Debris disk evolution around a stars

We report 24 and/or 70 mu m measurements of similar to 160 A-type main-sequence stars using the Multiband Imaging Photometer for Spitzer (MIPS). Their ages range from 5 to 850 Myr, based on estimates from the literature (cluster or moving group associations) or from the H-R diagram and isochrones. The thermal infrared excess is identified by comparing the deviation (similar to 3% and similar to 15% at the 1 sigma or level at 24 and 70 mu m, respectively) between the measurements and the synthetic Kurucz photospheric predictions. Stars showing excess infrared emission due to strong emission lines or extended nebulosity seen at 24 mu m are excluded from our sample; therefore, the remaining infrared excesses are likely to arise from circumstellar debris disks. At the 3 sigma, confidence level, the excess rate at 24 and 70 mu m is 32% and >= 33% (with an uncertainty of 5%), considerably higher than what has been found for old solar analogs and M dwarfs. Our measurements place constraints on the fractional dust luminosities and temperatures in the disks. We find that older stars tend to have lower fractional dust luminosity than younger ones. While the fractional luminosity from the excess infrared emission follows a general 1/t relationship, the values at a given stellar age vary by at least 2 orders of magnitude. We also find that (1) older stars possess a narrow range of temperature distribution peaking at colder temperatures, and (2) the disk emission at 70 mu m persists longer than that at 24 pm. Both results suggest that the debris disk clearing process is more effective in the inner regions.