Authors

E. Berger; G. Basri; J. E. Gizis; M. S. Giampapa; R. E. Rutledge; J. Liebert; E. Martin; T. A. Fleming; C. M. Johns-Krull; N. Phan-Bao;W. H. Sherry

Comments

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Abbreviated Journal Title

Astrophys. J.

Keywords

radio continuum : stars; stars : activity; stars : low-mass, brown; dwarfs; stars : magnetic fields; X-RAY-EMISSION; LOW-MASS STARS; L FIELD DWARFS; MAIN-SEQUENCE; BROWN; DWARF; RADIO-EMISSION; STELLAR ROTATION; M8.5 DWARF; SKY SURVEY; DME; STARS; Astronomy & Astrophysics

Abstract

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 alpha 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 less than or similar to 10(-5.7). The H alpha 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 x 10(4). The lack of radio variability during four rotations of LSR1835+32 requires a uniform stellar-scale field of similar to 10 G, and indicates that the H alpha 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 TVLM513 - 46546 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(-1), 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.

Journal Title

Astrophysical Journal

Volume

676

Issue/Number

2

Publication Date

1-1-2008

Document Type

Article

Language

English

First Page

1307

Last Page

1318

WOS Identifier

WOS:000254530600045

ISSN

0004-637X

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