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 periods , 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 confirming that L dwarfs are rapid rotators .
Radial velocities have also been measured to a precision of 1–2 km/s .
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 hrs for the L0.5 dwarf 2M0746+2000 published by Gelino et al .
( [ ] ) may be ruled out as the rotation period .
The period of 11.2 \pm 0.8 hrs for the L1.5 dwarf 2M1145+2317 obtained by Bailer-Jones & Mundt ( [ ] ) is consistent with the present v \sin i results so is plausibly the true rotation period .
The inclination of the rotation axis is constrained to be i =62 ^ { \circ } – 90 ^ { \circ } with an expectation value of 76 ^ { \circ } .
Alternatively the data set a lower limit on the radius of 0.1R _ { \odot } , which is within the range of radii predicted by models for brown dwarfs older than 0.5 Gyr .
Similarly , the period of 2.7 \pm 0.1 hrs detected by the same authors for 2M1334+1940 is also confirmed as the likely rotation period ; the inclination is i =27 ^ { \circ } – 44 ^ { \circ } ( < i > = 34 ^ { \circ } ) .
Where no variability or period was detected by the monitoring programs the likely reason is low contrast modulating surface features .
However , in three cases variability but no period was detected , even though the likely rotation period range inferred from v \sin i lies within the timescale to which the monitoring was sensitive .
This reinforces the ‘ masking hypothesis ’ of Bailer-Jones & Mundt ( [ ] ) , the idea that the evolution of photospheric features on timescales shorter than the rotation period obscure the regular modulation of the light curve .
As has been previously discussed , a likely candidate for such features is inhomogeneous dust clouds .