Magnetic braking in young late-type stars

Abstract

Context. The existence of rapidly rotating cool stars in young clusters implies a reduction of angular momentum loss rate for a certain period of the star's early life. Recently, the concentration of magnetic flux near the poles of these stars has been proposed as an alternative mechanism to dynamo saturation in order to explain the saturation of angular momentum loss. Aims. In this work we study the effect of magnetic surface flux distribution on the coronal field topology and angular momentum loss rate. We investigate if magnetic flux concentration towards the pole is a reasonable alternative to dynamo saturation. Methods. We construct a 1D wind model and also apply a 2-D self-similar analytical model, to evaluate how the surface field distribution affects the angular momentum loss of the rotating star. Results. From the 1D model we find that, in a magnetically dominated low corona, the concentrated polar surface field rapidly expands to regions of low magnetic pressure resulting in a coronal field with small latitudinal variation. We also find that the angular momentum loss rate due to a uniform field or a concentrated field with equal total magnetic flux is very similar. From the 2D wind model we show that there are several relevant factors to take into account when studying the angular momentum loss from a star. In particular, we show that the inclusion of force balance across the field in a wind model is fundamental if realistic conclusions are to be drawn from the effect of non-uniform surface field distribution on magnetic braking. This model predicts that a magnetic field concentrated at high latitudes leads to larger Alfvén radii and larger braking rates than a smoother field distribution. Conclusions. From the results obtained, we argue that the magnetic surface field distribution towards the pole does not directly limit the braking efficiency of the wind.

A spectroscopy study of nearby late-type stars, possible members of stellar kinematic groups

“…In the course of their evolution, rotation decreases due to the loss of angular momentum with stellar winds and magnetic breaking (Weber & Davis 1967;Jianke & Collier Cameron 1993;Aibéo et al 2007). Thus stellar rotation can be used to estimate stellar ages, and it is well known that solar-type stars follow a law of the form P Rot ∝ t 1/2 (Skumanich 1972).…”

The Effect of Magnetic Spots on Stellar Winds and Angular Momentum Loss

“…In both cases the rotation rate affects the magnetic field distribution. Aibéo et al (2007) have argued that re-distribution of coronal flux in stars does not significantly change the wind efficiency for the magnetic breaking. However, their model took into account only existing magnetic flux with no addition of new emerging flux (i.e.…”

“…Based on WD's model, a large number of more detailed and complex studies of AML in cool and hot stars, as well as in protostellar accretion disks, have been done both analytically (Mestel 1968;Holzwarth 2005) and numerically (Keppens & Goedbloed 2000;Matt & Pudritz 2008a,b;Ud-Doula et al 2009). In particular, Aibéo et al (2007) have shown that AML can be modified by a factor of 2-4 when introducing magnetic flux at high latitudes. They explain the change in AML as a result of a change in the mass flux or the size of the Alfvén surface, but they argue that the change in the magnetic flux tubes topology should not significantly affect magnetic breaking.…”

Stellar magnetism: empirical trends with age and rotation

“…However, the magnetic activity-rotation relation breaks for rapidly rotating stars, where the indicators of stellar magnetism saturate and become independent of rotation. A saturation of the dynamo operating inside the star, inhibiting the increase of magnetism with rotation rate, has been attributed to explain the activity saturation observed in low-period stars (Vilhu 1984), but alternative explanations also exist (e.g., MacGregor & Brenner 1991;Jardine & Unruh 1999;Aibéo, Ferreira & Lima 2007).…”