Spectroscopic rotation velocities of L dwarfs from VLT/UVES and their comparison with periods from photometric monitoring

The variability and rotation of ultra cool dwarfs (UCDs) provide important information on the atmospheres and evolution of these very low mass stars and brown dwarfs. As part of an ongoing program to investigate this, the projected rotation velocities, v $\sin i$, derived from high resolution VLT/UVES spectroscopy via cross correlation are presented for 16 field UCDs (M9V–L7.5V). This doubles the number of L dwarfs for which v $\sin i$ has been measured. All targets are found to have v $\sin i$ between 10 and 40 km s-1 confirming that L dwarfs are rapid rotators. Radial velocities have also been measured to a precision of 1–2 km s-1. From the random distribution of the rotation axes, i, and theoretically predicted radii, one-sided confidence intervals are placed on the rotation periods of individual objects. These are compared with published period data obtained from photometric monitoring programs. From this, the period of 31 h for the L0.5 dwarf 2M0746+200