5.4.5.Kelu1
To our knowledge,this L2.5V-type brown dwarf(T e?～2030K)shows the largest projected rotational velocity amongst all known ultracool dwarfs(Basri et al.(2000)),indicating that it is indeed a very fast rotator.Hence,it is expected that Kelu1shows an oblate shape,which coupled with the formation of dust clouds favor the detection of polarization.Kelu1has also been found to be photometrically variable with a periodicity of about1.8h(Clarke et al.(2002a));nevertheless,the amplitude of the I-band light curve is small(roughly6mmag).Further spectrocopic follow-up of this object reveals a modulated variability of the Hαline intensity;the line is always detected in rather low emission(Clarke et al.(2003)).Kelu1 appears overluminous in the diagram of Fig.5,which suggests that this object is either a binary of similar components or a young object or both.However,high-resolution direct imaging and radial velocity searches for companions have failed to resolve Kelu1(Mart′?n et al.(1999b);Clarke et al.(2003)),imposing upper limits on the separation of the binary(≤0.3arcsec)and on the mass of the companion(M sin i≤10M Jup). Youth is quite likely because the Li i resonance doublet is detected in the optical spectrum,which implies young ages and consequently relatively lower gravities than?eld dwarfs of similar types.Nevertheless, Kelu1is not as young as CFHT-BD-Tau4since no infrared excesses are observed in the former brown dwarf (Golimowski et al.(2004)),it is not located in any star-forming region,and the alkali lines of Kelu1’s optical spectrum appear relatively strong(McGovern et al.(2004)).Using evolutionary models as those illustrated in Fig.5,the age of Kelu1is estimated at a few hundred Myr,and its mass is in the interval30–50M Jup (35M Jup at the age of100Myr).As pointed out by Sengupta(2003),for a given rotational velocity,the degree of polarization is higher for objects with comparatively lower surface gravity.Interestingly,various
–14–
groups of authors provide di?erent pseudo-equivalent width measurements of the Li iλ670.8nm feature(from slightly less than1?A up to4.7?A,see Mart′?n et al.(1998),and references therein).This may be indicative of some sort of activity that changes the strength of the TiO molecular absorptions(e.g.di?erent coverage of dust clouds).It is also possible that Kelu1is burning lithium if its age is around a few hundred Myr. From our data,Kelu1shows a detectable polarization in the I-band.The fast rotation,the formation of clouds of condensates,and the relatively atmospheric low gravity may account for the measured polarization amplitude.
5.4.
6.2MASS J14122449+1633115
Our polarimetric data suggest that this L0.5V-type dwarf shows some polarization.However,the detection is marginal,with P/σP=3.0.Further data would be needed to con?rm or discard our result. Gelino et al.(2002)did not?nd any evidence for photometric variability in J1412+16,imposing an upper limit on the I-band variability of25mmag.In addition,this object shows moderate Hαemission(Kirkpatrick et al.(2000)).
5.4.7.2MASS J15074769?1627386
This L5V-type dwarf shows one of the highest degrees of I-band linear polarization in our sample (P/σP=4.5).Because of the proximity of this object(parallactic distance of7.3pc,Vrba et al.(2004)), the interstellar medium is not likely contributing to the observed polarization.From optical spectroscopic observations,J1507?16does not show Hαemission,and has e?ciently depleted its original lithium content. This suggests,as for J0036+18,that the age of J1507?16is older than several hundred Myr.From its location in Fig.5,the most likely age of this object is very similar to that of the great majority of?eld dwarfs with similar spectral types(～1–5Gyr).The L′and M′photometric data of Golimowski et al.(2004) indicate that there are no infrared excesses associated to this object.And Berger(2002)reported an upper limit on the radio emission of58μJy at8.46GHz,suggesting that if any magnetic?eld exists,it has to be rather weak,unable to polarize the photospheric optical photons.Thus,the measured polarization is very likely intrinsic to the physical properties of the ultracool atmosphere of J1507?16.According to Bailer-Jones (2004),the projected rotational velocity of this object is rather high,which would impose a deviation from sphericity in its shape.A signi?cant oblateness and a heterogeneous distribution of dust clouds on the surface might account for the relatively large degree of polarization.It is worth noting that while J1507?16did not show convincing photometric variability in the I-band monitoring program of Koen(2003),the CLOUDS collaboration(Goldman(2003))has detected a1%variation(I-band)over a few hours(Goldman2004,priv. communication).More polarimetric data are needed to study changes in the amplitude and direction of the polarization,which in combination with the I-band photometric light curves,would con?rm the presence of dust clouds in the photosphere.
