Simultaneous multiwavelength observations of magnetic activity in ultracool dwarfs. I. The complex behavior of the M8.5 dwarf TVLM 513-46546
Abbreviated Journal Title
radio continuum : stars; stars : activity; stars : low-mass; brown; dwarfs; stars : magnetic fields; X-RAY-EMISSION; H-ALPHA EMISSION; LOW-MASS STARS; RADIO-EMISSION; MAIN-SEQUENCE; BROWN DWARF; AD LEONIS; ROTATIONAL MODULATION; STELLAR; CORONAE; COOL STARS; Astronomy & Astrophysics
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 L-X/L-bol approximate to 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 H alpha and H beta 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 similar to 10(2) 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 similar to 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.
"Simultaneous multiwavelength observations of magnetic activity in ultracool dwarfs. I. The complex behavior of the M8.5 dwarf TVLM 513-46546" (2008). Faculty Bibliography 2000s. 123.