David E. Trilling

Debris Disk Evolution Around A Stars

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 what 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 2 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.

Decay of Planetary Debris Disks

We report new Spitzer 24 � m photometry of 76 main-sequence A-type stars. We combine these results with previously reportedSpitzer24 � m data and 24 and 25 � m photometry from theInfrared Space Observatoryand the InfraredAstronomySatellite.Theresultisasampleof266starswithmasscloseto2.5M� ,alldetectedtoatleastthe � 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 � 24 (or 25) � m photospheric color and then compared all stars in the sample tothatcolor.Becausewehaveexcludedstarswithstrongemissionlinesorextendedemission(associatedwithnearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emissionmore frequently andwithhigher fractional excess thandothe olderstars. However,asmanyas 50% oftheyoungerstarsdonotshowexcessemission.Thedeclineinthemagnitudeofexcessemission,forthosestarsthat show it, has a roughly t0/time dependence, with t0 � 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 endgame,andthatindividualeventsoftendominatetheradiometricpropertiesofadebrissystem.Thislatterbehavior agrees generally withwhat weknowabouttheevolution of thesolar system, andalsowiththeoretical models ofplanetary system formation. Subject headingg circumstellar matter — infrared: stars — planetary systems: formation Online material: machine-readable table

Orbital Evolution and Migration of Giant Planets: Modeling Extrasolar Planets

Giant planets in circumstellar disks can migrate inward from their initial (formation) positions. Radial migration is caused by inward torques between the planet and the disk, by outward torques between the planet and the spinning star, and by outward torques due to Roche lobe overflow and consequent mass loss from the planet. We present self-consistent numerical considerations of the problem of migrating giant planets. Summing torques on planets for various physical parameters, we find that Jupiter-mass planets can stably arrive and survive at small heliocentric distances, thus reproducing observed properties of some of the recently discovered extrasolar planets. Inward migration timescales can be approximately equal to or less than disk lifetimes and star spindown timescales. Therefore, the range of fates of massive planets is broad and generally comprises three classes: (I) planets that migrate inward too rapidly and lose all their mass; (II) planets that migrate inward, lose some but not all of their mass, and survive in very small orbits; and (III) planets that do not lose any mass. Some planets in class III do not migrate very far from their formation locations. Our results show that there is a wide range of possible fates for Jupiter-mass planets for both final heliocentric distance and final mass.

FREQUENCY OF DEBRIS DISKS AROUND SOLAR-TYPE STARS: FIRST RESULTS FROM A SPITZER MIPS SURVEY

We have searched for infrared excesses around a well-defined sample of 69 FGK main-sequence field stars. These starswereselectedwithoutregardto theirage,metallicity,oranypreviousdetectionof IRexcess; they have amedian ageof � 4Gyr.Wehavedetected70 � mexcessesaroundsevenstarsatthe3 � confidencelevel.Thisextraemissionis 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 � m, corresponding to dust with temperaturesk300Klocatedinsideof1AU.WhiledebrisdiskswithLdust/L? � 10 � 3 arerarearoundoldFGKstars,wefind that thediskfrequencyincreasesfrom2% � 2%forLdust/L? � 10 � 4 to12% � 5%forLdust/L? � 10 � 5 .Thistrendin 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. Although there is no correlation of IR excesswithmetallicity orspectraltype,there isaweak correlationwithstellarage,withstarsyoungerthanagigayear more likely to have excess emission.

Debris disks around Sun-like stars

We have observed nearly 200 FGK stars at 24 and 70 ?m 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 ?m for a sample of 213 Sun-like (FG) stars and -->16.4+ 2.8?2.9% at 70 ?m 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.

AN EXCESS DUE TO SMALL GRAINS AROUND THE NEARBY K0 V STAR HD 69830: ASTEROID OR COMETARY DEBRIS?

Spitzer photometry and spectroscopy of the star HD 69830 reveal an excess of emission relative to the stellar photosphere between 8 and 35 � m dominated by strong features attributable to crystalline silicates with an emitting surface area more than 1000 times that of our zodiacal cloud. The spectrum closely resembles that of the comet C/1995 O1 (Hale-Bopp). Since no excess is detected at 70 � m, the emitting material must be quite warm, be confined within a few AU of the star, and originate in grains with low, long-wavelength emissivity, i.e., grains much smallerthan70 � m/2� � 10 � m.Thestrongmineralogicalfeaturesareevidenceforevensmaller,possiblysubmicronsized grains. This small grain size is in direct contrast to the 10‐100 � m grains that dominate the relatively featureless spectra of our zodiacal dust cloud and most other main-sequence stars with excesses. The upper limit at 70 � ma lso implies that any Kuiper Belt analog must be either very cold or less massive than � 5 times our own Kuiper Belt. WithcollisionalandPoynting-Robertsondragtimesoflessthan1000yrforsmallgrains,theemittingmaterialmust either (1) be created through continual grinding down of material in a dense asteroid belt, or (2) originate in cometary debris arising from either a single ‘‘supercomet’’ or a very large number of individual comets arriving from a distant reservoir. In the case of a cometary origin for the emission, the mass requirements for continuous generation by many individual comets are unreasonable, and we favor the capture of a single super comet into a 0.5‐1 AU orbit, where it can evolve a large number of small grains over a 2 Myr period.

