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hd  23514 ( hii  1132 ) is an f6 star in the pleiades . it is one of only a handful of youthful stars , ages 35 to @xmath2100 myr , that are surrounded by warm dust particles located in the terrestrial planet zone and which absorb a percent or so of the bolometric luminosity of their central star @xcite . we have observed hd  23514 with an adaptive optics system at the keck observatory using the nirc2 camera to check the binarity of this unusually dusty star .    to the best of our knowledge , to date no substellar companion to a stellar primary has been found in the pleiades . however , while this paper was being reviewed , @xcite announced the discovery of a substellar companion to the pleiad hii  1348 . studies of substellar objects in the pleiades focus on free floating brown dwarfs or companions to low mass stars @xcite . with an adaptive optics survey of 144 solar type stars in the pleiades , @xcite was sensitive to objects with masses down to @xmath20.03  m@xmath1 at separations of @xmath21 . however , their lowest mass companion ( @xmath20.15  m@xmath1 ) is still above the substellar boundary ( they did not observe hd  23514 ) . our work has revealed one such substellar companion in the pleiades . we observed hd  23514 with the adaptive optics ( ao ) system and the nirc2 narrow camera at the 10-m keck telescope at mauna kea observatory . observations were performed in natural guide star mode . some images were obtained in angular differential imaging mode ( adi ) while others had the primary behind a 600  mas or 800  mas diameter coronographic spot . the rest of the images were taken with dithered observations in position angle ( pa ) mode where the field of view is rotated so as to keep the sky fixed on the nirc2 camera . all data were reduced in the standard manner ( flat fielding , dark subtraction , sky subtraction ) with iraf routines . the data was linearized prior to processing using an idl program . we have applied the nirc2 distortion solution presented in @xcite to refine our astrometry . we adopt a pixel scale of @xmath3 mas / pix and a @xmath4 offset from north for the camera columns @xcite . we did not apply any advanced loci processing to analyze the data . our observations are summarized in table  [ tab : data ] . relative photometry was performed with iraf s * apphot * tasks using aperture sizes that maximized the signal - to - noise of the companion . generally , this was 11.4 times the fwhm of a point source . the apparent magnitude for the companion at k@xmath5 is estimated by correcting the 2mass ks magnitude of @xmath6 for the primary as described by @xcite , namely k@xmath5k @xmath7(h  k ) and using the relationship between k and ks described by @xcite . hd  23514 then has a k@xmath5 magnitude of @xmath8 , not too different from the ks magnitude . for the coronographic data , the primary is attenuated by a factor of 28.57 and 27.80 for the 600 and 800  mas diameter spot , respectively ( r.  galicher , private communication , 2011 ) . multiple short exposures were obtained with integration times range from 10 - 60 seconds , as listed in table  [ tab : data ] , and the companion is clearly visible in most of the individual frames . the primary is saturated in the 2006 , 2007 , and 2008 ks and k@xmath5 long exposures . the individual frames were registered to shorter , unsaturated frames using iraf s * xregister * task . photometry for these paired observations is performed with the same aperture size , but takes into account the different exposure times . we measure the candidate companion at a separation of @xmath9  with position angle @xmath10  in our 2010 coronographic observations . the object has remained at the same location between epoch 2006 and 2010 ( see table  [ tab : meas ] and figure  [ fig : image ] ) . had this been a more distant background object , then , based on the primary s proper and parallactic motion ( @xmath11 mas / yr , @xmath12 mas / yr ; * ? ? ? * ) , after 5 years this object should have been separated by @xmath13  and had a pa of @xmath14  ( see figure  [ fig : location ] ) . as mentioned in section  [ results ] , the 2006 adi data is somewhat unreliable as evidenced by the large uncertainties in table  [ tab : meas ] . comparison from 2007 yields an expected position of a background source of @xmath15  with pa of @xmath16 . over this shorter 4-year baseline , a background object can be ruled out at the @xmath212-@xmath17 level . the consistent values for separation and pa for the