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

a r X i v :a s t r o -p h /0411531v 1 18 N o v 2004Accepted for publication in ApJ,March 2005

Optical linear polarization of late M-and L-type dwarfs

M.R .Zapatero Osorio

LAEFF-INTA,P.O .50727,E-28080Madrid,Spain mosorio@laeff.esa.es J.A .Caballero Instituto de Astrof′?sica de Canarias,E-38200La Laguna,Tenerife,Spain zvezda@ll.iac.es and V.J.S .B′e jar Instituto de Astrof′?sica de Canarias,GTC Project.E-38200La Laguna,Tenerife,Spain vbejar@ll.iac.es ABSTRACT We report on the linear polarimetric observations in the Johnson I -band ?lter of 44ultracool dwarfs with spectral types between M6and L7.5,corresponding to e?ective temperatures in the range 2800–1400K,and one M4.5-type star.Based on our mea-surements of polarization (P )and their associated error bars (σP ),eleven (10L and 1M)dwarfs appear to have signi?cant linear polarization (P/σP ≥3).For these,the

polarization degrees we have measured are in the interval P =0.2–2.5%.Because of the typical average uncertainty of our data,we can easily con?rm polarization of ultracool dwarfs that show degree of linear polarization greater than 0.4%.We have compared the two populations in our sample,the M-and L-type dwarfs,and have found evidence for a larger frequency of high I -band polarization in the coolest objects,supporting the presence of signi?cant amounts of dust in L-type atmospheres.The probable mechanism polarizing the far-red optical photons of ultracool dwarfs is related to the presence of heterogeneous dust clouds nonuniformly distributed across the visible photospheres and the asymmetric shape of the objects (rapid rotations impose deviations from sphericity).In some young ultracool dwarfs,surrounding dusty disks (or shells)may also yield po-larization.For polarimetric detections,a trend for slightly larger polarization from L0to L6.5may be present in our data,suggesting changes in the distribution of the grain properties and in the vertical height of the cloud layer.Faster rotations and important

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di?erences in metallicity and age within our sample could also account for this trend.

One of the targets is the peculiar brown dwarf2MASS J22443167+2043433(L6.5),for

which we have determined the largest I-band polarization degree in our study.We

discuss that the origin of such large polarization may lie in a surrounding dusty disk(or

shell)and/or rather large photospheric dust grains.Two of the likely polarized dwarfs

(CFHT-BD-Tau4,a very young,M7-type brown dwarf of the Taurus star-forming re-

gion,and2MASS J00361617+1821104,an L3.5?eld dwarf)were also observed in the

Johnson R-band?lter,allowing us to discuss qualitatively the size of the grains respon-

sible for the polarization.Our data support the presence of a circum(sub)stellar disk

around the young accreting brown dwarf CFHT-BD-Tau4.The higher degree of polar-

ization in the R-band as compared to the I-band indicates that the grain growth lies

in the submicron regime in the visible photosphere of2MASS J00361617+1821104(ef-

fective temperature of about1900K).Our polarimetric data do not obviously correlate

with activity(Hαand radio emission)or projected rotational velocity.Three polarized

early-to mid-L dwarfs have been photometrically monitored in the I-band,displaying

light curves with amplitudes below10mmag.

Subject headings:polarization—stars:atmospheres—stars:late-type—stars:low-mass,

brown dwarfs

1.Introduction

Very low-mass dwarf stars and brown dwarfs are characterized by e?ective temperatures(T e?)below ～2800K,corresponding to spectral types later than M6–M7.These objects are often called“ultracool”dwarfs in the literature(e.g.Liebert et al.(1999);Gizis et al.(2000)).During the last decade,a large population of ultracool dwarfs covering an interval of T e?between2800and800K have been discovered by di?erent groups of observers(see the reviews by Basri(2000)and Chabrier&Bara?e(2000)).They have been assigned spectral types late-M,L and T.A detailed description of the features used for spectral classi?cation is provided by the following works:Mart′?n et al.(1999a);Kirkpatrick et al.(1999);Geballe et al.(2002); and Burgasser et al.(2002a).Refractory elements,like Ti,Fe,Ca and V,are strongly depleted from the gas in the atmospheres of ultracool dwarfs,forming clouds of dust grains(Tsuji et al.(1996)).Observations of the coolest dwarfs of spectral class T in combination with theory suggest that,below～1300K,grains predominantly lie in a thin deck at the very deep photosphere,i.e.beyond the visible region of the atmosphere, because of complete gravitational settling(Allard et al.(2001);Marley et al.(2002)).However,for the L type,there is ine?cient sedimentation,and dust clouds play an important role in controlling opacity and the temperature structure of the atmosphere.The physical properties(e.g.shape,size),the number density and the precise species of the dust particulates remain highly unknown.Also unknown are the geometrical height and the location of the cloud layer within the atmospheres.

