–16–
monitoring of this brown dwarf will be particularly interesting.
5.4.11.DENIS-P J225210.73?173013.4
This is the coolest dwarf(L7.5V)in our sample.According to Kendall et al.(2004),J2252?17is probably located at8.3pc(if it is a single object),being one of the nearest L dwarfs to the Sun.The averaged value of all our polarimetric measurements does not indicate detection because they di?er by more than5σthe uncertainties.However,on the basis of our criterion,the data of one of the epochs support the presence of polarized photons in the I-band light from J2252?17.We note that only the averaged polarimetric value is plotted in Figs.1and2,and that the measurement with the highest P/σP is shown in Table4.The polarization degree of J2252?17is similar to those of mid-L dwarfs,but is signi?cantly smaller than the polarization degree of J2244+20(L6.5V).
6.Final remarks and conclusions
We have conducted Johnson I-band(850nm)polarimetric observations of35L-type(L0–L7.5)?eld dwarfs and10M dwarfs(M4.5–M9.5).Johnson R-band(641nm)polarimetric data were also collected for two of the brightest targets in our sample:the L3.5dwarf2MASS J00361617+1821104,for which M′e nard et al.(2002)have previously detected signi?cant polarization,and the M7dwarf CFHT-BD-Tau4,which is a brown dwarf member of the Taurus star-forming region.Because of the typical average uncertainty of our measurements(P),we can easily con?rm polarization of objects with P≥0.4%.Eleven(10L and1 M)dwarfs show linear polarization degrees that comply with the following criterion:P/σP≥3,whereσP is the error bar.For these,our measurements are in the interval P=0.2–2.5%.The observed polarization is intrinsic to the objects.We have compared the fraction of polarized M and L dwarfs in our sample,and have found it to be higher for the cool L types.This is a clear evidence for the presence of considerable amounts of dust in ultracool atmospheres.
We argue that the most viable origin of the observed polarization in our sample is photon scattering by heterogeneous dust clouds in a rotationally-induced oblate photosphere.In some cases,the possible presence of dusty disks or dust shells gives rise to comparatively high polariztion.The linear polarization degree of2MASS J00361617+1821104(L3.5,～1900K)appears to decrease with increasing wavelength(from 641up to850nm),suggesting that the grain growth lies in the submicron regime throughout the dwarf atmosphere.Our polarimetric data of the young brown dwarf CFHT-BD-Tau4support the presence of a circum(sub)stellar dusty disk,which was previously suspected to exist from the observed near-infrared excesses and persistent,strong Hαemission.2MASS J22443167+2043433(L6.5),which is a peculiar brown dwarf known for its very red colors and unusual near-infrared spectrum,shows the largest polarization degree in our sample(P=2.5±0.5%).Rather large photospheric dust grains(possibly related to high metallicity) and/or the presence of a disk may account for such a considerable polarimetric amplitude.
We have compared our polarimetric measurements to I-band photometric varibility and rotation(using spectroscopic v sin i values).No obvious correlation is seen between polarization and the projected rotational velocities.Three likely polarized L-type dwarfs spanning spectral types L2.5–L5show I-band light curves with rather small amplitudes(≤10mmag)over a few hours of photometric monitoring.More light curves and further polarimetric studies are needed to prove(or discard)any possible relation between photometric
–17–
variability and polarization.
We thank M.Alises,F.Hoyo,J.Aceituno and U.Thiele for their assistance with the observations at the2.2m Calar Alto Telescope.We thank the referee(S.Sengupta)for his very valuable comments.We also thank R.Rebolo for his support at the time we started this work.We are indebted to B.Goldman for sharing with us data prior to publication.This paper is based on observations collected at the Centro Astron′o mico Hispano Alem′a n(CAHA)at Calar Alto,operated jointly by the Max-Planck Institut f¨u r Astronomie and the Instituto de Astrof′?sica de Andaluc′?a(CSIC).This research has made use of the SIMBAD database, operated at CDS,Strasbourg,France,and of data products from the Two Micron All Sky Survey,which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology,funded by the National Aeronautics and Space Administration and the National Science Foundation.This work was partly carried out with support from the projects AYA2003-05355and Ram′o n y Cajal.
Facilities:CAHA2.2m(CAFOS).
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