object from 2006 to 2010 imply this is a companion to the system or is a nearby co - moving pleiades member , a possibility we address in section  [ pleiad ] . the quoted photometry and astrometry in table  [ tab : meas ] is the average of all the frames and the uncertainties are standard deviations of the individual measurements . the more precise astrometry present in the latter epochs is possible since both the primary and secondary are detected and are unsaturated in the same frame . a companion 6.8 magnitudes fainter than the primary f6 star at ks would be a late - m dwarf ( see section  [ results ] ) . the j  ks and h ks colors of this object are also consistent with being late - m ( @xmath2m79 ; see @xcite ) . the j h color is somewhat discrepant being bluer than expected ; however , as shown in figure  [ fig : cmd ] , the measurement is consistent given the uncertainties . evolutionary models for a 100-myr old f6 star like hd  23514 ( 1.35  m@xmath1 and @xmath18  k ; * ? ? ? * ) predict an absolute ks magnitude of about 2.5 @xcite giving a distance modulus of 5.65 and a distance of 135  pc . an additional distance estimate can be obtained by considering that this star is part of the pleiades , which lies 133.5  pc away ( @xcite ; see also table  3 in @xcite ) . the pleiades extends nearly 10  in diameter ( * ? ? ? * and references therein ) ; if we assume a spherical distribution , this amounts to a possible radial extent of 11.7  pc . hence , the distance to hd  23514 would be somewhere between 122  pc and 145  pc ; we adopt a distance of @xmath19  pc . considering our ks photometry , the companion has @xmath20ks = @xmath21 , which when comparing with the 2mass photometry for hd  23514 gives an apparent magnitude of ks = @xmath22 . for our adopted distance , the absolute ks magnitude of the companion is @xmath23 .    to estimate the mass of the companion we make use of low - mass / substellar evolutionary models . the dusty evolutionary models of @xcite take into account the formation of condensates in the atmosphere and are appropriate for low - temperature objects down to @xmath21300  k. with these models , a 100  myr object with absolute k magnitude 9.27 would have a mass somewhere 0.06 and 0.07 solar masses and effective temperature @xmath22700  k. the evolutionary models of @xcite use the nextgen dust - free atmospheres and are appropriate for objects with @xmath24  k. these models similarly predict @xmath25  k and mass @xmath20.050.06 solar masses when compared to the ks - band photometry . the j and h - band photometry suggest somewhat lower masses ( down to 0.05  solar masses ) and cooler temperatures ( @xmath22500  k ) . combining all results together , we can infer that the companion has a mass of @xmath26 solar masses . this is below the stellar / substellar boundary making the object a brown dwarf . the temperature , likewise , appears to be @xmath27  k , placing it at spectral type @xmath2m7 ( see figure  2 in @xcite ) , consistent with the near - infared colors @xcite . no other object is detected in the @xmath28 field of view . at the distance to hd  23514 , this amounts to a radius of 675  au . figure  [ fig : limit ] presents the 5@xmath17 magnitude limit as a function of distance from the primary for the two coronographic ks observations ( 2009 and 2010 ) . these are our best limits for hd  23514 . limits are obtained by performing aperture photometry over blank regions of the sky at annuli surrounding the primary . these limits are averaged over the multiple frames . no companion is detected with a limit of @xmath21112 magnitudes beyond 1.5 ( @xmath2200  au ) . this suggests an absolute magnitude limit of @xmath214 at ks , or a mass of nearly @xmath211 jupiter masses when compared to evolutionary models @xcite . the average ks magnitude for the secondary , 14.92 , differs from the k@xmath5 magnitude , 15.40 , by half a magnitude . a late - m star is expected to have h ks@xmath20.5 , which suggests k@xmath5ks@xmath20.1 , not enough to account the difference . however , the uncertainty in our k@xmath5 photometry is the highest of our measurements , at nearly 0.4 magnitudes . for the 2006 data , the uncertainty is a result of the larger aperture size ( @xmath29 fwhm ) needed as the m - type companion is smeared out in the adi exposures . for 2007 and 2008 , the primary is saturated and the secondary s flux ( and location ) is compared to separate , shorter exposures as described in section  [ observations ] . the high uncertainty , combined with the expected difference in the k@xmath5 and ks filters is enough to account for the discrepancy .      