Polar branches of stellar activity waves: dynamo models and observations

Abstract

Aims. Stellar activity data provide evidence of activity wave branches propagating polewards rather than equatorwards (the solar case). This evidence is especially pronounced for the well-observed subgiant HR 1099. Stellar dynamo theory allows polewards propagating dynamo waves for certain governing parameters. In this paper, we try to unite observations and theory. Methods. Taking into account the preliminary stage both of observations of polar activity branches and of the determination of the governing parameters for stellar dynamos, we restrict our investigation to the simplest mean-field dynamo models, while recognizing more modern approaches to be an essential development. Results. We suggest a crude preliminary systematization of the reported cases of polar activity branches. Then we present results of dynamo model simulations which contain magnetic structures with polar dynamo waves, and identify the models which look most promising for explaining the latitudinal distribution of spots in dwarf stars. Those models require specific features of stellar rotation laws, and so observations of polar activity branches may constrain internal stellar rotation. Specifically, we find it unlikely that a pronounced poleward branch can be associated with a solar-like internal rotation profile, while it can be more readily reproduced in the case of a cylindrical rotation law appropriate for fast rotators. We stress the case of the subgiant component of the active close binary HR 1099 which, being best investigated, presents the most severe problems for a dynamo interpretation. Our best model requires dynamo action in two layers separated in radius. This interpretation requires some change in the paradigm of stellar magnetic studies, as it explains surface manifestations in a subgiant as a joint effect of shallow and deep layers of the stellar convective zone, rather than of a surface magnetic field only, as appears to be the case in dwarf stars. Conclusions. Observations of polar activity branches provide valuable information for understanding stellar activity mechanisms and internal rotation, and thus deserve intensive observational and theoretical investigation. Current stellar dynamo theory seems sufficiently robust to accommodate the phenomenology.

Measuring stellar differential rotation with high-precision space-borne photometry

“…The amplitude of the DR determined by our method depends of course on the specific interval considered because spots can appear at different latitudes in late-type stars. The latitude range is wider in more active stars than in the Sun (see, e.g., Moss et al 2011), thus making the differences larger for CoRoT-6 and the young Sun-like stars in our sample. Specifically, from the modelling of another interval for CoRoT-6, we find ΔP/P = 0.259 ± 0.003, while for, say, KIC 8429280, another interval gives ΔP/P = 0.011 ± 3.73 × 10 −5 , where the standard deviations are derived from the a posteriori distributions given by the MCMC method.…”

Resonances for activity waves in spherical mean field dynamos

“…The magnetic field is continuous across the boundary between the two domains. A related model was discussed previously in Moss & Sokoloff (2007) and Moss et al (2011) in a more specifically astronomical context.…”

Polar branches of stellar activity waves: Dynamo models

“…Moss et al (2011) confirm that the sign of the dynamo number D ∼ α∂Ω/∂r is the main quantity which determines the direction of activity wave propagation, even though considerable finer detail can be found in the results, e.g. in the form of two branches at low and high latitudes, or of standing wave patterns.…”

“…Moss et al (2011) investigated how the recent and current observational data concerning poleward branches of stellar activity might be connected with ideas from stellar dynamo theory. They appreciated that the observational situation after these pioneering results still remains quite uncertain, and so studied just the most traditional forms of stel-lar dynamos, i.e.…”

“…Moss & Sokoloff (2007) demonstrated that for some choice of the dynamo governing parameters one shell can enslave the other and instead of propagating polewards the activity wave can propagate equatorwards. Moss et al (2011) obtained models with two dynamo active shells which provide standing rather than traveling waves. Moreover, Moss & Sokoloff (2008) and Moss et al (2010) investigated other dynamo systems with two domains of dynamo activity, i.e.…”