Primordial Disk Research Articles

A RESOLVED DEBRIS DISK AROUND THE CANDIDATE PLANET-HOSTING STAR HD 95086

Recently, a new planet candidate was discovered on direct images around the young (10-17 Myr) A-type star HD95086. The strong infrared excess of the system indicates that, similarly to HR8799, {\ss} Pic, and Fomalhaut, the star harbors a circumstellar disk. Aiming to study the structure and gas content of the HD95086 disk, and to investigate its possible interaction with the newly discovered planet, here we present new optical, infrared and millimeter observations. We detected no CO emission, excluding the possibility of an evolved gaseous primordial disk. Simple blackbody modeling of the spectral energy distribution suggests the presence of two spatially separate dust belts at radial distances of 6 and 64 AU. Our resolved images obtained with the Herschel Space Observatory reveal a characteristic disk size of ~6.0x5.4 arcsec (540x490 AU) and disk inclination of ~25 degree. Assuming the same inclination for the planet candidate's orbit, its re-projected radial distance from the star is 62 AU, very close to the blackbody radius of the outer cold dust ring. The structure of the planetary system at HD95086 resembles the one around HR8799. Both systems harbor a warm inner dust belt and a broad colder outer disk and giant planet(s) between the two dusty regions. Modelling implies that the candidate planet can dynamically excite the motion of planetesimals even out to 270 AU via their secular perturbation if its orbital eccentricity is larger than about 0.4. Our analysis adds a new example to the three known systems where directly imaged planet(s) and debris disks co-exist.

A TREND BETWEEN COLD DEBRIS DISK TEMPERATURE AND STELLAR TYPE: IMPLICATIONS FOR THE FORMATION AND EVOLUTION OF WIDE-ORBIT PLANETS

Cold debris disks trace the limits of planet formation or migration in the outer regions of planetary systems, and thus have the potential to answer many of the outstanding questions in wide-orbit planet formation and evolution. We characterized the infrared excess spectral energy distributions of 174 cold debris disks around 546 main-sequence stars observed by both Spitzer IRS and MIPS. We found a trend between the temperature of the inner edges of cold debris disks and the stellar type of the stars they orbit. This argues against the importance of strictly temperature-dependent processes (e.g. non-water ice lines) in setting the dimensions of cold debris disks. Also, we found no evidence that delayed stirring causes the trend. The trend may result from outward planet migration that traces the extent of the primordial protoplanetary disk, or it may result from planet formation that halts at an orbital radius limited by the efficiency of core accretion.

CONSTRAINING A MODEL OF TURBULENT CORONAL HEATING FOR AU MICROSCOPII WITH X-RAY, RADIO, AND MILLIMETER OBSERVATIONS

Many low-mass pre-main-sequence stars exhibit strong magnetic activity and coronal X-ray emission. Even after the primordial accretion disk has been cleared out, the star's high-energy radiation continues to affect the formation and evolution of dust, planetesimals, and large planets. Young stars with debris disks are thus ideal environments for studying the earliest stages of non-accretion-driven coronae. In this paper we simulate the corona of AU Mic, a nearby active M dwarf with an edge-on debris disk. We apply a self-consistent model of coronal loop heating that was derived from numerical simulations of solar field-line tangling and magnetohydrodynamic turbulence. We also synthesize the modeled star's X-ray luminosity and thermal radio/millimeter continuum emission. A realistic set of parameter choices for AU Mic produces simulated observations that agree with all existing measurements and upper limits. This coronal model thus represents an alternative explanation for a recently discovered ALMA central emission peak that was suggested to be the result of an inner "asteroid belt" within 3 AU of the star. However, it is also possible that the central 1.3 mm peak is caused by a combination of active coronal emission and a bright inner source of dusty debris. Additional observations of this source's spatial extent and spectral energy distribution at millimeter and radio wavelengths will better constrain the relative contributions of the proposed mechanisms.

A Trend Between Cold Debris Disk Temperature and Stellar Type: Implications for the Formation and Evolution of Wide-Orbit Planets

AbstractCold debris disks have the potential to answer many outstanding questions in wide-orbit planet formation and evolution. We characterized the infrared excess SEDs of 174 cold debris disks with Spitzer IRS and MIPS. We found a trend between the temperature of the disks and the stellar type of the stars they orbit. This argues against the importance of strictly temperature-dependent processes (e.g. ice lines) in setting the dimensions of cold debris disks. We also found no evidence that delayed stirring causes the trend. The trend may result from outward planet migration that traces the extent of the primordial protoplanetary disk, or from planet formation that halts at an orbital radius limited by the efficiency of core accretion. For the full details of this work, see Ballering et al. (2013).

