So far it is not possible to draw any conclusions about the relation between optical polarization and activity as measured from Hαemission,?ares,X-ray or radio emission.There are very few data on this respect in the literature,and what is found appears to be contradictory.Berger(2002)has detected radio ?ares and radio constant emission from BRI0021and J0036+18in our sample,suggesting that radio activity is present between M-types and L3.5V.This contrasts with the observed drop in persistent Hαactivity beyond spectral type M7(Burgasser et al.(2000)).Moreover,neither BRI0021nor J0036+18show Hαemission (less than0.5?A pseudoequivalent width),except for very rare occasions(a?are in BRI0021was detected once,Reid et al.(1999)).From our data and according to M′e nard et al.(2002),J0036+18exhibits some polarization while BRI0021does not.The high polarization measured in J1507?16does not correlate with the non-detection of radio emission(Berger(2002)).When compared to warmer stars,ultracool dwarfs
–11–
(except for the very young ones)appear to be rather inactive in X-rays despite of their rapid rotation and,in some cases,moderately strong activity at radio wavelengths(Mart′?n&Bouy(2002)).The source responsible
for polarization does not seem to be strongly related to magnetic?elds,since the intensity of these?elds
as inferred from radio and X-ray activity is quite low and the atmospheres of ultracool dwarfs are rather neutral.
Figure4displays our polarimetric measurements as a function of projected rotational velocities(v sin i), which have been taken from the literature(see references above,and Basri&Marcy(1995)).The great majority of the ultracool dwarfs display rapid rotation(typically v sin i～10–60km s?1),despite the fact
they do not seem to be very active objects(Basri(2000)).Fast rotational velocities would impose deviations
from sphericity in the shape of the objects(i.e.asymmetry),favoring the detection of polarization in dusty atmospheres(Sengupta&Krishan(2001)).We fail to observe any correlation in Fig.4.However,this may
be due to the uncertainty introduced by the unknown equatorial inclination of v sin i,and the relatively poor statistics,i.e.there are rather few objects for which polarimetric observations and velocities are available.
5.4.The likely and possible polarized dwarfs
Finally,we will discuss each of the polarized dwarfs separately.The astrometric parallaxes have been obtained for nine of our target dwarfs(Dahn et al.(2002);Vrba et al.(2004)),allowing us to produce
an HR diagram as the one depicted in Fig.5.To incorporate CFHT-BD-Tau4into the Figure,we have converted its apparent K-band magnitude into the absolute value using the cannonical distance to the Taurus star-forming region(140pc).Three isochrones of10,100and1000Myr from Chabrier&Bara?e (2000)are overplotted onto the data.The mass intervals covered by the Figure are as follows:7–40M Jup (Jovian masses,10Myr),20–70M Jup(100Myr),and50–90M Jup(1Gyr).These state-of-the-art evolutionary models provide magnitudes in the?lters of interest.To transform predicted T e?’s into spectral types,the calibrations given by Dahn et al.(2002)for late-M types and Vrba et al.(2004)for the L classes have been applied.
5.4.1.2MASS J00361617+1821104
According to M′e nard et al.(2002),this L3.5V dwarf(T e?～1900K)shows some polarization(P=0.20±0.03%) in the I Bessel-band,which is centred on768nm.On the contrary,our I Johnson data suggest that there is
very little or no polarization at redder wavelengths(850nm).At the same time,we do detect signi?cant polarization at short wavelengths,in the R-band(641nm).This is worthy of especial mention.The dust properties(e.g.refractive index and scattering cross-section)depend on wavelength,i.e.they vary from one wavelength to another.Additionally,there is a hint for the variability of the R-band polarization angle,
as inferred from Table4.Our data indicate that the degree of linear polarization is possibly a few times larger at around641nm than at about850nm.This and the measurement of M′e nard et al.(2002)suggest
that the polarization observed in J0036+18systematically decreases with increasing wavelength,providing evidence for the presence of dust grains in the photosphere of this L-type dwarf since this behavior may not
be accounted for by other mechanisms,e.g.magnetic?elds.This polarimetric property may also shed new
light on the size of the particles responsible for the observed polarization.
