We report 24 and/or 70 µm measurements of ∼160 A-type main-sequence stars using the Multiband Imaging Photometer for Spitzer (MIPS). Their ages range from 5 to 850 Myr based on estimates from the literature (cluster or moving group associations) or from the H-R diagram and isochrones. The thermal infrared excess is identified by comparing the deviation (∼3% and ∼15% at the 1-σ level at 24 and 70 µm, respectively) between the measurements and the synthetic Kurucz photospheric predictions. Stars showing excess infrared emission due to strong emission lines or extended nebulosity seen at 24 µm are excluded from our sample; therefore, the remaining infrared excesses are likely to arise from circumstellar debris disks. At the 3-σ confidence level, the excess rate at 24 and 70 µm is 32% and ≥33% (with an uncertainty of 5%), considerably higher than has been found for old solar analogs and M dwarfs. Our measurements place constraints on the fractional dust luminosities and temperatures in the disks. We find that older stars tend to have lower fractional dust luminosity than younger ones. While the fractional luminosity from the excess infrared emission follows a general 1/t relationship, the values at a given stellar age vary by at least two orders of magnitude. We also find that (1) older stars possess a narrow range of temperature distribution peaking at colder temperatures, and (2) the disk emission at 70 µm persists longer than that at 24 µm. Both results suggest that the debris-disk clearing process is more effective in the inner regions.

We have observed nearly 200 FGK stars at 24 and 70 microns with the Spitzer Space Telescope. We identify excess infrared emission, including a number of cases where the observed flux is more than 10 times brighter than the predicted photospheric flux, and interpret these signatures as evidence of debris disks in those systems. We combine this sample of FGK stars with similar published results to produce a sample of more than 350 main sequence AFGKM stars. The incidence of debris disks is 4.2 +2.0 −1.1 % at 24 microns for a sample of 213 Sun-like (FG) stars and 16.4 +2.8 −2.9 % at 70 microns for 225 Sun-like (FG) stars. We find that the excess rates for A, F, G, and K stars are statistically indistinguishable, but with a suggestion of decreasing excess rate toward the later spectral types; this may be an age effect. The lack of strong trend among FGK stars of comparable ages is surprising, given the factor of 50 change in stellar luminosity across this spectral range. We also find that the incidence of debris disks declines very slowly beyond ages of 1 billion years.Subject headings: circumstellar matter -planetary systems: formation -infrared: stars IntroductionPlanetary system formation must be studied through various indirect means due to the relative faintness of distant planets and the long timescales over which planetary system -2evolution takes place. One powerful technique has advanced substantially in the era of the Spitzer Space Telescope: investigations of dusty debris disks around mature, main sequence stars. Debris disks arise from populations of planetesimals that remain from the era of planet formation; the analogs in our Solar System are the asteroid belt and the Kuiper Belt. Any system that possesses a debris disk necessarily has progressed toward forming a planetary system to some degree.The many small bodies that inhabit a debris disk can, on occasion, collide, producing a shower of fragments that grind each other down to dust particles. These dust grains can be heated by the central star to temperatures ∼100 K, where they can be detected at wavelengths of 10-100 microns. Data from the IRAS and ISO satellites were used to identify and characterize debris disks (e.g., Aumann et al. 1984;Decin et al. 2000;Spangler et al. 2001;Habing et al. 2001;Decin et al. 2003). The Multiband Imaging Photometer for Spitzer (MIPS; Rieke et al. (2004)) offers substantially improved sensitivities at 24 and 70 microns and therefore can be used to advance the study of debris disks and measure the fraction of stars that possess colliding swarms of remnant planetesimals.A number of these surveys have been carried out using MIPS. Rieke et al. (2005) and Su et al. (2006) observed hundreds of A stars and found that the number of A stars showing thermal infrared excess suggestive of collisionally-produced dust decreases as a function of stellar age, from 50% or more at ages just past the gas dissipation age of 10 Myr to 30% at 500 Myr. The excess rates are lower for older and lower mass stars. Bryden et al. (2006) found th...

We have searched for infrared excesses around a well defined sample of 69 FGK main-sequence field stars. These stars were selected without regard to their age, metallicity, or any previous detection of IR excess; they have a median age of ∼4 Gyr. We have detected 70 µm excesses around 7 stars at the 3-σ confidence level. This extra emission is produced by cool material (< 100 K) located beyond 10 AU, well outside the "habitable zones" of these systems and consistent with the presence of Kuiper Belt analogs with ∼100 times more emitting surface area than in our own planetary system. Only one star, HD 69830, shows excess emission at 24 µm, corresponding to dust with temperatures > ∼ 300 K located inside of 1 AU. While debris disks with L dust /L ⋆ ≥ 10 −3 are rare around old FGK stars, we find that the disk frequency increases from 2 ± 2% for L dust /L ⋆ ≥ 10 −4 to 12 ± 5% for L dust /L ⋆ ≥ 10 −5 . This trend in the disk luminosity distribution is consistent with the estimated dust in our solar system being within an order of magnitude, greater or less, than the typical level around similar nearby stars.

This paper confronts a simple analytical model for the steady state evolution of debris disks due to collisions with Spitzer observations of dust around main-sequence A stars. It is assumed that every star has a planetesimal belt, the initial mass and radius of which are drawn from distributions. In the model disk mass is constant until the largest planetesimals reach collisional equilibrium, whereupon mass falls /t À1 age . We find that the detection statistics and trends seen at 24 and 70 m can be fitted well by the model. While there is no need to invoke stochastic evolution or delayed stirring to explain the statistics, a moderate rate of stochastic events is not ruled out. Potentially anomalous systems are identified by a high dust luminosity compared with the maximum permissible in the model (HD 3003, HD 38678, HD 115892, HD 172555); their planetesimals may have unusual properties ( high strength or low eccentricity), or this dust could be transient. The overall success of our model, which assumes planetesimals in all belts have the same strength, eccentricity, and maximum size, suggests the outcome of planet formation is reasonably uniform. The distribution of planetesimal belt radii, once corrected for detection bias, follows N (r) / r À0:8AE0:3 for 3Y120 AU. Since belt boundaries may be attributed to unseen planets, this provides a unique constraint on A star planetary systems. It is also shown that P-R drag may sculpt the inner edges of A star disks close to the Spitzer detection threshold (HD 2262, HD 19356, HD 106591, HD 115892). This model can be readily applied to the interpretation of future surveys, and predictions for the upcoming SCUBA-2 survey include that 17% of A star disks should be detectable at 850 m.

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