The Vega Debris Disk: A Surprise from Spitzer

We present high spatial resolution mid- and far-infrared images of the Vega debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The disk is well resolved, and its angular size is much larger than found previously. The radius of the disk is at least 43 (330 AU), 70 (543 AU), and 105 (815 AU) in extent at 24, 70, and 160 μm, respectively. The disk images are circular, smooth, and without clumpiness at all three wavelengths. The radial surface brightness profiles follow radial power laws of r-3 or r-4 and imply an inner boundary at a radius of 11 ± 2 (86 AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with the surface particle number density following an inverse radial power law. The disk radiometric properties are consistent with a range of models using grains of sizes ~1 to ~50 μm. The exact minimum and maximum grain size limits depend on the adopted grain composition. However, all of these models require an r-1 surface number density profile and a total mass of × 10-3 M⊕ in grains. We find that a ring, containing grains larger than 180 μm and at radii of 86-200 AU from the star, can reproduce the observed 850 μm flux, while its emission does not violate the observed MIPS profiles. This ring could be associated with a population of larger asteroidal bodies analogous to our own Kuiper Belt. Cascades of collisions starting with encounters among these large bodies in the ring produce the small debris that is blown outward by radiation pressure to much larger distances, where we detect its thermal emission. The relatively short lifetime (<1000 yr) of these small grains and the observed total mass, ~3 × 10-3 M⊕, set a lower limit on the dust production rate, ~1015 g s-1. This rate would require a very massive asteroidal reservoir for the dust to be produced in a steady state throughout Vegas life. Instead, we suggest that the disk we imaged is ephemeral and that we are witnessing the aftermath of a large and relatively recent collisional event, and a subsequent collisional cascade.

Debris Disks in Main-Sequence Binary Systems

We observed 69 A3-F8 main-sequence binary star systems using the Multiband Imaging Photometer for Spitzer on board the Spitzer Space Telescope. We find emission significantly in excess of predicted photospheric flux levels for 9 % and 40 % of these systems at 24 and 70 μm, respectively. Twenty-two systems total have excess emission, including four systems that show excess emission at both wavelengths. A very large fraction (nearly 60%) of observed binary systems with small (<3 AU) separations have excess thermal emission. We interpret the observed infrared excesses as thermal emission from dust produced by collisions in planetesimal belts. The incidence of debris disks around main-sequence A3-F8 binaries is marginally higher than that for single old AFGK stars. Whatever combination of nature (birth conditions of binary systems) and nurture (interactions between the two stars) drives the evolution of debris disks in binary systems, it is clear that planetesimal formation is not inhibited to any great degree. We model these dust disks through fitting the spectral energy distributions and derive typical dust temperatures in the range 100-200 K and typical fractional luminosities around 10-5, with both parameters similar to other Spitzer-discovered debris disks. Our calculated dust temperatures suggest that about half the excesses we observe are derived from circumbinary planetesimal belts and around one-third of the excesses clearly suggest circumstellar material. Three systems with excesses have dust in dynamically unstable regions, and we discuss possible scenarios for the origin of this short-lived dust.

PLANETS AND INFRARED EXCESSES: PRELIMINARY RESULTS FROM A SPITZER MIPS SURVEY OF SOLAR-TYPE STARS

As part of a large Spitzer MIPS Guaranteed Time Observation program, we have searched for infrared excesses due to debris disks toward 26 FGK field stars known from radial velocity (RV) studies to have one or more planets. While none of these stars show excesses at 24 � m, we have detected 70 � m excesses around six stars at the 3 � confidence level. The excesses are 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. These planet-bearing stars are, by selection for RV studies, typically older than 1 Gyr, and the stars identified here with excesses have a median age of 4 Gyr. We find a preliminary correlation of both the frequency and the magnitude of dust emission with the presence of known planets. These are thefirststarsoutside thesolarsystemidentifiedashaving both well-confirmed planetary systems and well-confirmed IR excesses. Subject headingg infrared: stars — Kuiper Belt — planetary systems: formation — planetary systems: protoplanetary disks

Far-infrared properties of M dwarfs

We report the mid- and far-infrared properties of nearby M dwarfs. Spitzer MIPS measurements were obtained for a sample of 62 stars at 24 μm, with subsamples of 41 and 20 stars observed at 70 and 160 μm, respectively. We compare the results with current models of M star photospheres and look for indications of circumstellar dust in the form of significant deviations of K-[24 μm] colors and 70 μm/24 μm flux ratios from the average M star values. At 24 μm, all 62 of the targets were detected; 70 μm detections were achieved for 20 targets in the subsample observed, and no detections were seen in the 160 μm subsample. No clear far-infrared excesses were detected in our sample. The average far-infrared excess relative to the photospheric emission of the M stars is at least 4 times smaller than the similar average for a sample of solar-type stars. However, this limit allows the average fractional infrared luminosity in the M-star sample to be similar to that for more massive stars. We have also set low limits (10-4 to 10-9 M⊕ depending on location) for the maximum mass of dust possible around our stars.