Mapping the Radio Coronae of Cool Stars andBrown Dwarfs

摘要

The pulsing radio emission detected from ultracool dwarfs can be used as a powerfuldiagnostic of magnetic field strengths and topologies at and below the substellar boundary. Studiesthus far have confirmed magnetic field strengths of 3 kG for two late M dwarfs and 1.7 kG for anL3.5 dwarf, the latter being the first confirmation of kG magnetic fields for an L dwarf. Ongoinglong term monitoring of the radio pulses will also investigate the stability of the associated large-scale magnetic fields over timescales > 1 year. We also present the preliminary results of a lengthyradio monitoring campaign of the rapidly rotating M4 star V374 Peg, with the resulting light curvesphased with magnetic maps previously obtained through Zeeman Doppler Imaging. The radioemission from V374 Peg is strongly modulated by the large scale dipolar magnetic field, with twoclear peaks in the radio light curve per period of rotation, occurring when the dipolar field lies in theplane of the sky. These results provide strong evidence that the electron cyclotron maser instabilityplays a pivotal role in the production of quiescent radio emission from V374 Peg, representing asignificant departure from the accepted model of gyrosynchrotron emission as the dominant sourceof quiescent radio emission from active M dwarfs.