Rotational evolution of VLM objects and brown dwarfs
Jochen Eislöffel (Thüringer Landessternwarte, Tautenburg)
Alexander Scholz (Thüringer Landessternwarte, Tautenburg)
The regulation of angular momentum is one of the key processes for our
understanding of stellar evolution. The rotational evolution of a solar-mass
star is mainly determined by the magnetic interaction with the circumstellar
disk and angular momentum loss through stellar winds, and depends thus
critically on the properties of the magnetic field of the star. For solar-mass
stars, hundreds of rotation periods have been measured,which set strong
constraints for models of rotational evolution. However, we are lacking a
comparable database for very low mass (VLM) objects, i.e. brown dwarfs and stars
with masses below 0.4 solar masses. In contrast to more massive stars, these VLM
objects are believed to be fully convective. This may lead to major differences
in terms of rotation and activity, since fully convective objects cannot host a
solar-type dynamo.
Here we report on our observational efforts to understand the rotational
evolution of VLM objects. By means of photometric monitoring, we determined 80
rotation periods for targets in five clusters, which form an age sequence from 3
to 750 Myr. We find that VLM objects rotate faster than their solar-mass
siblings in all evolutionary stages. Their rotational evolution is determined by
the hydrostatic contraction and exponential angular momentum loss, whereas the
influence of the disk is not significant. The photometric amplitudes of the
lightcurves are much lower than for more massive stars. This may be explained as
a consequence of symmetric spot distributions and low contrast between spots and
their environment. Most of these results can be explained with a change of the
magnetic field properties with decreasing mass. VLM objects probably exhibit
only small-scale, turbulent magnetic fields.