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the m8 brown dwarf 2massw j1207334 - 393254 ( hereafter 2m1207a ) is proving to be an important system for studying the formation of substellar objects .
it was discovered by @xcite in a search for brown dwarf members of the @xmath2 myr old tw hydrae assocation @xcite .
2m1207a is a very - low - mass substellar analog to a classical t tauri star : it has broad , variable h@xmath3 emission due to accretion @xcite , mid - infrared excess due to a disk @xcite , ultraviolet emission due to hot accreted gas and warm circumstellar molecular hydrogen gas @xcite , and forbidden oxygen emission due to an outflow@xcite . despite its youth
, it is not detected in x rays @xcite or radio @xcite , so is apparently relatively magnetically inactive .
@xcite discovered a red companion ( 2m1207b ) , 5 magnitudes fainter in the k band .
common proper motion confirms that this is a bound pair @xcite with a separation of @xmath4 .
the secondary has a late - l spectral type @xcite .
the inferred luminosity implies a mass @xmath5 @xcite , although @xcite suggest that the secondary is @xmath6 jupiter masses and viewed through an edge - on disk .
because the twa is a relatively nearby , loose association there has been some confusion on the distance to the system .
@xcite adopted a distance of 70 pc , on the basis of theoretical models of brown dwarf evolution .
the hipparcos distance of tw hya itself is @xmath7 @xcite .
@xcite used the moving cluster distance method to estimate the distance to 2m1207a to be @xmath8 pc , while @xcite used the same method , but an updated proper motion and a different group membership list to estimate @xmath9 pc . with uncertainties in the distance to the tw hya group of @xmath10 , firm conclusions about the natures of 2m1207
a and b , as well as other members of the group , have been elusive .
" here we present the first trigonometric parallax for 2m1207a .
we confirm that it is a member of the tw hya association and put put constraints on the planet candidate 2m1207b .
observations of 2m1207a in the @xmath11 band were obtained at the ctio 0.9 m telescope by the recons group via the smarts consortium .
there are 54 parallax frames obtained over 2.14 years .
the observing techniques and data reduction are fully described by @xcite .
the resulting relative parallax is @xmath12 .
@xmath13 photometry was obtained in july 2007 on five nights using the same telescope and reduced as described in @xcite .
we estimate the correction to absolute parallax to be @xmath14 on the basis of photometry of the seven reference stars ( table 1 . )
the absolute parallax is therefore @xmath15 , for a distance of @xmath16 .
the observed proper motion is @xmath17 at position angle @xmath18 degrees .
the distance and proper motion of 2m1207 is consistent with twa membership .
the position angle expected for motion towards @xcite s twa convergent point is 251.4 degrees , consistent with the measured proper motion . using @xcite s radial velocity of @xmath19 for 2m1207a , the ( u , v , w ) space velocities are ( @xmath20 ) @xmath21 , consistent with mamajek s centroid group value of ( @xmath22 ) @xmath21 . in particular , the measured distance rules out any association with the background lower centaurus crux discussed by @xcite . using @xcite s measurements and our distance , the projected separation is @xmath23 a.u .
_ the primary and its disk : _
@xcite found 2m1207a to be @xmath24 brown dwarf . because they used mamajek s value of 53 pc as the distance , this mass is not changed significantly by a distance increase of 2% : 2m1207a is best understood as a @xmath25 brown dwarf .
the disk parameters derived by @xcite also remain unchanged because they used the same distance .
the observed @xmath26 is consistent with the m8 spectral type and suggests the accretion rate at the time was @xmath27 ( see figure 4 of riaz & gizis 2007 . ) in figure  1 , we plot the h - r diagram of the local field population and 2m1207a . like the young m
dwarf au mic ( gl 803 ) , 2m1207a lies @xmath28 magnitudes above the main sequence in the @xmath29 vs @xmath30 diagram , confirming youth .    _
the secondary : _ the usual procedure for analyzing 2m1207b is to assume a bolometric correction appropriate to late - l dwarfs , and then fit the luminosity to evolutionary models .
@xcite estimated @xmath31 for 70 pc , @xcite estimated @xmath32 for 59 pc , and @xcite estimated @xmath33 for 53 pc .
the trigonometric parallax would therefore support the last two estimates .
@xcite , however , noted an inconsistency with this procedure .
they argued that their h and k - band near - infrared spectra of 2m1207b were best fit by an effective temperature of @xmath34k .
however , for the @xmath35 fits , the expected effective temperature is more like @xmath36 k. they suggest the best resolution is that 2m1207b is viewed through an edge - on gray disk , and that therefore it is more luminous than otherwise estimated .
2m1207b is then a @xmath37 planetary mass brown dwarf .
the wide separation and mass ratio ( @xmath38 ) suggests this planetary - mass object did not form through core accretion @xcite .    without rejecting the possibility of a edge - on disk
, we argue that available evidence does not rule out a low temperature for 2m1207b . in figure  2 , we plot colors and absolute magnitudes for late - m , l and t dwarfs with parallaxes @xcite .
all the previous attempts to fit 2m1207b noted that it is red compared to field brown dwarfs , which can be attributed to having more dust in the photosphere .
the faintness at j - band measured by @xcite ( @xmath39 ) is supported by the nicmos f110 m measurement of @xcite ( @xmath40 ) .
