Person:

Cook, Benjamin

Observations of magnetic activity indicators in solar-type stars exhibit a relationship with rotation with an increase until a “saturation” level and a moderate decrease in activity in the very fastest rotators (“supersaturation”). While X-ray data have suggested that this relationship is strongly violated in ultracool dwarfs (UCDs; spectral type &M7), the limited number of X-ray detections has prevented firm conclusions. In this paper, we analyze the X-ray activity-rotation relation in 38 ultracool dwarfs. Our sample represents the largest catalog of X-ray active ultracool dwarfs to date, including seven new and four previously-unpublished Chandra observations presented in a companion paper. We identify a substantial number of rapidly-rotating UCDs with X-ray activity extending two orders of magnitude below the expected saturation level and measure a “supersaturation”-type anticorrelation between rotation and X-ray activity. The scatter in UCD X-ray activity at a fixed rotation is ∼3 times larger than that in earlier-type stars. We discuss several mechanisms that have been proposed to explain the data, including centrifugal stripping of the corona, and find them to be inconsistent with the observed trends. Instead, we suggest that an additional parameter correlated with both X-ray activity and rotation is responsible for the observed effects. Building on the results of Zeeman-Doppler imaging of UCD magnetic fields and our companion study of radio/X-ray flux ratios, we argue that this parameter is the magnetic field topology, and that the large scatter in UCD X-ray fluxes reflects the presence of two dynamo modes that produce distinct topologies.

Although ultracool dwarfs (UCDs) are now known to generate and dissipate strong magnetic fields, a clear understanding of the underlying dynamo is still lacking. We have performed X-ray and radio observations of seven UCDs in a narrow range of spectral type (M6.5–M9.5) but spanning a wide range of projected rotational velocities (v sin i ≈ 3–40 km s−1). We have also analyzed unpublished archival Chandra observations of four additional objects. All of the newly-observed targets are detected in the X-ray, while only one is detected in the radio, with the remainder having sensitive upper limits. We present a database of UCDs with both radio and X-ray measurements and consider the data in light of the so-called Güdel-Benz relation (GBR) between magnetic activity in these bands. Some UCDs have very bright radio emission and faint X-ray emission compared to what would be expected for rapid rotators, while others show opposite behavior. We show that UCDs would still be radio-over-luminous relative to the GBR even if their X-ray emission were at standard rapid-rotator “saturation” levels. Recent results from Zeeman-Doppler imaging and geodynamo simulations suggest that rapidly-rotating UCDs may harbor a bistable dynamo that supports either a stronger, axisymmetric magnetic field or a weaker, non-axisymmetric field. We suggest that the data can be explained in a scenario in which strong-field objects obey the GBR while weak-field objects are radio-over-luminous and X-rayunder-luminous, possibly because of a population of gyrosynchrotron-emitting coronal electrons that is continuously replenished by low-energy reconnection events.