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Debris Disk Evolution around A Stars
We report 24 and/or 70 μm measurements of ~160 A-type main-sequencestars using the Multiband Imaging Photometer for Spitzer (MIPS). Theirages range from 5 to 850 Myr, based on estimates from the literature(cluster or moving group associations) or from the H-R diagram andisochrones. The thermal infrared excess is identified by comparing thedeviation (~3% and ~15% at the 1 σ level at 24 and 70 μm,respectively) between the measurements and the synthetic Kuruczphotospheric predictions. Stars showing excess infrared emission due tostrong emission lines or extended nebulosity seen at 24 μm areexcluded from our sample; therefore, the remaining infrared excesses arelikely to arise from circumstellar debris disks. At the 3 σconfidence level, the excess rate at 24 and 70 μm is 32% and >=33%(with an uncertainty of 5%), considerably higher than what has beenfound for old solar analogs and M dwarfs. Our measurements placeconstraints on the fractional dust luminosities and temperatures in thedisks. We find that older stars tend to have lower fractional dustluminosity than younger ones. While the fractional luminosity from theexcess infrared emission follows a general 1/t relationship, the valuesat a given stellar age vary by at least 2 orders of magnitude. We alsofind that (1) older stars possess a narrow range of temperaturedistribution peaking at colder temperatures, and (2) the disk emissionat 70 μm persists longer than that at 24 μm. Both results suggestthat the debris disk clearing process is more effective in the innerregions.
| UBV photometry of stars whose positions are accurately known.
Photometric indices V, B-V, and U-B were measured for about 560 stars ofthe AGK3R and NPZT catalogs between BD declinations 11 deg and 23 deg,using the 40-cm Cassegrain telescope of the Kvistaberg Observatory fromApril 1986 to May 1987. The observation procedure and the reductiontechnique were the same as in the earlier papers of this series by Oja(1984, 1985, 1986, and 1987). The mean errors were calculated from theinternal consistency of the data. The accuracy was found to be the sameas for the earlier parts of the survey.
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