As discussed by M′e nard et al.(2002),the intensity of the magnetic?eld that might be present in
–12–
J0036+18as inferred from its radio detection(135μJy at8.46GHz,Berger(2002)),is not powerful enough to induce a detectable linear polarization of the emergent optical radiation.In addition,J0036+18was not detected as variable(upper limits of9,16and25mmag on the I,J and K s variability,respectively)by the photometric monitoring programs of Gelino et al.(2002)and Caballero et al.(2003).On the other hand,J0036+18does not possess any infrared?ux excesses at3.8μm or4.7μm(Golimowski et al.(2004)), suggesting that there is no massive nor warm disk around this dwarf.Moreover,the optical spectrum of J0036+18does not show the Li i feature at670.8nm,which indicates that the age of the object is older than several hundred Myr(Chabrier&Bara?e(2000)).This is also consistent with the location of this dwarf in the HR diagram of Fig.5.The disk is very likely dissipated at these ages and does not contribute signi?cantly to the polarization.Hence,the most likely origin of the measured polarization in J0036+18 is related to the formation of dust clouds within the object’s atmosphere.From theoretical considerations, the degree of polarization due to single and multiple scattering will be more in the blue wavelengths if the grain size is very small(Sengupta&Krishan(2001)).By inspecting Fig.1of Sengupta(2003),the models that better reproduce the trend of the polarimetric observations of J0036+18are those computed for atmospheres with dust particle sizes less than1μm in diameter.The synthetic spectra provided by many groups(e.g.Tsuji et al.(1996),(2004);Allard et al.(2001))are obtained assuming size distributions of the grains between～0.006μm and0.25μm(submicron range),which is consistent with our result.However,the recent calculations of Woitke&Helling(2004)predict grain growths up to30and400μm at the deepest cloud layers.It would be desiderable to obtain more polarimetric data at longer and shorter wavelengths to provide a deeper study of the physical properties of the dust grains.If the particles are very small,no polarization is expected beyond1.3μm(i.e.the J-band).
5.4.2.2MASS J01410321+1804502
Recently con?rmed as a L4.5V dwarf by Wilson et al.(2003),as far as we know there is no more information on this object available in the literature.More data are required to con?rm the I-band polari-metric detection.The three measurements shown in Table2are consistent with each other within1σthe uncertainties,suggesting little variability in scales of days.
5.4.3.2MASS J01443536?0716142
Liebert et al.(2003)report the detection of a?are event in this L5V-type dwarf.This object displayed strong Hαemission which rapidly declined in about15minutes.Based on their spectroscopic data,the authors concluded that L-dwarfs are observed in strong?ares only occasionally.Further polarimetric obser-vations are needed to con?rm our(marginal)detection.The measurements obtained on four di?erent epochs indicate little variability within2σthe uncertainties.
5.4.4.CFHT-BD-Tau4
This is one of the warmest dwarfs in our sample(spectral class M7),and is also the youngest object. It is suspected to be a member of the Taurus star-forming region,with an age estimated at less than a few Myr.Its location in Fig.5clearly reveals that CFHT-BD-Tau4is overluminous as compared to the
–13–
?eld sequence.Previous photometric and spectroscopic studies identify this object as a very young brown dwarf,which is probably obscured by the presence of a surrounding shell from which the central object is intensively accreting.CFHT-BD-Tau4shows moderate extinction(A V～3mag),very strong and variable Hαemission(Mart′?n et al.(2001b);Jayawardhana et al.(2003)),X-ray activity(Mokler&Stelzer(2002)) and infrared excesses(Liu et al.(2003);Pascucci et al.(2003)),as do ordinary T Tauri stars.Gorlova et al.(2003)determined a rather low surface gravity for this object,which is consistent with very young ages. Additionally,the recent detection of millimeter dust emission(Klein et al.(2003))and the multiwavelength study of Pascucci et al.(2003)support the presence of circum(sub)stellar dust of about a few Earth masses in the form of a disk(the total mass of the disk is estimated at about1Jupiter mass,if we extrapolate the dust masses to disk masses assuming a gas-to-dust ratio of100).We detect signi?cant linear polarization in both R-and I-band wavelengths,which can be attributed to the disk around the central object,supporting the T Tauri scenario.We remark that other(older)late-M dwarfs in our sample do not show any evidence for linear polarization.Our data suggest that the degree of polarization of CFHT-BD-Tau4shows a trend in the sense that the amount of polarized radiation increases with wavelength.This trend is also observed in various T Tauri stars(e.g.V410Tau,Mekkaden(1999)).This behavior is di?erent from that of J0036+18, being a hint toward large grain sizes in the disk of CFHT-BD-Tau4.This result is in agreement with the very recent infrared observations of Apai et al.(2004).These authors determine that the silicate feature of CFHT-BD-Tau4is dominated by emission from2μm amorphous olivine grains.This brown dwarf provides compelling evidence that young substellar objects undergo a T Tauri-like accretion phase similar to that in low-mass stars.We do not detect photopolarimetric variability within1σthe error bars on scales of up to three days.However,it would be interesting to monitor this object(simultaneous photometry and polarimetry)in order to constrain the geometry of the circum(sub)stellar material.
