Rotational Periods of very Young Brown Dwarfs in Cha I

A photometric monitoring campaign of brown dwarfs in the Cha I star forming region in the i and R band revealed significant periodic variations of the three M6.5–M7 type brown dwarf candidates Cha Hα 2, 3 and 6 (Joergens et al. 2002). These are the first rotational periods for very young (1–5 Myr) brown dwarfs and among the first rotational periods for brown dwarfs at all. The relatively long periods of 2.2 to 3.4 days as well as v sin i values (Joergens & Guenther 2001) indicate that our targets are moderately fast rotators in contrast to very rapidly rotating old brown dwarfs. The periods for the Cha I brown dwarf candidates provide valuable data points in an as yet, in terms of rotational characteristic, almost unexplored region of the age-mass diagram. A comparison with rotational properties of older brown dwarfs indicates that most of the acceleration during the contraction phase takes place within the first 30 Myr or less of the lifetime of a brown dwarf. We have also determined periods for the two M5–M5.5 type very low-mass stars B 34 and CHXR 78C.