Rotation of Giant Stars

Abstract

The internal rotation of post-main sequence stars is investigated, in response to the convective pumping of angular momentum toward the stellar core, combined with a tight magnetic coupling between core and envelope. The spin evolution is calculated using model stars of initial mass 1, 1.5, and M 5  , taking into account mass loss on the giant branches. We also include the deposition of orbital angular momentum from a sub-stellar companion, as influenced by tidal drag along with the excitation of orbital eccentricity by a fluctuating gravitational quadrupole moment. A range of angular velocity profiles r ( ) W is considered in the envelope, extending from solid rotation to constant specific angular momentum. We focus on the backreaction of the Coriolis force, and the threshold for dynamo action in the inner envelope. Quantitative agreement with measurements of core rotation in subgiants and post-He core flash stars by Kepler is obtained with a two-layer angular velocity profile: uniform specific angular momentum where the Coriolis parameter Co 1 con  t º W (here con t is the convective time), and r r ( ) 1 W µ -where Co 1  . The inner profile is interpreted in terms of a balance between the Coriolis force and angular pressure gradients driven by radially extended convective plumes. Inward angular momentum pumping reduces the surface rotation of subgiants, and the need for a rejuvenated magnetic wind torque. The co-evolution of internal magnetic fields and rotation is considered in Kissin & Thompson, along with the breaking of the rotational coupling between core and envelope due to heavy mass loss.

Asteroseismic Signatures of Evolving Internal Stellar Magnetic Fields

“…The relatively slow core rotation rates indicate that strong angular momentum transport mechanisms are at work (Cantiello et al 2014), coupling the radiative cores with the convective envelope. The strong magnetic fields frequently found in the cores of RGB stars (Stello et al 2016b) may play an important role in this process (Maeder & Meynet 2014;Kissin & Thompson 2015). However, our work suggests that strong magnetic fields are restricted to mass coordinates of RGB stars that were convective on the MS. For stars of M  1.5 M e , the strong fields are restricted to the He core and cannot directly couple the core with the envelope.…”

Slowing the spins of stellar cores

“…We hope to explore mixing effects and make detailed predictions for surface abundances in future work. Our models assume that convection zones are nearly rigidly rotating, which may not be true for deep convective zones where asymmetric convective energy/AM fluxes may cause deeper layers of the convective envelope to rotate faster (Brun & Palacios 2009;Kissin & Thompson 2015a). Indeed, some degree of envelope differential rotation may be necessary to explain rotation rates of horizontal branch stars (Sills & Pinsonneault 2000).…”

Differential rotation in convective envelopes: constraints from eclipsing binaries

“…To briefly summarize our results, we find that at short periods the radial relative shear must be small, and in particular that it is too small to account for the seismicallyinferred rotation periods of stellar cores (Kissin & Thompson 2015). We likewise constrain the latitudinal shear to be small, though the uncertainties in this are larger than for the radial case.…”

“…Even for longer periods out to 10 d we find β ranging from 0 to 0.6. This is significant because Kissin & Thompson (2015) find that steep rotation profiles with β < −1 are needed to explain the rotation rates of red giant cores if the shear is primarily located in the convection zone. For orbital periods, and hence rotation periods, less than 6 d we disfavor such steep profiles at p < 10 −4 .…”

“…Despite much work there remains significant uncertainty as to the primary location of rotational shear in stars. In particular, whether shear is strongest in radiative or convective regions remains an open question (Cantiello et al 2014;Kissin & Thompson 2015;Eggenberger et al 2019), and one which has significant consequences for the spin periods of compact objects (Hermes et al 2017). In the Sun, helioseismic inversions provide a measurement of the shear in the convection zone and place some constraints on the upper parts of the radiative interior, though the rotation of the deep interior remains uncertain (Schou et al 1998;Antia & Basu 2010).…”