THE BURST MODE OF ACCRETION IN PRIMORDIAL PROTOSTARS

We study the formation and long-term evolution of primordial protostellar disks harbored by first stars using numerical hydrodynamics simulations in the thin-disk limit. The initial conditions are specified by pre-stellar cores with distinct mass, angular momentum, and temperature. This allows us to probe several tens of thousand years of the disk's initial evolution, during which we observe multiple episodes of fragmentation leading to the formation of gravitationally bound gaseous clumps within spiral arms. These fragments are torqued inward due to gravitational interaction with the spiral arms on timescales of 10^3 - 10^4 yr and accreted onto the growing protostar, giving rise to accretion and luminosity bursts. The burst phenomenon is fueled by continuing accretion of material falling onto the disk from the collapsing parent core, which replenishes the mass lost by the disk due to accretion, and triggers repetitive episodes of disk fragmentation. We show that the burst phenomenon is expected to occur for a wide spectrum of initial conditions in primordial pre-stellar cores and speculate on how the intense luminosities (~10^7 solar luminosities) produced by this mechanism may have important consequences for the disk evolution and subsequent growth of the protostar.

Protostars, multiplicity, and disk evolution in the Corona Australis region: aHerschelGould Belt Study

The CrA region and the Coronet cluster form a nearby (138 pc), young (1-2 Myr) star-forming region hosting a moderate population of YSO. We present Herschel PACS photometry at 100 and 160 micron, obtained as part of the Herschel Gould Belt Survey. The Herschel maps reveal the cluster members with high sensitivity and high dynamic range. Many of the cluster members are detected, including some embedded, very low-mass objects, several protostars, and substantial emission from the surrounding cloud. The Herschel data provide sufficient spatial resolution to detect small-scale details, such as bright filaments around the IRS5 protostar complex and a bubble-shaped rim associated with the Class I object IRS2. The disks around the Class II objects display a wide range of mid- and far-IR excesses consistent with different disk structures. We have modeled the disks using the RADMC radiative transfer code, finding an interesting mixture of objects for a young and presumably coeval cluster. Some of them are consistent with flared, massive, relatively primordial disks (SCrA, TCrA). Others display significant evidence for inside-out evolution, consistent with the presence of inner holes/gaps (G-85, G-87). Finally, we find disks with a dramatic small dust depletion (G-1, HBC677) that, in some cases, could be related to truncation or to the presence of large gaps in a flared disk (CrA-159). The derived masses for the disks around the low-mass stars are found to be below the typical values in Taurus, in agreement with previous Spitzer observations. Given the high degree of multiplicity and interactions observed among the protostars in the region, the diversity of disks may be a consequence of the early star formation history, which should also be taken into account when studying the disk properties in similar sparsely populated clusters.

Origin of apparent period variations in eclipsing post-common-envelope binaries

Apparent period variations detected in several eclipsing, close-compact binaries are frequently interpreted as being caused by circumbinary giant planets. This interpretation raises the question of the origin of the potential planets that must have either formed in the primordial circumbinary disk, together with the host binary star, and survived its evolution into a close-compact binary or formed in a post-common-envelope circumbinary disk that remained bound to the post-common-envelope binary (PCEB). Here we combine current knowledge of planet formation and the statistics of giant planets around primordial and evolved binary stars with the theory of close-compact binary star evolution aiming to derive new constraints on possible formation scenarios. We compiled a comprehensive list of observed eclipsing PCEBs, estimated the fraction of systems showing apparent period variations, reconstructed the evolutionary history of the PCEBs, and performed binary population models of PCEBs to characterize their main sequence binary progenitors. We reviewed the currently available constraints on the fraction of PCEB progenitors that host circumbinary giant planets. We find that the progenitors of PCEBs are very unlikely to be frequent hosts of giant planets (<~10 per cent), while the frequency of PCEBs with observed apparent period variations is very high (~90 per cent). The variations in eclipse timings measured in eclipsing PCEBs are probably not caused by first-generation planets that survived common-envelope evolution. The remaining options for explaining the observed period variations are second-generation planet formation or perhaps variations in the shape of a magnetically active secondary star. We suggest observational tests for both options.

