document: massive stars play a fundamental role in driving the energy flow and material cycles that influence the physical and chemical evolution of galaxies . despite receiving much attention 
   , their formation process remains enigmatic . 
   observationally , the large distances to the nearest examples and the clustered mode of formation make it difficult to isolate individual protostars for study . 
   it is still not certain , for instance , whether massive stars form via accretion ( similar to low mass stars ) or through mergers of intermediate mass stars . 
   advances in instrumentation , have enabled ( sub ) arcsecond resolution imaging at wavelengths less affected by the large column densities of material that obscure the regions at shorter wavelengths . 
   recent observations exploiting these capabilities have uncovered the environment surrounding _ individual _ massive protostellar systems . from analysis of @xmath42.3 @xmath0 m co bandhead emission , 
   @xcite have inferred keplerian disks very closely surrounding ( within a few au ) four massive young stellar objects , while interferometric , mm - continuum observations , find the mass - function of protostellar dust clumps lies close to a salpeter value down to clump radii of 2000au @xcite . 
   these high resolution observations point toward an accretion formation scenario for massive stars . 
   further discrimination between the two competing models is possible by examining the properties , in particular the young stellar populations , of hot molecular cores . 
   the mid - infrared ( mir ) window ( 7 - 25 @xmath0 m ) offers a powerful view of these regions . 
   the large column densities of material process the stellar light to infrared wavelengths , and diffraction limited observations are readily obtained . 
   recent observations indicate that class ii methanol masers exclusively trace regions of massive star formation  @xcite and are generally either not associated or offset from uchii regions  @xcite . 
   @xcite ( hereafter m05 ) have carried out multi - wavelength ( mm to mir ) observations toward five star forming complexes traced by methanol maser emission to determine their large scale properties . 
   they found that maser sites with weak ( @xmath510mjy ) radio continuum flux are associated with massive ( @xmath650m@xmath7 ) , luminous ( @xmath610@xmath8l@xmath7 ) and deeply embedded ( a@xmath940 mag ) cores characterising protoclusters of young massive ( proto)stars in an earlier evolutionary stage than uchii regions . 
   the spatial resolution of the observations ( @xmath68@xmath2 ) was , however , too low to resolve the sources inside the clumps . 
   details of the regions from observations in the literature are described in m05 . 
   we have since observed three of the m05 regions at high spatial resolution to uncover the embedded sources inside the cores at mir wavelengths . 
   the data were obtained with michelle . ] on the 8-m , gemini north telescope in queue mode , on the 18@xmath10 , 22@xmath11 and 30@xmath10 of march 2003 . 
   each pointing centre was imaged with four n band silicate filters ( centred on 7.9 , 8.8 , 11.6 and 12.5 @xmath0 m ) and the qa filter ( centred on 18.5 @xmath0 m ) with 300 seconds on - source integration time . 
   g173.49 and g188.95 were observed twice on separate nights and g192.60 observed once . 
   the n and q band observations were scheduled separately due to the more stringent weather requirements at q band . 
   the standard chop - nod technique was used with a chop throw of 15@xmath2 and chop direction selected from msx images of the region , to minimise off - field contamination . 
   the spatial resolution calculated from standard star observations was @xmath4 0.36@xmath2 at 10 @xmath0 m and @xmath4 0.57@xmath2 at 18.5 @xmath0 m . 
   the 32@xmath2x24@xmath2 field of view fully covered the dust emission observed by m05 in each region . 
   particular care was taken to determine the telescope pointing position but absolute positions were determined by comparing the mir data to sensitive , high resolution , cm continuum , vla images of the 3 regions ( minier et al . in prep ) . 
   similar spatial distribution and morphology of the multiple components allowed good registration between the images . 
   the astrometric uncertainty in the vla images is @xmath41@xmath2 . 
   flux calibration was performed using standard stars within 0.3 airmass of the science targets . 
   there was no overall trend in the calibration factor as a result of changes in airmass throughout the observations . 
   the standard deviation in the flux of standards throughout the observations was found to be 7.4 , 3.1 , 4.4 , 2.4 and 9% for the four n - band and 18.5 @xmath0 m filters respectively . the statistical error in the photometry 
   was dominated by fluctuations in the sky background . 