while the common motion with hd  23514a is suggestive , there exists the possibility that the observed object may be a more distant ( or closer ) free - floating pleiades brown dwarf . as a member of the pleiades , it would have about the same proper motion and thus appear to be co - moving with hd  23514 . the most robust way to ascertain this would be to observe the system over many years in order to detect ( or rule out ) orbital motion . however , statistical arguments suggest the brown dwarf is indeed bound to hd  23514 . @xcite used a group of surveys to estimate the spatial distribution of brown dwarfs in the pleiades . hd  23514 is located about 1.7 degrees from the center ( @xmath30 , @xmath31 ; @xcite ) . at that distance , @xcite suggest a brown dwarf ( mass @xmath32  m@xmath33 ) density of 2.7 per square degree . however , more recent studies performed by @xcite and @xcite suggest average brown dwarf ( mass @xmath34  m@xmath33 ) densities of 7.8 and 6.3 per square degree , respectively . @xcite surveyed the pleiades in fields located 0.75 to 3.5 degrees away from the field center , @xcite incorporated that survey and in addition also covered the center of the pleiades with ukidss . thus , the area covered by these deeper surveys is similar in radial extent as that in @xcite ( @xcite ; out to 2.2  degrees from the center ) , but the density of brown dwarfs is larger by a factor of about 2 than the average brown dwarf density of 4.2 per square degree found by @xcite . to account for these deeper surveys ( mass @xmath34  m@xmath33 ) , we scale the density provided by @xcite and find that , at hd  23514 s distance from the center of the pleiades , the density of brown dwarfs is about @xmath25 per square degree . the nirc2 field of view is @xmath35 , or an area of @xmath36 square degrees . the likelihood of finding a free - floating pleiades brown dwarf with mass @xmath37m@xmath33 in the field of view is thus about @xmath38 . the small likelihood derived lends weight to the idea this is a bound companion . there are few brown dwarf companions known to stellar primaries ( the well - known  brown dwarf desert " phenomenon ; @xcite ) . even though the frequency of brown dwarf companions at separations @xmath3930  au is small  estimated to range from 0.5% to nearly 20% ( see discussion in @xcite ) hd 23514b is more likely to be a bound companion than an isolated pleiades brown dwarf . if we assume the orbit is circular and face - on , we can use the measured projected separation to estimate the yearly orbital motion of the system . given a mass of 1.35m@xmath33 , we estimate a period of @xmath26000  years . this yields an angular motion of 0.06 degrees per year or a linear motion of @xmath22.8  mas per year . this is comparable to our astrometric precision in our later epochs ( see table  [ tab : meas ] ) , suggesting a baseline of at least 5 more years would be needed to confirm orbital motion . however , the yearly motion is likely to be smaller as the orbit may be inclined and/or eccentric . it is instructive to consider whether or not a substellar object at that separation will remain bound over the 100  myr lifetime of the pleiades . figure  [ fig : binde ] presents the binding energy of resolved stellar and substellar binaries in the pleiades ( from @xcite ) . note that since inclination and eccentricity are typically not available for these systems ( and certainly not for hd  23514 ) , the true binding energy may be somewhat different than plotted here . despite not being the most widely separated binary in the pleiades ( @xcite  present systems at @xmath2900  au ) , hd  23514 does have the lowest binding energy among the stellar mass binaries . however , substellar pleiades binaries with separations @xmath4020  au have comparable or lower binding energies . the similar binding energies suggests that stellar interactions within the pleiades , while more frequent than in the field , may not be enough to disrupt the hd  23514 system in timescales much shorter than 100  myr . we note that hip  78530 @xcite and 1rxs  j160929.1210524 @xcite both host substellar companions at wide separations in the 5  myr - old upper scorpius region . these are considered physically bound based on common proper motion , similar to the case of hd  23514 . however , both systems have estimated binding energies lower than that of hd  23514 . previous searches for brown dwarf companions in the pleaides have focused on low - mass stars or free - floating brown dwarfs ( e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? @xcite used adaptive optics to observe 144 g- and k - type stars in the pleiades . the lowest mass companion they found was 0.15  m@xmath1 , though they were sensitive to objects with masses down to @xmath20.03  m@xmath1 at separations of @xmath21 . a deeper study conducted by @xcite also looked at solar - type stars , but found no substellar companions among @xmath220 pleaides stars . hence , while there may be hundreds of free - floating brown dwarfs in the pleaides @xcite , this is the first brown dwarf detected as a companion to a sun - like star . the scarcity of companion brown dwarfs to the @xmath2300 solar - type stars in the pleiades @xcite is consistent with the findings of a brown dwarf desert among solar - type stars out to separations of @xmath21000  au @xcite . hd  23514 is surrounded by a substantial amount of warm ( @xmath2700  k ) dust in a thin ring located @xmath20.25  au from the star if the emission comes from large blackbody grains or at most a few au if the grains are smaller and radiate less efficiently @xcite . at a distance of 135  pc , the companion in figure  [ fig : image ] has a projected separation of about 360  au , far larger than the dust semi - major axis . only a handful of systems are known to contain substantial quantities ( @xmath41 ) of warm dust : cha - near member ef  cha @xcite , @xmath42  pic member hd  172555 @xcite , lower centaurus crux member hd  113766 @xcite , ab dor member hd  15407 @xcite , m47 member p1121 @xcite , and ngc2547 member id8 @xcite . bd+20  307 is another main - sequence star known to contain a large amount of warm dust @xcite , however it is much older ( @xmath21  gyr ) than these other systems @xcite . of these 8 systems , hd  23514 , hd  15407 , hd  172555 , bd+20  307 , and hd  113766 are known to be binary or triple systems . all are wide separation systems ( @xmath43100  au ) with the exception of bd+20  307 , whose separation is only about 0.05  au ( though with a high probability of a third more distant , and as yet unseen , object ; see @xcite ) . interestingly , hd  23514 , hd  15407 , and hd  172555 all exhibit signs of silica emission suggesting high - velocity impacts produced the dust in the system @xcite . binaries with separations of a few tens of au are less likely to contain substantial amounts of dust when compared with single stars or with wide separation binaries @xcite . however , if hd  23514 hosts a tertiary companion , say a close - in planet , whose initial inclination is different than that of the substellar companion , the kozai mechanism @xcite can work to disrupt its orbit by continuously changing the inclination and eccentricity . this can potentially stir up planetesimals in the system and thus enhance collisions in the terrestrial zone ( for example , see @xcite ) . @xcite suggests a catastrophic or even planet - planet collision to account for the large amount of dust in the system . such an event could take place if the eccentricity of a planet or large planetesimal is sufficiently increased through the kozai mechanism . a large eccentricity can lead to high - velocity impacts near periastron , which can account for the silica emission observed in the system . equation  36 in @xcite can be used to estimate the period of the eccentricity oscillation of this tertiary body . for a planet or planetesimal at 5  au , and assuming hd  23514b s orbit is circular and its projected separation is the semi - major axis , the period is @xmath280  myr . while there are many unknowns , it is interesting to note that this is comparable to the age of the system . further monitoring of the system , as well as deeper searches for any additional companions closer to the star , will be key in determining the relationship , if any , between the secondary companion and the dust around the primary . using adaptive optics imaging at keck , we have discovered a substellar companion to the dusty pleiades star hd  23514 . based on comparison with evolutionary models , we estimate the companion to have a mass of about @xmath26m@xmath33 and temperature @xmath27  k. to our knowledge , hii  1348 and hd  23514 are the only pleiades stars known to have brown dwarf companions @xcite . hd  23514 is one of the dustiest main - sequence stars known to date with @xmath0 . the warm dust present in the system is located within a few au of the primary . in contrast , the companion has a projected separation of @xmath2360  au . at that