Spitzer MIPS observations of stars in the β Pictoris moving group

We present Multiband Imaging Photometer for Spitzer (MIPS) observations at 24 and 70 mu m for 30 stars, and at 160 mu m for a subset of 12 stars, in the nearby (similar to 30 pc), young (similar to 12 Myr) beta Pictoris moving group (BPMG). In several cases, the new MIPS measurements resolve source confusion and background contamination issues in the IRAS data for this sample. We find that 7 members have 24 mu m excesses, implying a debris disk fraction of 23%, and that at least 11 have 70 mu m excesses (disk fraction of >= 37%). Five disks are detected at 160 mu m(out of a biased sample of 12 stars observed), with a range of 160/70 flux ratios. The disk fraction at 24 and 70 mu m, and the size of the excesses measured at each wavelength, are both consistent with an inside-out'' infrared excess decrease with time, wherein the shorter wavelength excesses disappear before longer wavelength excesses, and consistent with the overall decrease of infrared excess frequency with stellar age, as seen in Spitzer studies of other young stellar groups. Assuming that the infrared excesses are entirely due to circumstellar disks, we characterize the disk properties using simple models and fractional infrared luminosities. Optically thick disks, seen in the younger TW Hya and eta Cha associations, are entirely absent in the BPMG. Additional flux density measurements at 24 and 70 mu m are reported for nine Tucana-Horologium association member stars. Since this is < 20% of the association membership, limited analysis on the complete disk fraction of this association is possible.