Trans-Neptunian Objects as Natural Probes to the Unknown Solar System

Trans-Neptunian objects (TNOs) are icy/rocky bodies that move beyond the orbit of Neptune in a region known as the trans-Neptunian belt (or Edgeworth-Kuiper belt). In contrast to the predictions of accretion models that feature protoplanetary disk planetesimals evolving on dynamically cold orbits (with both very small eccentricities, e, and inclinations, i), in reality TNOs exhibit surprisingly wide ranges of orbital eccentricities and inclinations. Several theoretical models have addressed the origin and orbital evolution of the main dynamical classes of TNOs, but none have successfully reproduced them all. In addition, none have explained several objects on peculiar orbits, or provided insightful predictions, without which a model cannot be tested properly against observations. Based on extensive simulations of planetesimal disks with the presence of the four giant planets and huge numbers of modeled planetesimals, I explore in detail the dynamics of the TNOs, in particular their (un)stable regions over timescales comparable to the age of the solar system, and the role of resonances across the entire trans-Neptunian region. I also propose that, along with the orbital history of the giant planets, the orbital evolution of primordial embryos (massive planetesimals comparable to Mars-Earth masses) can explain the fine orbital structure of the trans-Neptunian belt, the orbits of Jovian and Neptunian Trojans, and possibly the current orbits of the giant planets. Those primordial embryos were ultimately scattered by the giant planets, a process that stirred both the orbits of the giant planets and the primordial planetesimal disk to the levels observed at 40-50 AU. In particular, the main constraints provided by the trans-Neptunian belt are optimally satisfied if at least one such primordial embryo (planetoid) survived in the outskirts of the solar system.

THEHERSCHELDIGIT SURVEY OF WEAK-LINE T TAURI STARS: IMPLICATIONS FOR DISK EVOLUTION AND DISSIPATION

As part of the "Dust, Ice, and Gas In Time (DIGIT)" Herschel Open Time Key Program, we present Herschel photometry (at 70, 160, 250, 350 and 500 micron) of 31 Weak-Line T Tauri star (WTTS) candidates in order to investigate the evolutionary status of their circumstellar disks. Thirteen stars in our sample had circumstellar disks previously known from infrared observations at shorter wavelengths, while eighteen of them had no previous evidence for a disk. We detect a total of 15 disks as all previously known disks are detected at one or more Herschel wavelengths and two additional disks are identified for the first time. The spectral energy distributions (SEDs) of our targets seem to trace the dissipation of the primordial disk and the transition to the debris disk regime. Seven of the 15 disks appear to be optically thick primordial disks, including two objects with SEDs indistinguishable from those of typical Classical T Tauri stars, four objects that have significant deficit of excess emission at all IR wavelengths, and one "pre-transitional" object with a known gap in the disk. Despite their previous WTTS classification, we find that the seven targets in our sample with optically thick disks show evidence for accretion. The remaining eight disks have weaker IR excesses similar to those of optically thin debris disks. Six of them are warm and show significant 24 micron Spitzer excesses, while the last two are newly identified cold debris-like disks with photospheric 24 micron fluxes, but significant excess emission at longer wavelengths. The Herschel photometry also places strong constraints on the non-detections, where systems with F70/F70,star > 5 - 15 and L,disk/L,star > 1xE-3 to 1xE-4 can be ruled out. We present preliminary models for both the optically thick and optically thin disks and discuss our results in the context of the evolution and dissipation of circumstellar disks.

Disc clearing of young stellar objects: evidence for fast inside-out dispersal

The time-scale over which and the modality by which young stellar objects (YSOs) disperse their circumstellar discs dramatically influences the eventual formation and evolution of planetary systems. By means of extensive radiative transfer (RT) modelling, we have developed a new set of diagnostic diagrams in the infrared colour-colour plane (K-[24] vs. K-[8]), to aid with the classification of the evolutionary stage of YSOs from photometric observations. Our diagrams allow the differentiation of sources with unevolved (primordial) discs from those evolving according to different clearing scenarios (e.g. homologous depletion vs. inside-out dispersal), as well as from sources that have already lost their disc. Classification of over 1500 sources in 15 nearby star-forming regions reveals that approximately 39 % of the sources lie in the primordial disc region, whereas between 31 % and 32 % disperse from the inside-out and up to 22 % of the sources have already lost their disc. Less than 2 % of the objects in our sample lie in the homogeneous draining regime. Time-scales for the transition phase are estimated to be typically a few 10^5 years independent of stellar mass. Therefore, regardless of spectral type, we conclude that currently available infrared photometric surveys point to fast (of order 10 % of the global disc lifetime) inside-out clearing as the preferred mode of disc dispersal.

