Cool Stars 14 - Submitted Abstract # 109
This version created on 05 October 2006

Pulsed Coherent Radio Emission at the Substellar Boundary


Gregg Hallinan, National University of Ireland Galway
Stephen Bourke, National University of Ireland Galway
Caoilfhionn Lane, National University of Ireland Galway
Gerry Doyle, Armagh Observatory
Antoaneta Antonova, Armagh Observatory
Walter Brisken, Nationa Radio Astronomy Observatory
Bob Zavala, US Naval Observatory
Richard Boyle, Vatican Observatory
Aaron Golden, National University of Ireland Galway

A number of ultracool dwarfs have been detected as relatively strong
sources at radio frequencies, despite low X-ray and H-alpha
luminosities.  The characteristics of this radio emission have led to
speculation that it is incoherent gyrosynchrotron radiation similar to
the radio emission detected from more massive stars with X-ray
emitting coronae.  Conversely, we have shown that the radio emission
from one such dwarf, TVLM 513-46546, is periodic with rotation,
arguing for a more directive coherent process from compact source
regions (Hallinan et al.  2006/astro-ph/0608556).  We have recently
conducted further simultaneous radio and photometric monitoring
observations of TVLM 513-46546(paper in prep.).  The radio
observations are characterized by extremely bright (~5 mJy), periodic
(~1.958 hours), 100% circularly polarized bursts of short duration (~4
minutes).  These bursts conclusively confirm the coherent nature of
the radio emission and require magnetic field strengths of 3kG for
TVLM 513-46546.  The photometric monitoring observations also
establish that these high strength fields produce stable magnetic
spots on the surface of TVLM 513-46546 which constitute the probable
source region of the coherent radio emission.  Until now, it was
believed that coherent emission from stellar sources was confined to
transient bursts or flares.  For TVLM 513-46546, the transient nature
of the emission is due to the rotation of the dwarf and the strong
beaming of the emission.  The resultant periodic light curve is
reminiscent of the coherent emission detected from pulsars,
highlighting TVLM 513-46546 as the prototype of an entirely new class
of radio emitting source.

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