Brown dwarfs

Rotational periods of very young brown dwarfs

Viki Joergens, Matilde Fernandez, Ralph Neuhäuser
[Joergens et al. 2003, ApJ]



The evolution of angular momentum in the substellar regime is observationally poorly constrained
due to the lack of measured rotation rates for brown dwarfs. So far, we only know that old brown
dwarfs tend to rotate very rapidly (>60 km/s).
Rotation rates can be studied by tracing brightness variations caused by surface spots as the object
rotates, which has been done extensively for T Tauri stars. In order to study the rotational properties
of very young brown dwarfs, twelve bona fide and candidate brown dwarfs in the ChaI star forming
region at an age of 1-5 Myr have been monitored photometrically at the D1.5m telescope at ESO,
La Silla. Three of them showed photometric variabilities with periods of 2 to 3 days (Joergens et al. 2003, subm.),
which are consistent with projected rotational velocities vsini of 8-26 km/s, measured from UVES/VLT
spectra (Joergens & Guenther 2001). These are therefore the first rotational periods for very young
brown dwarfs and among the first rotational periods for brown dwarfs at all.





The graphic shows as an example the phase folded i-band light curve of the M7-type very young brown dwarf candidate ChaHa6. The visible photometric variations are periodic with 3.4 days and are most certainly caused by spots on the surface, which modulate the brightness as the object rotates. The detected period is therefore are direct tracer of the rotational period of ChaHa6 and agrees very well with the projected spectroscopic velocity of 13 km/s measured from UVES/VLT spectra and a radius of 0.7 solar radii. The spot model illustrates the dimming of the total brightness when the cool spot comes insight and blocks part of the light. (Note that the size, latitude, the fact that it is only one big spot instead of groups of spots are chosen arbitrarily.)










The derived periods are valuable data
points in an, in terms of angular momentum evolution,
extremely interesting but observationally almost unexplored age and mass regime, where
phenomena, like acceleration due to contraction, magnetic braking due to interaction with a
disk as well as a temporal stop of the contraction due to the onset of Deuterium burning are
expected to occur.
The here presented results show that very young brown dwarfs are moderately fast rotators
in contrast to very rapidly rotating old brown dwarfs. 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.






Angular momentum evolution of brown dwarfs:
The diagram shows the few known rotational periods
for brown dwarfs vs. age. The rotational periods of
brown dwarfs older than about 40 Myr (from the literatur)
are apparently below one day indicating rapid rotation.
Very young brown dwarfs on the other hand, as the brown
dwarfs in ChaI at an age of 1-5 Myr, are still rotating
moderatly fast with periods around 3 days in agreement
with their ongoing contraction.









Viki Joergens / viki @ mpia.de