Two populations of transition discs?

Abstract We examine the distribution of transition discs as a function of millimetre (mm) flux. We confirm that as expected in any model in which most primordial discs turn into transition discs and in which mm flux declines with time, transition discs have lower mm fluxes on average than primordial discs. However, we find that the incidence of transition discs does not, as expected, fall monotonically towards large mm fluxes and also investigate the hypothesis that these mm bright transition discs may have a distinct physical origin. We find that mm bright transition discs occupy a separate region of parameter space. Transition discs in the bright mm subsample have systematically higher accretion rates than those in the faint mm subsample, along with being systematically weighted to earlier spectral types.

THE DISK POPULATION OF THE UPPER SCORPIUS ASSOCIATION

We present photometry at 3-24um for all known members of the Upper Scorpius association (~11 Myr) based on all images of these objects obtained with the Spitzer Space Telescope and the Wide-field Infrared Survey Explorer. We have used these data to identify the members that exhibit excess emission from circumstellar disks and estimate the evolutionary stages of these disks. Through this analysis, we have found ~50 new candidates for transitional, evolved, and debris disks. The fraction of members harboring inner primordial disks is <10% for B--G stars (M>1.2 Msun) and increases with later types to a value of ~25% at >=M5 (M<=0.2 Msun), in agreement with the results of previous disk surveys of smaller samples of Upper Sco members. These data indicate that the lifetimes of disks are longer at lower stellar masses, and that a significant fraction of disks of low-mass stars survive for at least ~10 Myr. Finally, we demonstrate that the distribution of excess sizes in Upper Sco and the much younger Taurus star-forming region (~1 Myr) are consistent with the same, brief timescale for clearing of inner disks.

PREDICTING THE CONFIGURATION OF A PLANETARY SYSTEM: KOI-152 OBSERVED BYKEPLER

The recent Kepler discovery of KOI-152 reveals a system of three hot super-Earth candidates that are in or near a 4:2:1 mean motion resonance. It is unlikely that they formed in situ, the planets probably underwent orbital migration during the formation and evolution process. The small semimajor axes of the three planets suggest that migration stopped at the inner edge of the primordial gas disk. In this paper we focus on the influence of migration halting mechanisms, including migration "dead zones", and inner truncation by the stellar magnetic field. We show that the stellar accretion rate, stellar magnetic field and the speed of migration in the proto-planetary disk are the main factors affecting the final configuration of KOI-152. Our simulations suggest that three planets may be around a star with low star accretion rate or with high magnetic field. On the other hand, slow type I migration, which decreases to one tenth of the linear analysis results, favors forming the configuration of KOI-152. Under such formation scenario, the planets in the system are not massive enough to open gaps in the gas disk. The upper limit of the planetary masses are estimated to be about $15,~19$, and $24 M_\oplus$, respectively. Our results are also indicative of the near Laplacian configurations that are quite common in planetary systems.

On the theory of disc photoevaporation

We discuss a hydrodynamical model for the dispersal of protoplanetary discs around young, low mass (<1.5 M_sun) stars by photoevaporation from the central object's energetic radiation, which considers the far-ultraviolet as well as the X-ray component of the radiation field. We present analytical scaling relations and derive estimates for the total mass-loss rates, as well as discussing the existence of similarity solutions for flows from primordial discs and discs with inner holes. Furthermore, we perform numerical calculations, which span a wide range of parameter space and allow us to provide accurate scalings of the mass-loss rates with the physical parameters of the systems (X-ray and FUV luminosity, stellar mass, disc mass, disc temperature and inner hole radius). The model suggest that the X-ray component dominates the photoevaporative mass-loss rates from the inner disc. The mass-loss rates have values in the range from 10e-11 to 10e-7 M_sun/yr and scale linearly with X-ray luminosity, with only a weak dependence on the other parameters explored. However, in the case of high FUV to X-ray (L_FUV/L_X>100) luminosity ratios, the FUV constricts the X-ray flow and may dominate the mass-loss. Simulations of low mass discs with inner holes demonstrate a further stage of disc clearing, which we call `thermal sweeping'. This process occurs when the mid-plane pressure drops to sufficiently low values. At this stage a bound, warm, X-ray heated region becomes sufficiently large and unstable, such that the remaining disc material is cleared on approximately dynamical time-scales. This process significantly reduces the time taken to clear the outer regions of the disc, resulting in an expected transition disc population that will be dominated by accreting objects, as indicated by recent observations.

