A Spitzer Study of Debris Disks in the Young Nearby Cluster NGC 2232: Icy Planets Are Common around ~1.5-3 M☉ Stars

Abstract

We describe Spitzer IRAC and MIPS observations of the nearby 25 Myr old open cluster NGC 2232. Combining these data with ROSAT All-Sky Survey observations, proper motions, and optical photometry/spectroscopy, we construct a list of highly probable cluster members. We identify one A-type star, HD 45435, that has definite excess emission at 4.5-24 μm indicative of debris from terrestrial planet formation. We also identify 2-4 late-type stars with possible 8 μm excesses and 8 early-type stars with definite 24 μm excesses. Constraints on the dust luminosity and temperature suggest that the detected excesses are produced by debris disks. From our sample of B and A stars, stellar rotation appears to be correlated with 24 μm excess, a result that would be expected if massive primordial disks evolve into massive debris disks. To explore the evolution of the frequency and magnitude of debris around A-type stars, we combine our results with data for other young clusters. The frequency of debris disks around A-type stars appears to increase from ~25% at 5 Myr to ~50%-60% at 20-25 Myr. Older A-type stars have smaller debris disk frequencies: ~20% at 50-100 Myr. For these ages, the typical level of debris emission increases from 5 to 20 Myr and then declines. Because 24 μm dust emission probes icy planet formation around A-type stars, our results suggest that the frequency of icy planet formation is ηi ≳ 0.5-0.6. Thus, most A-type stars (≈1.5-3 M☉) produce icy planets.

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