We present the first simultaneous, multiwavelength observations of an L dwarf, the L3.5 candidate brown dwarf 2MASS J00361617+1821104, conducted with the Very Large Array, the Chandra X-Ray Observatory, and the Kitt Peak 4 m telescope. We detect strongly variable and periodic radio emission (P ¼ 3 hr) with a fraction of about 60% circular polarization. No X-ray emission is detected to a limit of L X /L bol P 2 ; 10 À5 , several hundred times below the saturation level observed in early M dwarfs. Similarly, we do not detect H emission to a limit of L H /L bol P 2 ; 10 À7 , the deepest for any L dwarf observed to date. The ratio of radio to X-ray luminosity is at least 4 orders of magnitude in excess of that observed in a wide range of active stars (including M dwarfs), providing the first direct confirmation that late-M and L dwarfs violate the radio/X-ray correlation. The radio emission is due to gyrosynchrotron radiation in a large-scale magnetic field of about 175 G, which is maintained on timescales longer than 3 yr. The detected 3 hr period may be due to (1) the orbital motion of a companion at a separation of about 5 stellar radii, similar to the configuration of RS CVn systems, (2) an equatorial rotation velocity of about 37 km s À1and an anchored, long-lived magnetic field, or (3) periodic release of magnetic stresses in the form of weak flares. In the case of orbital motion, the magnetic activity may be induced by the companion, possibly explaining the unusual pattern of activity and the long-lived signal. We conclude that fully convective stars can maintain a large-scale and stable magnetic field, but the lack of X-ray and H emission indicates that the atmospheric conditions are markedly different than in early-type stars and even M dwarfs. Similar observations are therefore invaluable for probing both the internal and external structure of low-mass stars and substellar objects, and for providing constraints on dynamo models.

As part of our ongoing investigation of magnetic activity in ultracool dwarfs we present simultaneous radio, X-ray, UV, and optical observations of LSR1835+32 (M8.5), and simultaneous X-ray and UVobservations of VB10 (M8), both with a duration of about 9 hr. LSR1835+32 exhibits persistent radio emission, and H variability on timescales of 0.5-2 hr. The detected UV flux is consistent with photospheric emission, and no X-ray emission is detected to a deep limit of L X /L bol P10 À5:7 . The H and radio emission are temporally uncorrelated, and the ratio of radio to X-ray luminosity exceeds the correlation seen in F-M6 stars by >2 ; 10 4 . The lack of radio variability during four rotations of LSR1835+32 requires a uniform stellar-scale field of~10 G, and indicates that the H variability is dominated by much smaller scales, <10% of the chromospheric volume. VB10, on the other hand, shows correlated flaring and quiescent X-ray and UV emission, similar to the behavior of early M dwarfs. Delayed and densely sampled optical spectra exhibit a similar range of variability amplitudes and timescales. Along with our previous observations of the M8.5 dwarf TVLM513À46546 we conclude that late M dwarfs exhibit a mix of activity patterns, which points to a transition in the structure and heating of the outer atmosphere by large-scale magnetic fields. We find that rotation may play a role in generating the fields as evidenced by a tentative correlation between radio activity and rotation velocity. The X-ray emission, however, shows evidence for supersaturation at v sin i > 25 km s À1 , which could be the result of secondary effects such as inefficient heating or centrifugal stripping of extended coronal loops. These effects may underlie the severe violation of the radio/ X-ray correlation in ultracool dwarfs.

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 hours. 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 ≈ 10 −4.9 , the faintest to date for any object later than M5, and a possible weak 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, suggesting that, unlike in early M dwarfs, chromospheric heating may not be due to 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 is the first known example of such Balmer line behavior, which points to a co-rotating 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 ∼ 10 2 G, in good agreement with the value required for confinement of the X-ray emitting plasma. 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 multi-polar 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 may not be the result of coronal heating.

In searches for low-mass companions to late-type stars, correlation between radial velocity variations and line bisector slope changes indicates contamination by large starspots. Two young stars demonstrate that this test is not sufficient to rule out starspots as a cause of radial velocity variations. As part of our survey for substellar companions to T Tauri stars, we identified the ∼2 Myr old planet host candidates DN Tau and V836 Tau. In both cases, visible-light radial velocity modulation appears periodic and is uncorrelated with line bisector span variations, suggesting close companions of several M Jup in these systems. However, high-resolution, infrared spectroscopy shows that starspots cause the radial velocity variations. We also report unambiguous results for V827 Tau, identified as a spotted star on the basis of both visible-light and infrared spectroscopy. Our results suggest that infrared follow-up observations are critical for determining the source of radial velocity modulation in young, spotted stars.