   upper flux limits were calculated from the standard deviation of the sky background for each filter and a 3@xmath12 upper detection limit is used in table  1 . 
   similarly , a 3@xmath12 error value is quoted for the fluxes in table  1 ( typical values for the n and q band filters were 0.005 and 0.03 jy respectively ) . 
   the flux densities for the standard stars were taken from values derived on the gemini south instrument , t - recs which shares a common filter set with michelle . 
   regions confused with many bright sources were deconvolved using the lucy - richardson algorithm with 20 iterations . 
   this was necessary to resolve source structure and extract individual source fluxes . 
   the instrumental psf was obtained for each filter using a bright , non - saturated standard star . 
   the results were reliable and repeatable near the brighter sources when using different stars for the psf and observations of the objects taken over different nights . 
   as a further check , the standard stars were used to deconvolve other standards and reproduced point sources down to 1% of the peak value after 20 iterations , so only sources greater than 3% of the peak value were included in the final images . 
   the resulting deconvolutions are shown in fig 1 . 
   [ tab_sources ]    [ cols="^,^,^,^,^,^,^,^,^,^,^,^ " , ]      as the large scale clump dust and gas morphology appears simple and centrally peaked ( see m05 ) , we make the reasonable assumption that the protocluster centres coincide with the central peak of dust emission . 
   the spatial distribution of the point sources within the protocluster is similar between the clumps with close point sources toward the cluster centre . 
   the methanol masers are found closest to the brightest mir point source ( within the assumed 1@xmath2 pointing error from image registration ) . 
   these sources have temperatures sufficient to evaporate methanol ice from the dust grains into the gas phase ( @xmath690k ) as well as sufficient luminosity of ir photons to pump the masing transition 
   conditions models suggest are required for such emission  @xcite . 
   it is known that more massive stars favour cluster centres ( e.g.  @xcite ) , but it is unclear whether they form there or migrate in from outside . 
   we have used the simple - harmonic model of ballistic motion developed by  @xcite to consider the motion of sources within the cores . using the measured column density and radius from m05 ( listed in table  2 ) , 
   the time required for migration from the edge to the centre is @xmath4 @xmath13 years . 
   this is comparable to the predicted hmc lifetime of 10@xmath14 years @xcite so we can not rule out the possibility of migration within the clumps . 
   any sources having migrated to the centre in this way would have acquired a velocity of @xmath4  2  kms@xmath15 with respect to the clump . 
   massive stars in clusters are observed to have a high companion star fraction  @xcite . in the m16 cluster , 
   @xcite observed massive stars ( earlier than b3 ) with visual companions separated by 1000 - 3000au . 
   if multiple systems are bound from birth , it is likely some of the sources we have observed will belong to multiple systems , even though the companions may lie below the detection limit . 
   however , all three regions show two or more point sources at close angular separation ( see insets of figure 1 ) corresponding to linear separations of 1700 to 6000au . we can not determine whether these stars are physically bound or simply close due to projection effects but we can calculate the instrumental sensitivity required to confirm or deny the association . 
   assuming they are physically bound in a keplerian orbit , the maximum proper motions ( projection angle = 0@xmath16 ) of @xmath4 0.1 mas / year are too small to be detected on short temporal baselines . the maximum velocity difference ( projection angle = 90@xmath16 ) 
   @xmath4 2 kms@xmath15 is achievable by high spectral resolution observations of any line features . 
   the mass distribution of stars is generally well described as a power law through the initial mass function ( imf ) . 
   given the mass of gas available to form stars , we may estimate the likelihood that a cluster will end up with the most massive stars that are observed in it . 
   the fraction of gas that forms stars is given by the star formation efficiency ( sfe ) and is observationally found to be less than 50% in any cloud and to be @xmath17 33% for nearby embedded clusters @xcite . for a cluster whose total stellar mass is 120 ( 50 , 320 ) m@xmath7 ( equivalent to the gas mass determined for the three cores ) , @xcite estimate that the mean maximum mass that a star may have in it is 10 ( 5 , 20 ) m@xmath7 . 
   this is comparable to the largest observed mass in two out of the three cases . 
   however , we also observed several other stars in each cluster so can estimate the probability of generating stars of equal or greater mass than the remaining mass distribution . 
