Cool Stars 14 - Submitted Abstract # 402
This version created on 23 October 2006

Evidence for Mass-dependent Circumstellar Disk Evolution in the
5 Myr-old Upper Scorpius OB Association


John Carpenter, Caltech
Eric Mamajek, Harvard-CfA
Lynne Hillenbrand, Caltech
Michael Meyer, University of Arizona


We present 4.5, 8, and 16um photometry from the Spitzer Space
Telescope for 204  stars in the Upper Scorpius OB Association.
The data are used to investigate  the frequency and properties
of circumstellar disks around stars with masses  between ~ 0.1
and 20 Msun at an age of ~ 5 Myr. We identify 35 stars that
have  emission at 8um or 16um in excess of the stellar
photosphere. The lower mass  stars (~ 0.1-1.2 Msun) appear
surrounded by primordial optically thick disks  based on the
excess emission characteristics. Stars more massive than ~ 1.8 
Msun have lower fractional excess luminosities suggesting that
the inner ~ 10  AU of the disk has been largely depleted of
primordial material. None of the G  and F stars (~ 1.2-1.8 Msun)
in our sample have an infrared excess at  wavelengths <= 16um.
These results indicate that the mechanisms for dispersing 
primordial optically thick disks operate less efficiently on
average for low  mass stars, and that longer time scales are
available for the buildup of  planetary systems in the
terrestrial zone for stars with masses < 1 Msun. 

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