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the large and small magellanic cloud ( lmc , smc ) are the closest undisrupted neighboring dwarf galaxies to our own .
they have four to five times lower metallicities than the milky way ( mw ) , while their gas - to - dust ratio is much higher , forming environments resembling those of the early universe .
the magellanic clouds ( mcs ) show clear evidence for energetic star formation activity with h  i shells @xcite , @xcite , h  ii regions @xcite , @xcite , and molecular clouds @xcite , @xcite , all linked to ongoing star formation , as it is observed in young stellar systems , the _ stellar associations _ @xcite , @xcite , .
both mcs contain a variety of such stellar systems , the age and imf of which become very important sources of information on their recent star formation .
they provide a rich sample of targets for the comprehensive study of current star formation in low - metallicity environments . considering that the mcs are so close to us ( @xmath1 50 kpc and 60 kpc ) ,
they are indeed ideal laboratories for a detailed study of clustered star formation and the imf in the early universe , and _ hubble _ s contribution is fundamental in such a study .     and @xmath2 ( @xmath3- and @xmath4-equivalent ) of the lmc star - forming region lh  95/n  64 .
this sharp image , presented at the 2006 general assembly of the international astronomical union , reveals a large number of low - mass infant stars coexisting with young massive ones . these observations ,
being the deepest ever taken towards the lmc , allow us to explore the scientific gain that can be achieved for mcs studies using high spatial resolution photometry from _ hubble_. image credit : nasa , esa and d. a. gouliermis ( mpia ) .
acknowledgments : davide de martin ( esa / hubble).[fig - ima ] ]    , @xmath3 cmd of the stars detected with acs / wfc in the region of lh  95/n  64 . these observations , with a detection limit of @xmath5  28.4 mag ( @xmath0  -2.truept  0.5  m ) , reveal a unique sample of @xmath1  2,450 pms stars , easily distinguished as a secondary red sequence , almost parallel to the faint part of the main sequence .
these stars are found to be concentrated in the central part of the association and in surrounding compact clusters , and their spatial distribution is in excellent coincidence with the loci of the brightest ms stars @xcite.[fig - cmd ] ]
stellar associations contain the richest sample of young bright stars in a galaxy .
consequently our knowledge on the young massive stars of the mcs has been collected from photometric and spectroscopic studies of young stellar associations @xcite .
however , the picture of these stellar systems changed when _
hubble _ observations revealed that mcs associations are not mere aggregates of young bright stars alone , but they also host large numbers of faint pms stars @xcite , @xcite . although nearby galactic ob associations are known to be significant hosts of such stars @xcite , @xcite , @xcite , pms studies in the mcs with _
hubble _ were focused only on the surrounding field of the supernova 1987a @xcite , cluster ngc  1850 @xcite , and the star - burst of 30 doradus @xcite , all in the lmc .
however , these studies are limited by crowding , even at the angular resolution facilitated by _ hubble_.    to learn more about low - mass pms stars in the mcs , one has to study less crowded regions like young stellar associations .
indeed , an investigation on the main - sequence imf of the lmc association lh  52 with hst / wfpc2 observations by @xcite revealed @xmath1 500 low - mass candidate pms stars easily distinguishable in the @xmath6 , @xmath3 color - magnitude diagram ( cmd ) @xcite .
more recently , deeper observations with the wide - field channel ( wfc ) of acs of another lmc association ( the star - forming region lh  95/n  64 ) revealed the coexistence of pms stars and early - type stars in such stellar systems ( figure [ fig - ima ] ) .
these one - of - a - kind observations dramatically changed the picture we had for stellar associations in the mcs by revealing a unique rich sample of pms stars in lh  95/n  64 ( figure [ fig - cmd ] ) .
the spatial distribution of these low - mass members demonstrates the existence of significant substructure ( `` subgroups '' ) , as in the case of galactic ob associations .
this stellar sub - clustering has its origins possibly in short - lived parental molecular clouds within a giant molecular cloud complex .
each of these `` _ pms clusters _ '' in lh  95/n  64 includes a few early - type stars .
such stars have been identified as candidate herbig ae / be ( haebe ) stars due to their strong h  emission .
near - ir spectroscopic study with vlt / sinfoni ( eso program 078.d-0200 ) will clarify their nature .
, @xmath7 being the standard deviation of the background surface density .
this map demonstrates the existence of statistically significant concentrations of pms stars outside the main body of the association ngc  346 ( located at the center ) .
such pms clusters are suspected to be the product of sequential star formation triggered by the action of the ob stars in ngc  346 , which shape the southern part , and a supernova , which affects the northern part of the region @xcite .
