Optical Linear Polarization of Late M- and L-Type Dwarfs

(Abridged). We report on the linear polarimetric observations in the Johnson I 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 have compared the M- and L-dwarf populations finding evidence for a larger frequency of high I-band polarization in the coolest objects, supporting the presence of significant amounts of dust in L-dwarfs. The probable polarizing mechanism is related to the presence of heterogeneous dust clouds nonuniformly distributed across the visible photospheres and the asymmetric shape of the objects. In some young ultracool dwarfs, surrounding dusty disks may also yield polarization. For polarimetric detections, a trend for slightly larger polarization from L0 to L6.5 may be present in our data, suggesting changes in the distribution of the grain properties, vertical height of the clouds, metallicity, age, and rotation speed. One of our targets is the peculiar brown dwarf (BD) 2MASS J2244+20 (L6.5), which shows the largest I-band polarization degree. Its origin may lie in a surrounding dusty disk or rather large photospheric dust grains. The M7 young BD CFHT-BD-Tau 4 and the L3.5 field dwarf 2MASS J0036+18 were also observed in the Johnson R filter. Our data support the presence of a circum(sub)stellar disk around the young accreting BD. Our data also support a grain growth in the submicron regime in the visible photosphere of J0036+18 (1900 K). The polarimetric data do not obviously correlate with activity or projected rotational velocity. Three polarized early- to mid-L dwarfs display I-band light curves with amplitudes below 10 mmag.Comment: Accepted for publication in ApJ (March 2005), 35 pages, 5 figure

Some Polarimetry I Have Done at University of Wisconsin

Observations were made of double stars, and the cluster NGC 1502 and the line of sight to it with the University of Wisconsin spectropolarimeter (HPOL). Observations of more stars with ISM polarization were made with the Wisconsin Ultraviolet Photopolarimeter (WUPPE) (ASTRO-2 Shuttle mission) besides those which appeared in the first WUPPE ISM paper (AJ 112, 2126). -- 3 -- 1. Double Stars For my PhD thesis (CWRU 1976) I observed more than 30 wide, bright, distant physical double stars. Previous work (e.g. Krzeminski and Serkowski 1967) had yielded values of fractions of a parsec for the upper limit of the microscale of fluctuation in the polarizing properties of the interstellar medium. If differences in polarization between stars in pairs were found, a direct estimate of the microscale could be made; if not, a lower limit could be estimated. From this data I placed a lower limit of 10000 AU on the microscale. The limit could not be placed much higher since only about one third of th..

( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE FIRST ULTRAVIOLET AND OPTICAL SPECTROPOLARIMETRY OF THE B[e] STAR HD 50138

We report the Ðrst ultraviolet spectropolarimetry of the B[e] star HD 50138, obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment aboard the Astro-2 Space Shuttle mission in 1995 March. The data cover the spectral range 1500È3200 A �. Spectropolarimetry obtained contemporaneously in the range of 3800È10,000 A � in the visualÈtoÈnear-IR region are presented as well. The presence of intrinsic polarization is detected from the UV to the near-IR. Strong evidence of a thin gaseous disk around the star is found. An almost Ñat wavelength dependence of the intrinsic polarization in the optical spectral region indicates that electron scattering, rather than dust scattering, is the dominant polarizing mechanism, although a small contribution due to dust scattering cannot be completely ruled out. A small inclination of the disk away from edge-on with respect to the line of sight is suggested. Comparison with similar polarimetric observations obtained for di†erent objects that have the same circumstellar geometry shows that the envelope of HD 50138 probably has a rather large density contrast between the equatorial and polar regions that does not produce a position angle Ñip like that observed in other objects, notably HD 45677

Ultraviolet Interstellar Polarization of Galactic Starlight I, Observations by the Wisconsin Ultraviolet Photo Polarimeter Experiment

The Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE) flew twice as part of NASA's Astro Spacelab missions in 1990 December and 1995 March. A systematic survey of the interstellar polarization in the ultraviolet was one of the main projects on both flights. The program was carefully crafted to 1) sample the galactic plane as uniformly as practicable, 2) explore sight lines of diverse chemical composition and morphology, and 3) measure the shape of the UV polarization through the full range of known wavelengths of peak polarization in the optical. We present here Astro-2 data for 20 previously unobserved sight lines and combine these with previously published UV data and with optical observations from the University of Wisconsin's ground based facilities and elsewhere. We thus have spectropolarimetry from 1500 A to 10,000 A for 35 galactic objects in which the polarization appears to be dominated by the interstellar component. The extrapolation of the empirical Serkowski formul..