbalance the torque applied by the Reynolds stresses (from which we have subtracted the contribution from viscous effect).…”

“…high mass and very low mass stars) interested readers may refer to (e.g. Browning et al 2004;Brun et al 2005;Dobler et al 2006;Brown et al 2008;Christensen et al 2009;Featherstone et al 2009;Augustson et al 2011;Cai et al 2011;Morin et al 2011;Schrinner et al 2012, and references therein). Convection plays a key role in transporting the star's heat by having strong correlations between vertical motions and temperature fluctuations.…”

Towards understanding the global magnetism of the Sun and solar-like stars

“…Similarly Nandy & Martens (2007) have explored the solar-stellar connection with B-L mean field dynamo models and have reached the same conclusion: only a positive scaling of the amplitude of meridional flows with the rotation rate can reconcile the models with observations of magnetism of solar-like stars. However recent theoretical work by Ballot et al (2007) and Brown et al (2008) indicate that this is unlikely to be the right scaling. Instead 3-D simulations reveal that the amplitude of the meridional flow weakens as the rotation rate is increased.…”

“…The ω-effect is a direct measure of the differential rotation ΔΩ established in the star. It is well known both theoretically and observationally that the differential rotation in the convective envelope of solar-type stars is directly connected to the star's rotation rate Ω 0 (Donahue et al 1996;Barnes et al 2005;Ballot et al 2007;Brown et al 2008;Küker & Rüdiger 2008). However, the exact scaling n r (i.e.…”

“…In Jouve et al (2009) we have explored how mean field Babcock-Leighton dynamo models may be used to explain the observational relation between the rotation and magnetic activity period of solar-like stars by incorporating scaling law for the amplitude of the meridional circulation coming from the 3-D simulations of Brown et al (2008). In this 3-D study they found that the energy and amplitude of the meridional circulation decreases with the rotation rate.…”