Optical linear polarization in ultra cool dwarfs

A tool to probe dust in the ultra cool dwarf atmospheres

¹ University of Central Florida, Department of Physics, Orlando, 32816-2385, USA e-mail: tata@physics.ucf.edu
² Centro de Astrobioloíga (CAB-CSIC), Ctra. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
³ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁴ Indian Institute of Astrophysics, Koramangala, Bangalore, 560034 India
⁵ TIARA-ASIAA/National Tsing Hua University, Hsinchu, Taiwan
⁶ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 106, Taiwan, China
⁷ Department of Physics, HCMIU, Vietnam National University Administrative Building, Block 6, Linh Trung Ward, Thu Duc District, HCM, Vietnam

Received: 11 February 2009
Accepted: 13 October 2009

Abstract

Aims. Recent studies have detected linear polarization in L dwarfs in the optical I band. Theoretical models have been developed to explain this polarization. These models predict higher polarization at shorter wavelengths. We discuss the polarization in the R and I band of 4 ultra cool dwarfs.

Methods. We report linear polarization measurements of 4 ultra cool dwarfs in the R and I bands using the Intermediate dispersion Spectrograph and Imaging System (ISIS) mounted on the 4.2 m William Herschel Telescope (WHT).

Results. As predicted by theoretical models, we find a higher degree of polarization in the R band when compared to polarization in the I band for 3/4 of these ultra cool dwarfs. This suggests that dust scattering asymmetry is caused by oblateness . We also show how these measurements fit the theoretical models. A case for variability of linear polarization is found, which suggests the presence of randomly distributed dust clouds. We also discuss one case for the presence of a cold debris disk.