   we did this by running monte - carlo simulations to populate 10@xmath14 clusters using @xcite , @xcite and @xcite imfs until the available gas mass was exhausted . 
   we only considered clusters which contained a star of at least equal mass to the most massive observed . 
   the simulations show that even using the salpeter form of the imf ( most biased toward forming high - mass stars ) and allowing 50% of the gas to form stars , it is difficult to generate the observed mass distributions ( probabilities @xmath18 10@xmath19 , 10@xmath20 , 10@xmath15 for the three cores respectively ) . by itself , this may not be significant for a single cluster . 
   however , since the probability is low for all three sources studied , it is unlikely that the mass distribution of the most massive stars can be produced by sampling a standard form of the imf from the reservoir of gas available for star formation . 
   this conclusion would not hold if there was a substantial stellar mass already in the cluster that remains unseen , or if much of the original gas mass had already been dispersed from the core due to star formation . 
   the former requires a sfe close to unity and given the relatively quiescent state of the cores , the latter seems unlikely . 
   a larger sample of young , massive protoclusters is required to draw general conclusions . however , in all three hot molecular cores traced by methanol maser emission we have found :    * multiple , mir sources which can be separated into three morphological types : unresolved point sources ( p ) ; unresolved point source with weak surrounding extended emission ( pe ) and extended sources ( e ) . * the point sources lie at close angular separations . 
   future high spatial and spectral resolution observations may be able to determine whether or not they are physically bound . * 
   the methanol masers are found closest to the brightest mir point source ( within the assumed 1@xmath2 pointing accuracy ) . 
   * cooler , extended sources dominate the luminosity . * 
   the time scale for a source at the core edge to migrate to the centre is comparable to the hot molecular core lifetime , so it is not possible to rule out large protostellar motions within the core . 
   * from the derived gas mass of the core and mass estimates for the sources , monte carlo simulations show that it is difficult to generate the observed distributions for the most massive cluster members from the gas in the core using a standard form of the imf . 
   this conclusion would not hold , however , if most of the original gas has already formed stars , or has been dispersed such that the original core mass is much greater than now observed . 
   s.l . would like to thank alistair glass , scott fisher , tony wong and melvin hoare for helpful discussion of the data and scientific input . 
   we thank the anonymous referee for the thorough response and insightful comments . 
   this work was made possible by funding from the australian research council and unsw . 
   the gemini observatory is operated by the association of universities for research in astronomy , inc . 
   , under a cooperative agreement with the nsf on behalf of the gemini partnership : nsf ( usa ) , pparc ( uk ) , nrc ( canada ) , conicyt ( chile ) , arc ( australia ) , cnpq ( brazil ) and conicet ( argentina ) . 
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   ser . 243 : from darkness to light : origin and evolution of young stellar clusters statistical properties of visual binaries as tracers of the formation and early evolution of young stellar clusters .
summary: we present high resolution , mid - infrared images toward three hot molecular cores signposted by methanol maser emission ; g173.49 + 2.42 ( s231 , s233ir ) , g188.95 + 0.89 ( s252 , afgl-5180 ) and g192.60 - 0.05 ( s255ir ) . each of the cores was targeted with michelle on gemini north using 5 filters from 7.9 to 18.5 @xmath0 m . 
   we find each contains both large regions of extended emission and multiple , luminous point sources which , from their extremely red colours ( @xmath1 ) , appear to be embedded young stellar objects . 
   the closest angular separations of the point sources in the three regions are 0.79 , 1.00 and 3.33@xmath2 corresponding to linear separations of 1,700 , 1,800 and 6,000au respectively . 
   the methanol maser emission is found closest to the brightest mir point source ( within the assumed 1@xmath2 pointing accuracy ) . 
   mass and luminosity estimates for the sources range from 3 - 22 m@xmath3 and 50 - 40,000 l@xmath3 . assuming the mir sources are embedded objects and 
   the observed gas mass provides the bulk of the reservoir from which the stars formed , it is difficult to generate the observed distributions for the most massive cluster members from the gas in the cores using a standard form of the imf . 
   masers  stars : formation  techniques : high angular resolution  stars : early type  stars : mass function  infrared : stars .