Magnetic Activity and Rotation in Brown Dwarfs and Low Mass Stars

Abstract

One of the triumphs of the last 2 decades has been the establishment of the relation between stellar rotation and magnetic activity in solar-type stars. Rapid rotation produces strong activity, which in turn provides magnetic braking to reduce rotation. A solar-type dynamo cannot operate in fully convective stars, so it is of interest to study mid and late M stars. Hints that a dramatic change occurs in very low-mass stars and substellar objects appeared in 1995. The past 7 years have seen substantial progress on this question, with the conclusion that the rotation-activity connection indeed breaks down. As one goes to the bottom of the main sequence and below, the amount of magnetic activity takes a sudden fall, with a concomitant increase in the spindown times of the objects. We summarize these results, and some theoretical work which helps explain them. We also present some remaining mysteries, such as why very young objects seem excessively active, and flaring in objects with no other signs of magnetic activity.