5.4.8.2MASS J17073334+4301304
It shows strong Hαemission in the spectrum of Cruz et al.(2003).As pointed out by these authors, this L0.5V dwarf should be monitored to check whether it was observed during a period of unusually high
–15–
activity.We note that the Li i absorption line atλ670.8nm is not apparent in Fig.8of Cruz et al.(2003), indicating that the age of this object is larger than several hundred Myr.Our two measurements taken on di?erent epochs do not suggest polarimetric variability.
5.4.9.2MASS J21580457?1540098
Despite the fact that the averaged polarimetric value shown in Table3does not comply with our criterion of polarization,we consider this L4V dwarf to be a likely polarized source.The two separated measurements listed in Table2are indicative of the presence of polarized photons.However,these measurements di?er by more than3σthe uncertainties,suggesting some kind of polarimetric variability.The baseline of the two epochs of observations is0.96yr.To the best of our knowledge,this dwarf has not been photometrically monitored by any group of observers.We note that the averaged polarimetric value is plotted in Figs.1 and2,and that only the most likely detection(largest P/σP)is listed in Table4.The pseudo-equivalent widths of the atomic lines observed in low-resolution near-infrared spectra are very similar to those of?eld dwarfs(McLean et al.(2003)),suggesting that J2158?15has the typical age of other?eld objects of related spectral classes.
5.4.10.2MASS J22443167+2043433
This L6.5V dwarf is a quite peculiar object.It displays the highest I-band linear polarization degree in our sample(P=2.5±0.5%),for which we have determined a relatively high P/σP.As pointed out in the literature,J2244?01is nearly0.3–0.5mag redder in optical,near-infrared and infrared colors than all other L dwars(Dahn et al.(2002);Golimowski et al.(2004);Knapp et al.(2004)).In addition,McLean et al.(2003) have found that this object has unusual near-infrared spectral features(very weak atomic lines and the peak ?ux in the J-band is less than at H and K).In contrast,the optical low-resolution spectrum of J2244?01 depicted in Fig.25of McLean et al.(2003)appears similar to other dwarfs of related spectral types.The weak atomic lines observed in the near-infrared spectrum may suggest a low photospheric gravity,i.e.a young age.However,this is not supported by the strong alkali lines(K i,Na i,Cs i)of the optical spectrum. Using the optical spectroscopic data kindly provided by J.Davy Kirkpatrick(as in McLean et al.(2003)), we have imposed an upper limit on the Hαemission(Table4),and have con?rmed the brown dwarf nature of J2244?01by detecting Li i(Rebolo et al.(1992)).It has been suggested(Golimowski et al.(2004), and references therein)that the observed spectroscopic and photometric properties of this brown dwarf can be attributed to heterogeneous,very thick cloud decks(that may cause an unusually strong veiling in the near-infrared wavelengths)resulting from a high metal abundance.Thick dust clouds would give rise to multiple scattering of photons,which in turn would yield less polarization than single scattering(Sengupta (2003)).Nevertheless,even the single scattering models of Sengupta(2003)show di?culties in explaining the signi?cant amplitude of our polarimetric measurement.On the assumption that polarization is intrinsic to the object atmosphere and the presence of dust clouds,one possible way to account for the observed high polarization is by invoking rather large-size grains.In that case,J2244+20should show polarization in the near-infrared wavelengths as well.Other possibility is related to the presence of a surrounding dusty disk or shell.As can be seen in the literature,many T Tauri stars show similar polarization degrees to that of J2244+20.Therefore,based on our data,we cannot discard that this dwarf is a young object.The determination of the astrometric parallax would help constrain its age.In addition,further photopolarimetric
