Circumstellar disks are thought to experience a rapid "transition" phase in their evolution that can have a considerable impact on the formation and early development of planetary systems. We present new and archival high angular resolution (0. ′′ 3 ≈ 40-75 AU) Submillimeter Array (SMA) observations of the 880 µm (340 GHz) dust continuum emission from 12 such transition disks in nearby star-forming regions. In each case, we directly resolve a dust-depleted disk cavity around the central star. Using two-dimensional Monte Carlo radiative transfer calculations, we interpret these dust disk structures in a homogeneous, parametric model framework by reproducing their SMA continuum visibilities and spectral energy distributions. The cavities in these disks are large (R cav = 15-73 AU) and substantially depleted of small (∼µm-sized) dust grains, although their mass contents are still uncertain. The structures of the remnant material at larger radii are comparable to normal disks. We demonstrate that these large cavities are relatively common among the millimeter-bright disk population, comprising at least 1 in 5 (20%) of the disks in the bright half (and ≥26% of the upper quartile) of the millimeter luminosity (disk mass) distribution. Utilizing these results, we assess some of the physical mechanisms proposed to account for transition disk structures. As has been shown before, photoevaporation models do not produce the large cavity sizes, accretion rates, and disk masses representative of this sample. It would be difficult to achieve a sufficient decrease of the dust optical depths in these cavities by particle growth alone: substantial growth (to meter sizes or beyond) must occur in large (tens of AU) regions of low turbulence without also producing an abundance of small particles. Given those challenges, we suggest instead that the observations are most commensurate with dynamical clearing due to tidal interactions with low-mass companions -young brown dwarfs or giant planets on long-period orbits. Subject headings: circumstellar matter -protoplanetary disks -planet-disk interactions -planets and satellites: formation -submillimeter: planetary systems

Sound velocities determined in iron, shock compressed to pressures between 77 GPa and 400 GPa, indicate that two phase transitions exist on the Hugoniot. A discontinuity in sound velocities at 200 +_ 2 GPa may mark the transition of e iron to 7 iron. A second discontinuity at 243 _+ 2 GPa is believed to indicate the onset of melting. The calculated temperature at melting lies between 5000 K and 5700 K. When extrapolated from the Hugoniot melting point, the Lindemann criterion yields an estimate of 5800 +_ 500 K for the melting of pure iron at the inner core boundary pressure of 330 GPa. The product of density times the thermodynamic Grfineisen parameter in liquid iron, calculated from the present data, is 19.6 +_ 0.8 Mg m-3. A temperature profile ranging from 3800 K at the core-mantle boundary to 5000 K at the earth's center is calculated using the present data. Sound velocities for e iron provide a better match to seismic velocities for the earth's inner core than do those of 7 iron. A comparison between liquid iron velocities and the velocity profile through the outer core provides further evidence for alloying of iron with "lighter elements" in the core.

To study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low-luminosity YSOs (L $ 0:1Y10 L ) using 3Y38 m Spitzer and ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well-studied massive YSOs (L $ 10 5 L ). The long-known 6.0 and 6.85 m bands are detected toward all sources, with the Class 0Y type YSOs showing the deepest bands ever observed. The 6.0 m band is often deeper than expected from the bending mode of pure solid H 2 O. The additional 5Y7 m absorption consists of five independent components, which, by comparison to laboratory studies, must be from at least eight different carriers. Much of this absorption is due to simple species likely formed by grain surface chemistry, at abundances of 1%Y30% for CH 3 OH, 3%Y8% for NH 3 , 1%Y5% for HCOOH, $6% for H 2 CO, and $0.3% for HCOO À relative to solid H 2 O. The 6.85 m band has one or two carriers, of which one may be less volatile than H 2 O. Its carrier(s) formed early in the molecular cloud evolution and do not survive in the diffuse ISM. If an NH þ 4 -containing salt is the carrier, its abundance relative to solid H 2 O is $7%, demonstrating the efficiency of low-temperature acid-base chemistry or cosmic-rayYinduced reactions. Possible origins are discussed for enigmatic, very broad absorption between 5 and 8 m. Finally, the same ices are observed toward massive and low-mass YSOs, indicating that processing by internal UV radiation fields is a minor factor in their early chemical evolution.

