Optical Linear Polarization of Late M and L Type Dwarfs

(Abridged). We report on the linear polarimetric observations in the JohnsonI filter of 44 M6-L7.5 ultracool dwarfs (2800-1400 K). Eleven (10 L and 1 M)dwarfs appear to have significant linear polarization (P = 0.2-2.5%). We havecompared the M- and L-dwarf populations finding evidence for a larger frequencyof high I-band polarization in the coolest objects, supporting the presence ofsignificant amounts of dust in L-dwarfs. The probable polarizing mechanism isrelated to the presence of heterogeneous dust clouds nonuniformly distributedacross the visible photospheres and the asymmetric shape of the objects. Insome young ultracool dwarfs, surrounding dusty disks may also yieldpolarization. For polarimetric detections, a trend for slightly largerpolarization from L0 to L6.5 may be present in our data, suggesting changes inthe distribution of the grain properties, vertical height of the clouds,metallicity, age, and rotation speed. One of our targets is the peculiar browndwarf (BD) 2MASS J2244+20 (L6.5), which shows the largest I-band polarizationdegree. Its origin may lie in a surrounding dusty disk or rather largephotospheric dust grains. The M7 young BD CFHT-BD-Tau 4 and the L3.5 fielddwarf 2MASS J0036+18 were also observed in the Johnson R filter. Our datasupport the presence of a circum(sub)stellar disk around the young accretingBD. Our data also support a grain growth in the submicron regime in the visiblephotosphere of J0036+18 (1900 K). The polarimetric data do not obviouslycorrelate with activity or projected rotational velocity. Three polarizedearly- to mid-L dwarfs display I-band light curves with amplitudes below 10mmag.Comment: Accepted for publication in ApJ (March 2005), 35 pages, 5 figure