[ fig - cmap ] ]      a spatial behavior similar to the pms population of lh  95/n  64 is seen in pms stars of the association ngc  346 in the smc @xcite , from observations with acs / wfc .
ngc  346 is located in the brightest h  ii region of the smc , n  66 , and acs uncovered the richness of this region in pms stars @xcite , @xcite .
the surface density map of the region of ngc  346/n  66 constructed from star counts of the pms stars in the observed field is shown in figure [ fig - cmap ] .
apart from the association itself ( seen as the central large concentration ) there are at least five distinct concentrations of pms stars with surface stellar density  -2.truept  3@xmath7 above the background ( where @xmath7 is the standard deviation of the background density ) , which fit the description of `` pms clusters '' .
the size of each cluster is defined by the isopleth corresponding to the local mean density around it , and the time - scale within which each pms cluster was presumably formed is defined by their individual cmds .
although the loci of the pms stars in the cmd exhibit a broadening , which prevents an accurate estimation of their age , it was found that the pms clusters located away from ngc  346 to the north , represent the most recent star formation activity in the region @xcite .    , @xmath3 cmd of all stars detected with acs / wfc imaging in the area of ngc  346 ( 06 around its center ) . pms isochrone models by for ages 0.5 to 15  myr are overplotted to demonstrate that the observed broadening of the pms stars can be easily misinterpreted as an age - spread .
simulations showed that this spread can be explained as the result of interstellar reddening of @xmath8  mag alone , or of two star formation events ( a true age - spread ) @xmath15  myr apart if the reddening is lower .
typical photometric uncertainties in brightness and color are also shown @xcite .
[ fig - pms ] ]      the loci of pms stars in the cmd often show a widening , which could be evidence for an age - spread @xcite .
the low - mass population in subgroups within ob associations of the mw exhibits little evidence for significant age - spreads on time - scales  -2.truept  10  myr @xcite .
although this time - scale is in agreement with a scenario of rapid star formation and cloud dissipation , age differences of the order of 10  myr may be very important for understanding of how sequential star formation proceeds .
moreover , there are several factors apart from age - spread , such as variability and binarity , which can cause considerable deviations of the positions of the pms stars in the cmd @xcite .
a broadening in the cmd is also observed for the pms stars of ngc  346/n  66 ( figure [ fig - pms ] ) .
simulations showed that apart from photometric uncertainties , binarity and variability , reddening seems to play the most important role in the observed widening of the pms stars , providing false evidence for an age - spread if the region suffers from high extinction @xcite .
young stellar systems , which host newborn pms stars , naturally provide the testbed for a comprehensive study of the stellar imf .
a coherent sample of pms stars is found with _ hubble _ from acs imaging in the vicinity of another smc association , ngc  602 , located in the h  ii region n  90 @xcite .
the region of ngc  602/n  90 includes no distinct subgroups , and therefore , being less complicated than ngc  346/n  66 , is more suitable for the investigation of the low - mass imf . for the construction ,
though , of this imf a mass - luminosity relation derived from evolutionary models can not be used due to the spread of the pms stars , also apparent in the cmd of ngc  602 .
instead , counting the pms stars between evolutionary tracks , which represent specific mass ranges , seems to be the most accurate method for the construction of their mass spectrum ( figure [ fig - imf ] ) .    ]
the coexistence of h  ii regions and pms stars in stellar associations of the mcs indicate that star formation may be still active in their vicinity .
indeed , observations with the _ spitzer _ space telescope revealed objects classified as candidate young stellar objects ( ysos ) in such regions @xcite , @xcite , and _ hubble _
s contribution has been very important in disentangling their nature @xcite .
the region of ngc  602/n  90 is also found with _
spitzer _ to host possible ysos , and the comparison of the loci of these ir - bright sources with the _ hubble _ images interestingly showed pms stars to be their optical counterparts @xcite .
a variety of objects is discovered to coincide with these candidate ysos , such as single highly embedded sources , small compact pms clusters , as well as features similar to `` elephant trunks '' , all located at the periphery of ngc  602 , along the dust ridges of the molecular cloud presumably blown - away by the action of the association itself @xcite .
d. a. gouliermis kindly acknowledges the support of the german research foundation through the individual grant 1659/1 - 1 .
based on observations made with the nasa / esa hubble space telescope , obtained from the data archive at the space telescope science institute .
stsci is operated by the association of universities for research in astronomy , inc . under nasa contract
nas 5 - 26555 .
the magellanic clouds ( mcs ) offer an outstanding variety of young stellar associations , in which large samples of low - mass stars ( with @xmath0  -2.truept  1  m ) currently in the act of formation can be resolved and explored sufficiently with the _ hubble _ space telescope . 
 these pre - main sequence ( pms ) stars provide a unique snapshot of the star formation process , as it is being recorded for the last 20 myr , and they give important information on the low - mass initial mass function ( imf ) of their host environments . 
 we present the latest results from observations with the _ advanced camera for surveys _ ( acs ) of such star - forming regions in the mcs , and discuss the importance of _ hubble _ for a comprehensive collection of substantial information on the most recent low - mass star formation and the low - mass imf in the mcs .