You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the pleiades cluster has been the major target of several recent surveys for brown dwarfs ( bds ) ( hambly et al .
1993 ( hhj ) ; jameson & skillen 1989 ( js ) ; schultz 1997 ; stauffer et al .
1989 , 1994a ; williams et al . 1996
; zapatero osorio 1997 ; zapatero osorio et al .
1997a , b ( zrm , zmr ) ; festin 1997 ; cossburn et al .
its nearness ( @xmath7 pc , mermilliod et al .
@xcite ) and youth ( 120 myr , basri et al .
@xcite ) makes the rapidly cooling bds still rather bright and easy to detect .
the first bona fide pleiades bd was reported by rebolo et al .
this object , known as teide1 , passed the lithium test @xcite and should thereby have a maximum mass of 0.06 m@xmath6 . by now , on the order of 10 bds have been confirmed in the pleiades .
the present observations were designed to probe low - mass bds in the pleiades and to provide accurate first - epoch data for future proper motion determination .
this paper is an extension of the @xmath8 survey described in festin @xcite .
for a teide1-type object , the flux rises from 10 to almost 100% of its peak value between @xmath9 and @xmath10 .
therefore , although providing a rather short wavelength baseline , @xmath11 was chosen as the primary temperature indicator .
its efficiency is proved by the large gap between the pleiades sequence and the background stars in the @xmath9 vs @xmath11 diagram .
most observational effort was spent on obtaining high - quality images in @xmath9 ( fig .
[ figseeing ] ) to get good first - epoch coordinates and to clean out galaxies . to minimize the effects of nonuniform pixels and rotation angle , each @xmath9 field
was observed at four different field angles ( 0 , 90 , 180 and 270 degrees ) .
complementary photometry was taken in @xmath12 .
+    all observations were carried out at the 2.5 m nordic optical telescope ( not ) , la palma .
the survey covers 648 arcmin@xmath0 in @xmath13 ( of which 240 arcmin@xmath0 were also covered in @xmath14 ) + 200 arcmin@xmath0 in @xmath15 only .
the total area is 848 arcmin@xmath0 near the centre of the pleiades ( fig .
[ figfield ] ) .
a summary of the observations is given in table  [ tabobsoverview ] .
.observations [ cols= " < , < , < , < , < " , ]     figure  [ figimf ] suggests a rising mf towards lower masses with a power - law index between 0 and 1 , but there are several uncertainties one should be aware of .
the observed number of objects is small , but it remains clear that the lf drops from @xmath16 to the survey limit .
we can not say how many pleiades bds that are lost in unresolved binaries .
however , we estimate a likely upper limit by assuming that the 30% multiple fraction for m dwarfs @xcite applies to all our single pleiades candidates which are brighter than the bd limit at @xmath17 . if those companions all are below the bd limit , 8 bds would have been lost . of course
, we do not know their possible distribution , but adding 8 bds to the lf still leaves it close to the @xmath18 curve . from this crude discussion it follows that lost companions are unlikely to raise the mf index significantly above 1 .
there is also a normalization error , which does not affect the falling trend , however .
the model in baraffe et al .
@xcite is state - of - the - art and based on the latest generation of non - grey atmospheres and should provide the best possible mass - luminosity relations for the pleiades bd sequence .
since it has not yet been possible to derive an empirical mass - luminosity relation for substellar objects , we do not try to estimate the uncertainties in mass that may arise from the model .
we have performed a deep 850 arcmin@xmath0 @xmath1 survey in the central area of the pleiades cluster .
photometry in @xmath9 , @xmath10 or @xmath14 is presented for 32 previously known and 7 new likely members .
four of those are below the bd limit , and the faintest one would be the lowest mass bd found in the pleiades so far , if its membership could be confirmed .
the overall agreement in the photometry with other surveys is satisfactory .
teide1 is an exception .
we find @xmath19 , @xmath200.5 mag fainter than zrm . based on rgo archive data and a roughly constant @xmath10 magnitude
we conclude that the reason for this discrepancy most likely sits in the photometry of zrm and not in intrinsic variability .      after splitting ppl15 and two other probable binaries into components ,
the pleiades lf was compared to model lfs derived from the most recent theoretical mass - luminosity relations .
the observed lf supports an mf - index between 0 and 1 .
thus , even if the mf seems to rise for low - mass bds in the pleiades , it is not steep enough to leave more than a few percent of the cluster s mass in bds , which is consistent with dynamical findings @xcite .
i would like to thank the not opc for allocating time for this project which is an essential part in my coming thesis .
thanks also to n.c .
hambly who provided proper motions for some of my candidates and to i. baraffe who provided the model on ascii format .
this work was supported by the nordic optical telescpe scientific association and the swedish natural science research council ( nfr ) .
this research has made use of the rgo la palma archive .
f. , hauschildt p. , 1997
, model atmospheres : brown dwarfs from the stellar perspective . in : r. rebolo , e. martin , m.  r. zapatero osorio ( eds . ) , brown dwarfs and extrasolar planets : asp conf .
, in press                                                    g. , 1997 , searching for new brown dwarf candidates in a pleiades ijk imaging survey in : r. rebolo , e. martin , m.  r. zapatero osorio ( eds . ) , brown dwarfs and extrasolar planets : asp conf .
the brown dwarf population in the pleiades cluster has been probed in a deep 850 arcmin@xmath0 @xmath1 survey . 
 the survey is complete to @xmath2 in 76% of the area and to @xmath3 in the remaining 24% . 
 photometry of 32 previously known members is presented together with 8 new candidates , four of which are below the brown dwarf limit . 
 the faintest one is the lowest mass brown dwarf candidate found hitherto in the pleiades ( @xmath4 , @xmath5 m@xmath6 ) . 
 the derived pleiades luminosity function is compared to the most recent theoretical mass - luminosity relations and is consistent with a power - law index in the mass function between 0 and 1 to the limit of this survey .