Spitzerobservations of NGC 2264: the nature of the disk population

NGC2264 is a young cluster with a rich circumstellar disk population which makes it an ideal target for studying the evolution of stellar clusters. Our goal is to study its star formation history and to analyse the primordial disk evolution of its members. The study presented is based on data obtained with Spitzer IRAC and MIPS, combined with deep NIR ground-based FLAMINGOS imaging and previously published optical data. We build NIR dust extinction maps of the molecular cloud associated with the cluster, and determine it to have a mass of 2.1x10^3Msun above an Av of 7mag. Using a differential K_s-band luminosity function of the cluster, we estimate the size of its population to be 1436$\pm$242 members. The star formation efficiency is ~25%. We identify the disk population: (i) optically thick inner disks, (ii) anaemic inner disks, and (iii) disks with inner holes, or transition disks. We analyse the spatial distribution of these sources and find that sources with thick disks segregate into sub-clusterings, whereas sources with anaemic disks do not. Furthermore, sources with anaemic disks are found to be unembedded (Av<3mag), whereas the clustered sources with thick disks are still embedded within the parental cloud. NGC2264 has undergone more than one star-forming event, where the anaemic and extincted thick disk population appear to have formed in separate episodes. We also find tentative evidence of triggered star-formation in the Fox Fur Nebula. In terms of disk evolution, our findings support the emerging disk evolution paradigm of two distinct evolutionary paths for primordial optically thick disks: a homologous one where the disk emission decreases uniformly at NIR and MIR wavelengths, and a radially differential one where the emission from the inner region of the disk decreases more rapidly than from the outer region (forming transition disks).

THE HUBBLE WIDE FIELD CAMERA 3 TEST OF SURFACES IN THE OUTER SOLAR SYSTEM: THE COMPOSITIONAL CLASSES OF THE KUIPER BELT

We present the first results of the Hubble Wide Field Camera 3 Test of Surfaces in the Outer Solar System (H/WTSOSS). The purpose of this survey was to measure the surface properties of a large number of Kuiper belt objects and attempt to infer compositional and dynamical correlations. We find that the Centaurs and the low-perihelion scattered disk and resonant objects exhibit virtually identical bifurcated optical colour distributions and make up two well defined groups of object. Both groups have highly correlated optical and NIR colours which are well described by a pair of two component mixture models that have different red components, but share a common neutral component. The small, $H_{606}\gtrsim5.6$ high-perihelion excited objects are entirely consistent with being drawn from the two branches of the mixing model suggesting that the colour bifurcation of the Centaurs is apparent in all small excited objects. On the other hand, objects larger than $H_{606}\sim5.6$ are not consistent with the mixing model, suggesting some evolutionary process avoided by the smaller objects. The existence of a bifurcation amongst all excited populations argues that the two separate classes of object existed in the primordial disk before the excited Kuiper belt was populated. The cold classical objects exhibit a different type of surface which has colours that are consistent with being drawn from the red branch of the mixing model, but with much higher albedos.

A METHOD TO CONSTRAIN THE SIZE OF THE PROTOSOLAR NEBULA

Observations indicate that the gaseous circumstellar disks around young stars vary significantly in size, ranging from tens to thousands of AU. Models of planet formation depend critically upon the properties of these primordial disks, yet in general it is impossible to connect an existing planetary system with an observed disk. We present a method by which we can constrain the size of our own protosolar nebula using the properties of the small body reservoirs in the solar system. In standard planet formation theory, after Jupiter and Saturn formed they scattered a significant number of remnant planetesimals into highly eccentric orbits. In this paper, we show that if there had been a massive, extended protoplanetary disk at that time, then the disk would have excited Kozai oscillations in some of the scattered objects, driving them into high-inclination (i > 50 deg), low-eccentricity orbits (q > 30 AU). The dissipation of the gaseous disk would strand a subset of objects in these high-inclination orbits; orbits that are stable on Gyr time scales. To date, surveys have not detected any Kuiper Belt Objects with orbits consistent with this dynamical mechanism. Using these non-detections by the Deep Ecliptic Survey (DES) and the Palomar Distant Solar System Survey we are able to rule out an extended gaseous protoplanetary disk (R_D > 80 AU) in our solar system at the time of Jupiter's formation. Future deep all sky surveys such as the Large Synoptic Survey Telescope (LSST) will all us to further constrain the size of the protoplanetary disk.

