2009A&A...502..929G


Query : 2009A&A...502..929G

2009A&A...502..929G - Astronomy and Astrophysics, volume 502, 929-936 (2009/8-2)

Polarisation of very-low-mass stars and brown dwarfs. I. VLT/FORS1 optical observations of field ultra-cool dwarfs.

GOLDMAN B., PITANN J., ZAPATERO OSORIO M.R., BAILER-JONES C.A.L., BEJAR V.J.S., CABALLERO J.A. and HENNING T.

Abstract (from CDS):

Ultra-cool dwarfs of the L spectral type (Teff=1400-2200K) are known to have dusty atmospheres. Asymmetries of the dwarf surface may arise from rotationally-induced flattening and dust-cloud coverage, and may result in non-zero linear polarisation through dust scattering. We aim to study the heterogeneity of ultra-cool dwarfs' atmospheres and the grain-size effects on the polarisation degree in a sample of nine late M, L and early T dwarfs. We obtain linear polarimetric imaging measurements using FORS1 at the Very Large Telescope, in the Bessel I filter, and for a subset in the Bessel R and the Gunn z filters. We measure a polarisation degree of (0.31±0.06)% for LHS102BC. We fail to detect linear polarisation in the rest of our sample, with upper-limits on the polarisation degree of each object of 0.09% to 0.76% (95% of confidence level), depending on the targets and the bands. For those targets we do not find evidence of large-scale cloud horizontal structure in our data. Together with previous surveys, our results set the fraction of ultra-cool dwarfs with detected linear polarisation to 30+10–6% (1-σ errors). From the whole sample of well-measured objects with errors smaller than 0.1%, the fraction of ultra-cool dwarfs with polarisation degree larger than 0.3% is smaller than 16% (95% confidence level). For three brown dwarfs, our observations indicate polarisation degrees different (at the 3-σ level) than previously reported, giving hints of possible variations. Our results fail to correlate with the current model predictions for ultra-cool dwarf polarisation for a flattening-induced polarisation, or with the variability studies for a polarisation induced by an heterogeneous cloud cover. This stresses the intricacy of each of those tasks, but may arise as well from complex and dynamic atmospheric processes.

Abstract Copyright:

Journal keyword(s): stars: low-mass, brown dwarfs - stars: atmospheres - polarization - stars: individual: LHS 102B

Simbad objects: 15

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Number of rows : 15
N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2023
#notes
1 L 362-29 B BD* 00 04 34.8671722377 -40 44 06.463544237       19.61 16.88 L5+L5 103 0
2 LSPM J0036+1821 BD* 00 36 16.1118891182 +18 21 10.286665966       18.451 16.053 L3.5 204 0
3 CPD-69 177 WD* 03 10 31.0195960176 -68 36 03.380768388 10.757 11.413 11.394 11.47 11.558 DA3.0 255 0
4 NGC 2024 Cl* 05 41 43 -01 50.5           ~ 1124 1
5 NAME Kelu-1 BD* 13 05 40.1642434199 -25 41 05.712165150         16.85 L2+L3.5 247 0
6 EGGR 360 WD* 13 47 24.3696354602 +10 21 38.013189353   15.49   14.8   DA7.1 92 0
7 2MASSW J1507476-162738 BD* 15 07 47.6758255542 -16 27 40.113982846     22.136 18.928 16.579 L5V 186 0
8 EGGR 118 WD* 16 17 55.2411185208 -15 35 52.154367684 12.086 13.215 13.425 13.558 13.714 DA1.7 92 0
9 CD-28 13479 * 17 43 19.5888438432 -28 40 32.665638756 11.17 11.36 10.61 10.46   B1II-III:nn 31 0
10 BD-12 5133 * 18 40 01.6961497488 -12 24 06.777653508 11.25 11.39 10.71 10.70   OB- 36 0
11 2MASSI J2158045-155009 BD* 21 58 04.5716969232 -15 50 09.793729140       20.771 18.582 L4 46 0
12 SIPS J2200-3038 ** 22 00 02.01888 -30 38 32.7120       19.2 16.7 M9+L0 29 0
13 * eps Ind B BD* 22 04 10.5951812451 -56 46 58.239102418     24.12 20.8 15.60 T1V+T6V 152 1
14 2MASS J22443167+2043433 BD* 22 44 31.67376 +20 43 43.3020           L6-L8gamma 88 0
15 2MASS J22521073-1730134 BD* 22 52 10.7512306563 -17 30 13.889743018         17.9 L4.5+T3.5 50 0

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2023.02.09-10:22:31

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