FREQUENCY OF DEBRIS DISKS AROUND SOLAR-TYPE STARS: FIRST RESULTS FROM A SPITZER MIPS SURVEY

We have searched for infrared excesses around a well-defined sample of 69 FGK main-sequence field stars. These starswereselectedwithoutregardto theirage,metallicity,oranypreviousdetectionof IRexcess; they have amedian ageof � 4Gyr.Wehavedetected70 � mexcessesaroundsevenstarsatthe3 � confidencelevel.Thisextraemissionis produced by cool material (<100 K) located beyond 10 AU, well outside the ‘‘habitable zones’’ of these systems and consistent with the presence of Kuiper Belt analogs with � 100 times more emitting surface area than in our own planetary system. Only one star, HD 69830, shows excess emission at 24 � m, corresponding to dust with temperaturesk300Klocatedinsideof1AU.WhiledebrisdiskswithLdust/L? � 10 � 3 arerarearoundoldFGKstars,wefind that thediskfrequencyincreasesfrom2% � 2%forLdust/L? � 10 � 4 to12% � 5%forLdust/L? � 10 � 5 .Thistrendin the disk luminosity distribution is consistent with the estimated dust in our solar system being within an order of magnitude greater or less than the typical level around similar nearby stars. Although there is no correlation of IR excesswithmetallicity orspectraltype,there isaweak correlationwithstellarage,withstarsyoungerthanagigayear more likely to have excess emission.