Name/Affiliation: Peter K. G. Williams (Harvard)
The emergence of aurorae in ultra-cool stars: insights from radio variability
Although early studies suggested that magnetism fades out in very cool stars and brown dwarfs ("ultra-cool dwarfs": UCDs), radio observations have demonstrated that at least some of these objects can generate stable, large-scale, kilogauss-strength magnetic fields. Periodic, polarized radio bursts strongly suggest the emergence of aurorae in some objects with spectral types later than ~M7, raising several profound questions. What is the origin of the energized auroral particles? What is their effect upon the outer layers of the stellar atmosphere? The answers to these questions have implications not only for the chemistry and dynamics of UCD atmospheres, but also for the nature of the poorly-understood underlying dynamo process. I will present new results from a detailed VLA study of 2MASSW J1047539+212423, which at a spectral type of T6.5 is the latest-type object yet detected in the radio. Its radio bursts are indeed periodic, adding it to the roster of likely auroral emitters. I will place these results in the context of our recent work to clarify the relationship between rotation and magnetic activity, as traced by radio and X-ray emission, in the ultra-cool regime. The recent upgrade of the VLA provides a powerful new instrument for pursuing these studies.