Spitzer MIPS observations of stars in the β Pictoris moving group

Abstract

We present Multiband Imaging Photometer for Spitzer (MIPS) observations at 24 and 70 μm for 30 stars, and at 160 μm for a subset of 12 stars, in the nearby (~30 pc), young (~12 Myr) β 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 μm excesses, implying a debris disk fraction of 23%, and that at least 11 have 70 μm excesses (disk fraction of ≥37%). Five disks are detected at 160 μm (out of a biased sample of 12 stars observed), with a range of 160/70 flux ratios. The disk fraction at 24 and 70 μm, 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 η Cha associations, are entirely absent in the BPMG. Additional flux density measurements at 24 and 70 μm 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.

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