distance , the companion s gravity is expected to have little influence over the evolution of the warm dust seen around the primary . however , if a tertiary object were present in the system very close to the star , the kozai mechanism can affect its orbit and possibly enhance collisions that generate dust . we thank j.  stauffer for useful discussions on the pleiades , r.  galicher for the nirc2 coronographic attenuation , and s.  yelda for assistance in nirc2 reduction . we thank our anonymous referee for a prompt review and constructive suggestions . the data presented herein were obtained at the w.m . keck observatory , which is operated as a scientific partnership among the california institute of technology , the university of california and the national aeronautics and space administration . the observatory was made possible by the generous financial support of the w.m . keck foundation . the authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of mauna kea has always had within the indigenous hawaiian community . we are most fortunate to have the opportunity to conduct observations from this mountain . this research was supported in part by nasa grants to ucla . lafrenire , d. , jayawardhana , r. , & van kerkwijk , m.  h.  2008 , , 689 , l153 lafrenire , d. , jayawardhana , r. , & van kerkwijk , m.  h.  2010 , , 719 , 497 lafrenire , d. , jayawardhana , r. , janson , m. , et al . 2011 , , 730 , 42                                                        2006 - 12 - 10 & k@xmath5 & 30 & 2 & 60 & 18 & adi mode , primary saturated + 2006 - 12 - 10 & k@xmath5 & 0.2 & 100 & 20 & 9 & adi mode , secondary not detected + 2006 - 12 - 10 & k@xmath5 & 0.005 & 50 & 0.25 & 12 & adi mode , secondary not detected + 2007 - 10 - 25 & k@xmath5 & 30 & 2 & 60 & 14 & primary saturated + 2007 - 10 - 25 & k@xmath5 & 0.018 & 20 & 0.36 & 8 & secondary not detected + 2007 - 10 - 25 & k@xmath5 & 0.005 & 50 & 0.25 & 6 & secondary not detected + 2008 - 11 - 04 & ks & 20 & 1 & 20 & 8 & primary saturated + 2008 - 11 - 04 & ks & 0.5 & 20 & 10 & 15 & secondary detected in 9/15 frames + 2009 - 11 - 01 & h & 1 & 60 & 60 & 7 & + 2009 - 11 - 02 & ks c600 & 30 & 1 & 30 & 14 & coronographic ; 600  mas diameter spot + 2010 - 10 - 30 & j & 0.053 & 200 & 10.6 & 5 & + 2010 - 10 - 30 & h & 0.053 & 200 & 10.6 & 10 & + 2010 - 10 - 30 & ks & 0.1 & 200 & 20 & 5 & + 2010 - 10 - 30 & ks c800 & 6 & 2 & 12 & 10 & coronographic ; 800  mas diameter spot +      2006 - 12 - 10 & k@xmath5 & 45 & @xmath44 & @xmath45 & @xmath46 & @xmath47 + 2007 - 10 - 25 & k@xmath5 & 25 & @xmath48 & @xmath49 & @xmath50 & @xmath51 + 2008 - 11 - 04 & ks & 20 & @xmath52 & @xmath53 & @xmath54 & @xmath55 + 2009 - 11 - 01 & h & 25 & @xmath56 & @xmath57 & @xmath58 & @xmath59 + 2009 - 11 - 02 & ks c600 & 20 & @xmath60 & @xmath61 & @xmath62 & @xmath63 + 2010 - 10 - 30 & j & 25 & @xmath64 & @xmath65 & @xmath66 & @xmath67 + 2010 - 10 - 30 & h & 25 & @xmath68 & @xmath69 & @xmath70 & @xmath71 + 2010 - 10 - 30 & ks & 25 & @xmath72 & @xmath73 & @xmath74 & @xmath75 + 2010 - 10 - 30 & ks c800 & 25 & @xmath76 & @xmath77 & @xmath78 & @xmath79 +        . the uncertainty in these two measurements are comparable to the symbol size and they effectively lie on top of each other in the figure . the black line indicates the relative motion a stationary background source would have throughout these observations with circles denoting the expected location at the time of our observations . we adopt a distance of @xmath19  pc ( see section  [ results ] ) . , width=529 ]        ) as a function of separation for our 2010 and 2009 coronographic data . limits are computed using the same aperture size by performing aperture photometry on blank regions of the sky in annuli around the primary . the limits are averaged over all frames . companions @xmath211 - 12 magnitudes fainter at ks than the primary could have been detected at separations larger than 1.5 . mass limits in solar masses from the dusty evolutionary model of @xcite are indicated . , width=529 ]
with adaptive optics imaging at keck observatory , we have discovered a substellar companion to the f6 pleiades star hd  23514 , one of the dustiest main - sequence stars known to date ( @xmath0 ) . 
 this is one of the first brown dwarfs discovered as a companion to a star in the pleiades . 
 the 0.06  m@xmath1 late - m secondary has a projected separation of @xmath2360  au . 
 the scarcity of substellar companions to stellar primaries in the pleiades combined with the extremely dusty environment make this a unique system to study .