Rotation and Magnetic Activity in Brown Dwarfs

Abstract

Brown dwarfs and very low mass (VLM) stars are the last frontier on the map of the angular momentum histories of star-like objects. Until the advent of 8-m class telescopes, it was impossible to obtain spectra with enough resolution to detect their rotation. The very existence of brown dwarfs was only established then. It was immediately apparent that there are differences between VLM objects and their heftier stellar cousins. Field VLM objects were found to be very rapidly rotating, yet they did not display the strong magnetic activity that would be expected of convective objects in that case. We now have a good preliminary understanding of the situation near the substellar boundary. I summarize the rotational data (both spectroscopic and photometric) on VLM objects, and how rotation and temperature fit into the production of magnetic activty. I also report more recent work on VLM objects when they are very young. There is increasing evidence that they form much like stars, beginning (when they become visible) as relatively slow rotators for the most part, followed by spin-up as they contract. Their disk lifetimes may be shorter, and they are more magnetically active when they are young. The subsequent angular momentum history of VLM objects is different from solar-type stars, as the usual magnetic braking mechanisms do not operate as in the stellar case. Much of the new work reported here is from the thesis of Subanjoy Mohanty, and was supported by NSF/AST-0098468.