Size and Spatial Distributions of Sub-km Main-Belt Asteroids

摘要

ThispaperpresentstheresultofthefirstsystematicinvestigationofverysmallMain-BeltAsteroids(sub-kmMBAs)usingtheSubaruPrime-FocusCamera(Suprime-Cam)withan$$8,mathrm{K} imes10,mathrm{K}$$mosaicCCDarrayattachedtothe8.2mSubaruTelescopeatopMaunaKea,Hawaii.WecallthissurveySMBAS(Sub-kmMain-BeltAsteroidSurvey).Observationswerecarriedouton2001February22and25(HST)anda$$sim3.0,mathrm{deg}^{2}$$skyareaneartheoppositionandneartheeclipticwassearched.Wedetected1111movingobjectsdownto$$Rsim26,mathrm{mag}$$(includingveryslowTrans-NeptunianObjects).Inthissurvey,wecouldnotdeterminetheexactorbitsofthemovingobjects,becauseoftheirshortobservationalarcofonly2hours.Instead,westatisticallyestimatedthesemi-majoraxis($$a$$)andinclination($$I$$)ofeachmovingobjectfromitsapparentsky-motionvector,andthenobtainedthesizeandspatialdistributionsofsub-kmMBAs.ThemainresultsofSMBASare:(1)TheskynumberdensityofMBAsisfoundtobe$$sim290,mathrm{deg}^{-2}$$downto$$Rsim24.4,mathrm{mag}$$(forMBAs)neartheoppositionandneartheecliptic.(2)Theslopeofthecumulativesizedistributionforsub-kmMBAsrangingfrom0.5kmto1kmindiameterisfairlyshallower($$sim1.2$$)thanthatforlargeMBAsofmorethan$$sim5,mathrm{km}$$indiameter($$sim1.8$$),whichwasobtainedfrompastasteroidsurveys.Thismeansthatthenumberofsub-kmMBAsismuchmoredepletedthanaresultextrapolatedfromthesizedistributionsforlargeasteroids.(3)Thedepletionofsub-kmMBAsisclearerintheoutermain-beltthanintheinnermain-belt.(4)ItseemsthatSMBASasteroidsdistributemorewidelyinthe$$I$$-directionintheouterzone($$a=2.8hbox{--}3.1,mathrm{AU}$$)ofthemain-beltthanknownlargeasteroidsdo.WealsodiscussthepossiblecausesforthecharacteristicsofthedistributionsofSMBAS-observedsmallasteroids.