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M. E. Putman (U. Colorado), A. J. Burgasser (UCLA)
We report the results of a radio monitoring program conducted at the Australian Telescope Compact Array to search for quiescent and flare emission from nearby late-type M and L dwarfs. Seven objects spanning spectral types M7 to L4.5 were observed simultaneously at 4.8 and 8.6 GHz (6 and 3 cm) for continuous periods of 9.5 to 12 hours. Of these, three were detected in quiescent emission (~0.2 mJy) in at least one band, one was marginally detected at 4.8 GHz, and three were undetected down to limiting sensitivites of 0.1 mJy. The brightness temperatures (108-109 K) and spectral indices (-1 < alpha < 0) of the quiescent emission indicate optically thin gyrosynchrotron emission from mildly relativistic electrons in weak (10-100 G) magnetic fields, consistent with prior results. One of these objects, DENIS 1048-3956, was detected in two spectacular flares, one at 4.8 GHz with a peak flux density of ~20 mJy, and one ten minutes later at 8.6 GHz with a peak flux density of ~40 mJy. These flares are over twenty times brighter than the flaring radio emission detected from LP 944-20, and result from a very different emission mechansim than that of the quiescent emission (also detected at 4.8 GHz in this object). Based on the high brightness temperature (~1012 K), short period (~2 minutes), high linear polarization, and apparently narrow spectral bandwidth, the DENIS 1048-3956 radio flare is likely the result of coherent electron sychrotron maser emission, although other forms of coherent emission may explain the possible temporal and frequency drift observed. We discuss these results in the context of magnetic field generation and the sustainability of hot coronae in cool dwarf star and brown dwarf atmospheres.
This research is supported by NASA through Hubble Fellowship grant HST-HF-01137.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
The author(s) of this abstract have provided an email address for comments about the abstract: adam@astro.ucla.edu
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.