Evidence of Terrestrial-Temperature Grains Around 'A' Stars in Young Open Clusters
Session 49 -- Associations and Clusters
Display presentation, Wednesday, 9:20-6:30, Pauley Room

## [49.12] Evidence of Terrestrial-Temperature Grains Around 'A' Stars in Young Open Clusters

An-Su Yuan, D.E. Backman (Franklin and Marshall College)

We present IRAS coadded survey 12 and 25 $\mu$m photometry of more than 100 'A' main sequence stars in 4 open clusters/groups: alpha Persei, the Pleiades, Ursa Major, and the Hyades. These range in age from about 50 to 700 Myr, spanning the era of "heavy bombardment" in our solar system. The most up-to-date cluster membership lists based on space motions and age indicators were used. Identical measurements were made at "sky" positions several arcmin from each star to check diffuse and point-source background contamination.

Few stars have significant individual excesses, but when summed together the 12 and 25 $\mu$m flux densities for the younger cluster samples showed significant excess above summed expected photospheric flux densities, as if warm circumstellar material disappears during the span of ages examined. Color temperatures indicate the material is not ambient general cluster medium illuminated by the stars but rather is material within a few A.U. of them.