Recently,Sengupta&Krishan(2001)argued that detectable polarization could arise because of dust scattering in the nonspherical atmosphere of L dwarfs.The size of the grains in the upper photosphere is expected to be in the submicron range;hence,polarization should occur at optical wavelengths.Nonzero polarization can also occur in the infrared if the particle size is large.The pioneering work by M′e nard et al.

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(2002)con?rmed the presence of red optical polarization in L dwarfs.Their work also suggested that a large fraction(about50%)of ultracool dwarfs shows signi?cant polarized I-band radiation(I Bessel?lter centred on768nm).These authors measured degrees of linear polarization of the order of0.1–0.2%in three L-type ?eld dwarfs,and attributed such polarization to the probable presence of dust clouds in the atmosphere of their targets.Sengupta(2003)tried to constrain the size of the grains by?tting models to the I-band observations of M′e nard et al.(2002).

Under the hypothesis that the linear polarization observed by M′e nard et al.(2002)is due to dust scattering,there are several factors that make net disk-integrated polarization more likely.First,nonspherical grains in the atmospheres.Second,the lack of symmetry in the shape of the objects,which leads to incomplete cancellation of the polarization of the radiation.The very fast rotation observed in the great majority of the known ultracool dwarfs(e.g.Basri(2000))gives them the shape of an oblate ellipsoid.Third,and of especial interest,dust clouds may evolve rapidly because of intense vertical motions,which are due to convective heat transport from the interior(S′a nchez-Lavega(2001)).Evidence for time evolution of the emergent?ux of ultracool dwarfs in intervals of one to several rotation periods is provided by the photometric monitoring carried out by various groups(e.g.Bailer-Jones&Mundt(2001);Mart′?n et al.(2001a);Gelino et al.(2002); Zapatero Osorio et al.(2003);see also the discussion by Bailer-Jones&Lamm(2003)).The rapidly evolving heterogeneous distribution of dust clouds may contribute to variations in the degree of polarization.In this respect,measurements of linear polarization can help identify the scattering mechanism,it can pinpoint an obscure source,and it can give information on the physical properties of the grains or the scattering medium. Depending on the geometry,polarimetric monitoring can also shed new light on the“weather”of ultracool dwarfs.

In this paper,we report on the linear polarimetric measurements in the R-and I-bands of44ultracool dwarfs with spectral types in the range M6–L7.5,and one M4.5V dwarf.We detect signi?cant polarized optical radiation in several of them,and discuss measured and zero linear polarization as a function of the known spectroscopic and photometric properties of the target sample.

2.Target selection

The total of45program objects are provided in Table1ordered by increasing right ascension.The great majority of them were selected from the DENIS and2MASS near-infrared sky surveys and the optical Sloan survey(“discovery”papers are the following:Delfosse et al.(1997);Kirkpatrick et al.(1999),(2000); Reid et al.(2000);Gizis et al.(2000);Gizis(2002);Liebert et al.(2003);McLean et al.(2003);Dahn et al.(2002);Hawley et al.(2002);Wilson et al.(2003);Cruz et al.(2003);Kendall et al.(2004)).Many (33)have spectral types in the range L0–L7.5V(as measured from optical and near-infrared spectra),with estimated e?ective temperatures from～2500K down to～1400K(Vrba et al.(2004)).Additional program objects with L classes are Kelu1(L2.5V,Ruiz,Leggett,&Allard(1997)),and the recently discovered low-metallicity L-type dwarf LSR J1610?0040(L′e pine et al.(2003)).This makes a total of35L dwarfs in our study.For comparison purposes,we also observed ten mid-M to late-M dwarfs,including BRI0021–0214 (M9.5V,Irwin,McMahon,&Reid(1991))and the M7-type young brown dwarf CFHT-BD-Tau4(Mart′?n et al.(2001b)).From now on,we will use“common”abridged names to refer to the various objects.

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