2008ApJ...673.1080B -
Astrophys. J., 673, 1080-1087 (2008/February-1)
Simultaneous multiwavelength observations of magnetic activity in ultracool dwarfs. I. The complex behavior of the M8.5 dwarf TVLM 513-46546.
BERGER E., GIZIS J.E., GIAMPAPA M.S., RUTLEDGE R.E., LIEBERT J., MARTIN E., BASRI G., FLEMING T.A., JOHNS-KRULL C.M., PHAN-BAO N. and SHERRY W.H.
Abstract (from CDS):
We present the first simultaneous radio, X-ray, ultraviolet, and optical spectroscopic observations of the M8.5 dwarf TVLM 513-46546, with a duration of 9 hr. These observations are part of a program to study the origin of magnetic activity in ultracool dwarfs, and its impact on chromospheric and coronal emission. Here we detect steady quiescent radio emission superposed with multiple short-duration, highly polarized flares; there is no evidence for periodic bursts previously reported for this object, indicating their transient nature. We also detect soft X-ray emission, with LX/Lbol~10–5.1, the faintest to date for any object later than M5, and a possible X-ray flare. TVLM 513-46546 continues the trend of severe violation of the radio/X-ray correlation in ultracool dwarfs, by nearly 4 orders of magnitude. From the optical spectroscopy we find that the Balmer line luminosity exceeds the X-ray luminosity by a factor of a few, ruling out chromospheric heating by coronal X-ray emission. More importantly, we detect sinusoidal Hα and Hβ equivalent width light curves with a period of 2 hr, matching the rotation period of TVLM 513-46546. This behavior points to a corotating chromospheric hot spot or an extended magnetic structure, with a covering fraction of about 50%. This feature may be transitory based on the apparent decline in light-curve peak during the four observed maxima. From the radio data we infer a large-scale and steady magnetic field of ∼102 G. A large-scale field is also required by the sinusoidal Balmer line emission. The radio flares, on the other hand, are produced in a component of the field with a strength of ∼3 kG and a likely multipolar configuration. The overall lack of correlation between the various activity indicators suggests that the short-duration radio flares do not have a strong influence on the chromosphere and corona, and that the chromospheric emission is not the result of coronal heating.
Abstract Copyright:
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Journal keyword(s):
Radio Continuum: Stars - Stars: Activity - Stars: Low-Mass, Brown Dwarfs - Stars: Magnetic Fields
Simbad objects:
8
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