ASpitzerStudy of Debris Disks in the Young Nearby Cluster NGC 2232: Icy Planets Are Common around ∼1.5–3M_⊙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 4.5Y24 m indicative of debris from terrestrial planet formation. We also identify 2Y4 late-type stars with possible 8 m excesses and 8 early-type stars with definite 24 m 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 m 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%Y60% at 20Y25 Myr. Older A-type stars have smaller debris disk frequencies: $20% at 50Y100 Myr. For these ages, the typical level of debris emission increases from 5 to 20 Myr and then declines. Because 24 m dust emission probes icy planet formation around A-type stars, our results suggest that the frequency of icy planet formation is i k 0:5Y0.6. Thus, most A-type stars (%1.5Y3 M ) produce icy planets.

Debris Disks in the Upper Scorpius Ob Association

“…The possible peak in the excess fraction result was noted by Currie et al (2008b), and the decline in the excess fraction toward older ages was found by Rieke et al (2005). No such increase is apparent for the G/K stars.…”

“…NGC 2232. Currie et al (2008b) estimated a 5σ and 10σ detection limit of 10.5 mag and 9.75 mag for their 24 μm survey. They also find that the number of sources detected at 24 μm falls rapidly for sources fainter than [24] = 10.5.…”

“…The compiled surveys include clusters and associations between ages of 2 and 757 Myr, and field stars between ages of 3 and 5000 Myr. The young associations and clusters that have been surveyed previously are either sparsely populated with only ∼20 members (η Cha, Gautier et al 2008; TW Hydra moving group, Low et al 2005) or are at large distances such that Spitzer is sensitive to the stellar photosphere for only A-type stars or earlier (IC 348, Lada et al 2006;σ Ori, Hernández et al 2007; Orion OB1a and OB1b, Hernández et al 2006;NGC 2262, Currie et al 2008b.…”

“…First, we did not include surveys where the observations were not sensitive to the photosphere for spectral types A or later, either because the regions are more distant than ∼ 900 pc (Trumpler 37 and NGC 7160, Sicilia-Aguilar et al 2006;h & χ Persei, Currie et al 2008a;NGC 2262, Currie et al 2008b, or the observations were exclusively of late spectral types (λ Orionis; Barrado y Navascués et al 2007). MIPS 24 μm observations of the γ Velorum cluster were not incorporated; while the observations were sensitive to the photosphere for stars early than ∼F8, the contamination from field stars is non-negligible (Hernández et al 2008).…”

“…For a distance of 340 pc, an age of 25 Myr, and a color excess of E(B − V ) = 0.07, J = 10 corresponds roughly to a A8 star, which we adopt as the completeness limit of the survey. The rms scatter in the K− [24] color for stars brighter than the completeness limit and not containing an infrared excess (as defined in Currie et al 2008b) is 0.09 mag, which was adopted as the photometric uncertainty.…”