Astrophys. J., 594, 971-981 (2003/September-2)
Rotational periods of very young brown dwarfs and very low mass stars in Chamaeleon I.
JOERGENS V., FERNANDEZ M., CARPENTER J.M. and NEUHAEUSER R.
Abstract (from CDS):
We have studied the photometric variability of very young brown dwarfs and very low mass stars (masses well below 0.2 M☉) in the Cha I star-forming region. We have determined photometric periods in the Gunn i and R bands for the three M6.5-M7 type brown dwarf candidates Cha Hα 2, Cha Hα 3, and Cha Hα 6 of 2.2-3.4 days. These are the longest photometric periods found for any brown dwarf so far. If interpreted as rotationally induced, they correspond to moderately fast rotational velocities, which is fully consistent with their vsini values and their relatively large radii. We have also determined periods for the two M5-M5.5 type very low mass stars B34 and CHXR 78C. In addition to the Gunn i- and R-band data, we have analyzed JHKSmonitoring data of the targets, which have been taken a few weeks earlier and confirm the periods found in the optical data. Upper limits for the errors in the period determination are between 2 and 9 hr. The observed periodic variations of the brown dwarf candidates as well as of the T Tauri stars are interpreted as modulation of the flux at the rotation period by magnetically driven surface features, on the basis of a consistency with vsini values as well as R-i color variations typical for spots. Furthermore, the temperatures even for the brown dwarfs in the sample are relatively high (>2800 K) because the objects are very young. Therefore, the atmospheric gas should be sufficiently ionized for the formation of spots on one hand, and the temperatures are too high for significant dust condensation and hence variabilities due to clouds on the other hand. A comparison with rotational properties of older brown dwarfs shows that most of the acceleration of brown dwarfs takes place within the first 30 Myr or less. If magnetic braking plays a role, this suggests that the disk dissipation for brown dwarfs occurs between a few and 36 Myr.
Stars: Activity - Stars: Fundamental Parameters - Stars: Individual: Constellation Name: Chamaeleon Hα 1 - Stars: Individual: Constellation Name: Chamaeleon Hα 2 - Stars: Individual: Constellation Name: Chamaeleon Hα 3 - stars: individual (Chamaeleon Hα 4, Chamaeleon Hα 5) - Stars: Individual: Constellation Name: Chamaeleon Hα 6 - stars: individual (Chamaeleon Hα 7, Chamaeleon Hα 8) - Stars: Individual: Constellation Name: Chamaeleon Hα 9 - stars: individual (Chamaeleon Hα 10, Chamaeleon Hα 11) - Stars: Individual: Constellation Name: Chamaeleon Hα 12 - Stars: Individual: Alphanumeric: B34 - stars: individual (CHXR 73) - stars: individual (CHXR 78C) - Stars: Late-Type - Stars: Low-Mass, Brown Dwarfs - Stars: Rotation
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