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 their atmospheres and evolution. As part of an ongoing program to investigate this, projected rotation speeds, vsini, are presented for 16 field UCDs (M9V-L7.5V). This doubles the number of L dwarfs for which vsini has been measured. All targets are found to have vsini between 10 and 40 km/s, confirming that L dwarfs are rapid rotators. From the random distribution of the rotation axes, i, one-sided confidence intervals are placed on the rotation periods of individual object. These are compared with published period data obtained from photometric monitoring. From this, the period of 31 hrs for the L0.5 dwarf 2M0746+2000 published by Gelino et al. (2002) may be ruled out as the rotation period. The period of 11.2 +/- 0.8 hrs for the L1.5 dwarf 2M1145+2317 obtained by Bailer-Jones & Mundt (2001) is plausibly the rotation period. The inclination of the rotation axis is i = 62-90 deg with an expectation value of 76 deg. Alternatively the data set a lower limit on the radius of 0.1 Rsol. Similarly, the period of 2.7 +/- 0.1 hrs for 2M1334+1940 is the likely rotation period; the inclination is i = 27-44 deg (mean i = 34 deg). Where no variability or period was detected by the monitoring programs the likely reason is low contrast surface features. However, in three cases variability but no period was detected, despite sensitivity to the likely rotation periods inferred from vsini. This reinforces the `masking hypothesis', the idea that the evolution of photospheric features obscures regular modulation of the light curve. As has been previously discussed, a likely candidate for such features is inhomogeneous dust clouds.