Debris Disk Evolution around A Stars

We report 24 and/or 70 um measurements of ~160 A-type main-sequence starsusing the Multiband Imaging Photometer for Spitzer (MIPS). Their ages rangefrom 5 to 850 Myr based on estimates from the literature (cluster or movinggroup associations) or from the H-R diagram and isochrones. The thermalinfrared excess is identified by comparing the deviation (~3% and ~15% at the1-\sigma level at 24 and 70 um, respectively) between the measurements and thesynthetic Kurucz photospheric predictions. Stars showing excess infraredemission due to strong emission lines or extended nebulosity seen at 24 um areexcluded from our sample; therefore, the remaining infrared excesses are likelyto arise from circumstellar debris disks. At the 3-sigma confidence level, theexcess rate at 24 and 70 um is 32% and >=33% (with an uncertainty of 5%),considerably higher than has been found for old solar analogs and M dwarfs. Ourmeasurements place constraints on the fractional dust luminosities andtemperatures in the disks. We find that older stars tend to have lowerfractional dust luminosity than younger ones. While the fractional luminosityfrom the excess infrared emission follows a general 1/t relationship, thevalues at a given stellar age vary by at least two orders of magnitude. We alsofind that (1) older stars possess a narrow range of temperature distributionpeaking at colder temperatures, and (2) the disk emission at 70 um persistslonger than that at 24 um. Both results suggest that the debris-disk clearingprocess is more effective in the inner regions.Comment: 17 pages, accepted for publication in Ap