Debris Disks in the Upper Scorpius OB Association

Abstract

We present MIPS 24 μm and 70 μm photometry for 205 members of the Upper Scorpius OB Association. These data are combined with published MIPS photometry for 15 additional association members to assess the frequency of circumstellar disks around 5 Myr old stars with spectral types between B0 and M5. Twelve stars have a detectable 70 μm excess, each of which also has a detectable 24 μm excess. A total of 54 stars are identified with a 24 μm excess more than 32% above the stellar photosphere. The MIPS observations reveal 19 excess sources—8 A/F/G stars and 11 K/M stars—that were not previously identified with an 8 μm or 16 μm excess. The lack of short-wavelength emission and the weak 24 μm excess suggests that these sources are debris systems or the remnants of optically thick primordial disks with inner holes. Despite the wide range of luminosities of the stars hosting apparent debris systems, the excess characteristics are consistent with all stars having dust at similar orbital radii after factoring in variations in the radiation blowout particle size with spectral type. The results for Upper Sco are compared to similar photometric surveys from the literature to re-evaluate the evolution of debris emission. After considering the completeness limits of published surveys and the effects of stellar evolution on the debris luminosity, we find that the magnitude of the 24 μm excess around F-type stars increases between ages of 5 and 17 Myr as found by previous studies, but at 2.6σ confidence. For B7-A9 and G0-K5 stars, any variations in the observed 24 μm excess emission over this age range are significant at less than ≾ 2σ confidence.

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