@xcite measured @xmath41 and @xmath42 for 2m1207b . on the other hand , it is clear from mohanty et al.s ( 2007 ) ( @xmath43 ) spectrum that there is deep water absorption but little methane absorption . we conclude that 2m1207b has a spectral energy distribution that is l - type , but very red .
reversing the usual procedure , in figure  3 we plot the required k - band bolometric correction required to fit the @xcite models at an age of 10 myr . observed bolometric corrections for field l and t dwarfs from @xcite as a function of temperature for an assumed age of @xmath44 are also shown .
the l to t transition is believed to occur at @xmath45k , and can be marked in figure 3 by the change in bolometric corrections .
the transition is related to the change in dust properties in photosphere ( see kirkpatrick 2005 for a review ) , and some of the first fits to late - l dwarf spectra gave incorrect values of @xmath46k due to the failure of cooler models to resemble the real spectra .
this simply reflects the extreme difficulty of modelling the temperature range 1200 - 1400k , and indeed in light of this , no models succeed in fully explaining both the blue hook and brightening in j - band of field t dwarfs .
analysis of the luminosity ( see kirkpatrick ) has been the most reliable way to derive temperatures .
this history suggests to us that the existing fit must be viewed with caution  while apparently very good , it is inconsistent with the absolute magnitudes unless an edge - on disk is invoked .
although mohanty et al . show that the dusty models do fit the observed color of 2m1207b , it must be noted that the same models predict very red colors for field l dwarfs , which are not observed .
indeed , @xcite note that `` the dusty and cond models represent extreme situations which bracket the more likely intermediate case resulting from complex , and presently not understood , thermochemical and dynamical processes . ''
we think it plausible that existing dusty models fail to properly model the dust in low surface gravity dwarfs , and that 2m1207b might therefore be @xmath47k , as expected for the @xmath48 model . as an example how this might occur ,
we note that @xcite invokes a parameter , @xmath49 , that characterizes the thickness of the clouds , and argues that the wide range of colors for field objects near @xmath50k is due to changes in this parameter in otherwise similar brown dwarfs . in one case , @xcite is able to fit an l6.5 dwarf with @xmath51k or @xmath52k ( without methane absorption ) by varying @xmath49 by only 100k .
evidently an extremely red color like 2m1207b could be obtained for a low @xmath53 with @xmath54k  that is , a very thick cloud compared to field l dwarfs .
this would be the opposite situation from field t dwarfs , where the cloud becomes thinner ( @xmath49 increases . ) similarly , in the @xcite models , a parameter , @xmath55 , represents sedimentation , and redder colors are produced by smaller values of @xmath55 ( i.e. , less precipitation and thicker clouds . )
regardless of how the proper degree of dust is produced , if our speculation that @xmath56k is correct , the implied @xmath57 would require that more of 2m1207b s energy is escaping at wavelengths longward of 3 microns than in field l dwarfs .
in any case , the observed colors and spectrum do not match any field brown dwarf , so there is not much doubt that atmosphere is dustier , but a low temperature remains speculative .
unfortunately , there is a third problem with estimating the mass of 2m1207b .
@xcite have investigated the dependence of the structure models to the initial conditions , and found that the luminosity is very sensitive to the initial conditions for up to 100 myr . in specifically discussing the case of 2m1207b
, they note that for a `` warm start '' rather than usually assumed `` hot start , '' the best fit mass is @xmath58 rather than @xmath59 .
the situation , therefore , is that given the now known distance , 2m1207b may be @xmath60 _ if _ current structural models are correct , the red color implies a cool temperature , and there is no disk , but both @xcite and @xcite present plausible scenarios in which the mass is higher .
we have measured the trigonometric parallax of 2m1207 and found that the distance and space motion are consistent , as expected , with membership in the tw hydrae association . indeed
, 2m1207 now has a more precise distance determination than tw hya itself .
there are no difficulties in modelling the primary : it is a @xmath1 mass brown dwarf that is accreting from a circumstellar disk .
the faint secondary remains problematic . because we do not know the appropriate initial conditions ,
do not have a model atmosphere that reproduces the colors , and do not know whether or not it is observed through a disk , a case can be made that 2m1207b s mass is as low as @xmath61 or as high as @xmath62 . our best estimate is @xmath48 if 2m1207b is not viewed through a disk .
further study of this planetary mass object is needed ; we particularly need to know if it has an effective temperature of @xmath63k , @xmath64k , or even less .
we thank eric mamajek and davy kirkpatrick for useful discussions .
we thank charlie finch for his initial reduction of the parallax data .
we thank the anonymous referee for a discussion of the tsuji paper .
we also thank the members of the smarts consortium , without whom the parallax observations could not have been made .
support for this work was provided by nasa research grant nng06gj03 g .
the recons parallax program has been supported by the nasa / nsf nstars project , nasa s space interferometry mission ( sim ) , the national science foundation ( grant ast 05 - 07711 ) , and georgia state university . research has benefitted from the m , l , and t dwarf compendium housed at dwarfarchives.org and maintained by chris gelino , davy kirkpatrick , and adam burgasser .
rrr @xmath65 & @xmath66 & + @xmath67 & @xmath68 & + @xmath69 & @xmath70 & + @xmath71 & @xmath72 & @xmath73 + @xmath74 & @xmath75 & @xmath76 + @xmath77 & @xmath78 & @xmath79 + @xmath80 ( mas ) & @xmath81 + @xmath82 ( mas ) & @xmath83 + @xmath84 ( mas ) & @xmath85 + @xmath29 & @xmath86 & + @xmath87 & @xmath88 & @xmath89 + @xmath90 & @xmath91 & @xmath92 + @xmath93 & @xmath94 & @xmath95 + mass ( @xmath96 ) & @xmath97 & @xmath98 +
we have measured a trigonometric parallax to the young brown dwarf 2massw j1207334 - 393254 . 
 the distance ( @xmath0 ) and space motion confirm membership in the tw hydrae association . 
 the primary is a @xmath1 brown dwarf . 
 we discuss the `` planetary mass '' secondary , which is certainly below the deuterium - burning limit but whose colors and absolute magnitudes pose challenges to our current understanding of planetary - mass objects .