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.

There is currently debate over whether the dust content of planetary systems is stochastically regenerated or originates in planetesimal belts evolving in quasi-steady state. In this paper a simple model for the steady state evolution of debris disks due to collisions is developed and confronted with the properties of the emerging population of 7 sun-like stars that have hot dust at < 10 AU. The model shows that there is a maximum possible disk mass at a given age, since more massive primordial disks process their mass faster. The corresponding maximum dust luminosity is f max = 0.16 × 10 −3 r 7/3 t −1age , where r is disk radius in AU and t age is system age in Myr. The majority (4/7) of the hot disks exceed this limit by a factor ≫ 1000 and so cannot be the products of massive asteroid belts, rather the following systems must be undergoing transient events characterized by an unusually high dust content near the star: η Corvi, HD69830, HD72905 and BD+20307. It is also shown that the hot dust cannot originate in a recent collision in an asteroid belt, since there is also a maximum rate at which collisions of sufficient magnitude to reproduce a given dust luminosity can occur in a disk of a given age. For the 4 transient disks, there is at best a 1:10 5 chance of witnessing such an event compared with 2% of stars showing this phenomenon. Further it is shown that the planetesimal belt feeding the dust in these systems must be located further from the star than the dust, typically at ≫ 2 AU. Other notable properties of the 4 hot dust systems are: two also have a planetesimal belt at > 10 AU (η Corvi and HD72905); one has 3 Neptune mass planets at < 1 AU (HD69830); all exhibit strong silicate features in the mid-IR. We consider the most likely origin for the dust in these systems to be a dynamical instability which scattered planetesimals inwards from a more distant planetesimal belt in an event akin to the Late Heavy Bombardment in our own system, the dust being released from such planetesimals in collisions and possibly also sublimation. Further detailed study of the planet, planetesimal and dust populations in these rare objects has the potential to uncover the chaotic evolutionary history of these systems and to shed light on the history of the solar system.

We report new Spitzer 24 m photometry of 76 main-sequence A-type stars. We combine these results with previously reported Spitzer 24 m data and 24 and 25 m photometry from the Infrared Space Observatory and the Infrared Astronomy Satellite. The result is a sample of 266 stars with mass close to 2.5 M , all detected to at least the $7 level relative to their photospheric emission. We culled ages for the entire sample from the literature and/or estimated them using the H-R diagram and isochrones; they range from 5 to 850 Myr. We identified excess thermal emission using an internally derived K À 24 (or 25) m photospheric color and then compared all stars in the sample to that color. Because we have excluded stars with strong emission lines or extended emission (associated with nearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emission more frequently and with higher fractional excess than do the older stars. However, as many as 50% of the younger stars do not show excess emission. The decline in the magnitude of excess emission, for those stars that show it, has a roughly t 0 /time dependence, with t 0 $ 150 Myr. If anything, stars in binary systems (including Algoltype stars) and k Boo stars show less excess emission than the other members of the sample. Our results indicate that (1) there is substantial variety among debris disks, including that a significant number of stars emerge from the protoplanetary stage of evolution with little remaining disk in the 10-60 AU region and (2) in addition, it is likely that much of the dust we detect is generated episodically by collisions of large planetesimals during the planet accretion end game, and that individual events often dominate the radiometric properties of a debris system. This latter behavior agrees generally with what we know about the evolution of the solar system, and also with theoretical models of planetary system formation.

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...