We present projected rotational velocities and new measurements of the rotational profile of some 180 nearby stars with spectral types A-F. The overall broadening profile is derived analysing spectral line shape from hundreds of spectral lines by the
method of least-squares deconvolution. Rigid and differential rotation can be distinguished in 56 cases. Ten stars with significant differential rotation rates are identified. As of now, 33 differential rotators detected by line profile analysis have been confirmed. The frequency of differential rotators decreases towards high effective temperature and rapid rotation. There is evidence for two populations of differential rotators with a gap in between at spectral type early-F. The gap can only partly be explained by an upper bound found for the horizontal shear of F stars. Apparently, the physical conditions of differential rotation change at early-F spectral types.
We continue our studies on stellar latitudinal differential rotation. The presented work is a sequel of the work of Reiners et al. who studied the spectral line broadening profile of hundreds of stars of spectral types A through G at high rotational
speed (vsini > 12 km/s). While most stars were found to be rigid rotators, only a few tens show the signatures of differential rotation. The present work comprises the rotational study of some 180 additional stars. The overall broadening profile is derived according to Reiners et al. from hundreds of spectral lines by least-squares deconvolution, reducing spectral noise to a minimum. Projected rotational velocities vsini are measured for about 120 of the sample stars. Differential rotation produces a cuspy line shape which is best measured in inverse wavelength space by the first two zeros of its Fourier transform. Rigid and differential rotation can be distinguished for more than 50 rapid rotators (vsini > 12 km/s) among the sample stars from the available spectra. Ten stars with significant differential rotation rates of 10-54 % are identified, which add to the few known rapid differential rotators. Differential rotation measurements of 6 % and less for four of our targets are probably spurious and below the detection limit. Including these objects, the line shapes of more than 40 stars are consistent with rigid rotation.
Very little is known about magnetic fields of extrasolar planets and brown dwarfs. We use the energy flux scaling law presented by Christensen et al. (2009) to calculate the evolution of average magnetic fields in extrasolar planets and brown dwarfs
under the assumption of fast rotation, which is probably the case for most of them. We find that massive brown dwarfs of about 70 M_Jup can have fields of a few kilo-Gauss during the first few hundred Million years. These fields can grow by a factor of two before they weaken after deuterium burning has stopped. Brown dwarfs with weak deuterium burning and extrasolar giant planets start with magnetic fields between ~100G and ~1kG at the age of a few Myr, depending on their mass. Their magnetic field weakens steadily until after 10Gyr it has shrunk by about a factor of 10. We use observed X-ray luminosities to estimate the age of the known extrasolar giant planets that are more massive than 0.3M_Jup and closer than 20pc. Taking into account the age estimate, and assuming sun-like wind-properties and radio emission processes similar to those at Jupiter, we calculate their radio flux and its frequency. The highest radio flux we predict comes out as 700mJy at a frequency around 150MHz for $tau$Boob, but the flux is below 60mJy for the rest. Most planets are expected to emit radiation between a few Mhz and up to 100MHz, well above the ionospheric cutoff frequency.
We present first results from a multi-object spectroscopy campaign in IC2602, the Hyades, the Pleiades, and the Coma cluster using VLT/FLAMES. We analysed the data for radial velocity, rotational velocity, and H-alpha activity. Here, we highlight thr
ee aspects of this study in the context of rotational braking and the rotation-activity relationship among low mass stars. Finally we discuss the cluster membership of sources in IC2602.
We show individual high resolution spectra of components A, B, and C of the nearby late-M type multiple system LHS 1070. Component A is a mid-M star, B and C are known to have masses at the threshold to brown dwarfs. From our spectra we measure rotat
ion velocities and the mean magnetic field for all three components individually. We find magnetic flux on the order of several kilo-Gauss in all components. The rotation velocities of the two late-M objects B and C are similar (vsini = 16km/s), the earlier A component is spinning only at about half that rate. This suggests weakening of net rotational braking at late-M spectral type, and that the lack of slowly rotating late-M and L dwarfs is real. Furthermore, we found that magnetic flux in the B component is about twice as strong as in component C at similar rotation rate. This indicates that rotational braking is not proportional to magnetic field strength in fully convective objects, and that a different field topology is the reason for the weak braking in low mass objects.