Dust discs around intermediate-mass and Sun-like stars in the 16 Myr old NGC 1960 open cluster

We present an analysis of Spitzer IRAC (3.6--8um) and MIPS (24um) imaging of members of the 16(+10/-5)Myr old open cluster NGC 1960 (M36). Models of terrestrial planet formation indicate that rocky planets are likely to achieve their final masses at around 10-30Myr, and thus this cluster is at an interesting epoch for planet formation. We find 21 B-F5 type stars and 14 F6-K9 type stars which have 24um excess emission, and thus determine that >30% of B-F5 type stars and >23% of F6-K9 type stars in this cluster have 24um excess emission. These excess frequencies are similar to those observed in other clusters of similar age. Three early type stars have excesses at near-infrared wavelengths. Analysis of their SEDs confirms that these are true debris discs and not remnant primordial or transitional discs. None of the 61 sun-like stars have confirmed near-infrared excess, and we can place a limit on the frequency of 8um excess emission around sun-like stars of <7%. All of the detected excesses are consistent with emission from debris discs and are not primordial.

Disc frequencies for brown dwarfs in the Upper Scorpius OB association: implications for brown dwarf formation theories

We have investigated the brown dwarf (BD) and stellar disc fractions in the Upper Scorpius OB Association (USco) and compared them with several other young regions. We have compiled the most complete sample of of all spectroscopically confirmed BDs in USco, and have made use of the WISE catalog to identify the disc candidates. We report on the discovery of 12 new BD discs in USco, with spectral type (SpT) between M6 and M8.5. The WISE colors for the new discs are similar to the primordial (transition) discs earlier detected in USco. Combining with previous surveys, we find the lowest inner disc fractions (~20-25%) for a wide range in stellar masses (~0.01-4.0 Msun) in the USco association. The low disc fractions for high-mass stars in USco (and the other clusters) are consistent with an evolutionary decline in inner disc frequency with age. However, BD disc fractions are higher than those for the stars in 1-3 Myr clusters, but very low in the ~5 Myr old USco. Also, primordial BD discs are still visible in the ~10 Myr TW Hydrae association, whereas the higher mass stars have all transitioned to the debris stage by this age. The disc frequencies for BDs do not show any dependence on the stellar density or the BD/star number ratio in a cluster. We suggest that the large differences in the observed BD disc fractions between regions may well be due to different BD formation mechanisms and therefore different initial disc fractions/properties. We also present a WISE SED classification scheme, based on the Ks and WISE bands of 3.4-12micron, to distinguish between the Class I/II and the Class III sequence. Our work includes a comparison of the sensitivities of WISE and Spitzer disc surveys. We estimate that WISE can be incomplete for discs at SpT later than M8 in distant clusters such as SOri. WISE should be able to recover the M8-M9 discs in the nearby young clusters.

THE LATE STAGES OF PROTOPLANETARY DISK EVOLUTION: A MILLIMETER SURVEY OF UPPER SCORPIUS

We present deep 1.2 millimeter photometry of 37 stars in the young (5 Myr) Upper Scorpius OB association, sensitive to ~4 x 10^-3 Mjup of cool millimeter dust. Disks around four low- and solar-mass stars are detected, as well as one debris disk around an intermediate mass star, with dust masses ranging from 3.6 x 10^-3 -- 1.0 x 10^-1 Mjup. The source with the most massive disk exhibits a transition-disk spectral energy distribution. Combining our results with previous studies, we find the millimeter-detection fraction of Class II sources has significantly decreased from younger ages, and comparison with near-infrared and Halpha measurements indicates the present disks have undergone significant evolution in composition or structure at all radii. The disks of Upper Scorpius represent the tail-end of the depletion of primordial disks; while a few near-solar mass stars may still sustain giant planet formation, this process has finished around higher mass stars