Summary. A Debye model using two cut‐off frequencies corresponding to compressional and shear velocities is used to calculate mineral entropies. This model permits entropy and heat capacity in the Earth to be calculated from seismic profiles, and iteration yields temperature profiles along an isentrope. With an adiabatic temperature profile it is possible to obtain Grüneisen's parameter and thermal expansion as a function of depth. Only in the lower mantle is the calculated Grüneisen's parameter along an isentrope approximately proportional to volume.

We present velocity-resolved spectro-astrometric imaging of the 4.7 µm rovibrational lines of CO gas in protoplanetary disks using the CRIRES high resolution infrared spectrometer on the Very Large Telescope (VLT). The method as applied to three disks with known dust gaps or inner holes out to 4-45 AU (SR 21, HD 135344B and TW Hya) achieves an unprecedented spatial resolution of 0.1 − 0.5 AU. While one possible gap formation mechanism is dynamical clearing by giant planets, other equally good explanations (stellar companions, grain growth, photo-evaporation) exist. One way of distinguishing between different scenarios is the presence and distribution of gas inside the dust gaps. Keplerian disk models are fit to the spectro-astrometric position-velocity curves to derive geometrical parameters of the molecular gas. We determine the position angles and inclinations of the inner disks with accuracies as good as 1-2 • , as well as the radial extent of the gas emission. Molecular gas is detected well inside the dust gaps in all three disks. The gas emission extends to within a radius of 0.5 AU for HD 135344B and to 0.1 AU for TW Hya, supporting partial clearing by a < 1 − 10 M Jup planetary body as the cause of the observed dust gaps, or removal of the dust by extensive grain coagulation and planetesimal formation. The molecular gas emission in SR 21 appears to be truncated within ∼ 7 AU, which may be caused by complete dynamical clearing by a more massive companion. We find a smaller inclination angle of the inner disk of TW Hya than that determined for the outer disk, suggestive of a disk warp. We also detect significant azimuthal asymmetries in the SR 21 and HD 135344B inner disks.

Abstract:The statistics of discovered exoplanets suggest that planets form efficiently. However, there are fundamental unsolved problems, such as excessive inward drift of particles in protoplanetary disks during planet formation. Recent theories invoke dust traps to overcome this problem. We report the detection of a dust trap in the disk around the star Oph IRS 48 using observations from the Atacama Large Millimeter/submillimeter Array (ALMA). The 0.44-millimeter-wavelength continuum map shows high-contrast crescent-shaped emission on one side of the star originating from millimeter-sized grains, whereas both the mid-infrared image (micrometer-sized dust) and the gas traced by the carbon monoxide 6-5 rotational line suggest rings centered on the star. The difference in distribution of big grains versus small grains/gas can be modeled with a vortex-shaped dust trap triggered by a companion.Main Text: While the ubiquity of planets is confirmed almost daily by detections of new exoplanets (1), the exact formation mechanism of planetary systems in disks of gas and dust around young stars remains a long-standing problem in astrophysics (2). In the standard coreaccretion picture, dust grains must grow from submicron sizes to ~10 M Earth rocky cores within the ~10 Myr lifetime of the circumstellar disk. However, this growth process is stymied by what is usually called the "radial drift and fragmentation barrier": Particles of intermediate size (~1 m at 1 AU, or ~1 mm at 50 AU from the star) acquire high drift velocities toward the star with respect to the gas (3,4). This leads to two major problems for further growth (5): First, highvelocity collisions between particles with different drift velocities cause fragmentation. Second, even if particles avoid this fragmentation, they will rapidly drift inward and thus be lost into the star before they have time to grow to planetesimal size. This radial drift barrier is one of the most persistent issues in planet formation theories. A possible solution is dust trapping in so-called pressure bumps: local pressure maxima where the dust piles up. One example of such a pressure bump is an anticyclonic vortex which can trap dust particles in the azimuthal direction (6-10).Using the Atacama Large Millimeter/submillimeter Array (ALMA), we report a highly asymmetric concentration of millimeter-sized dust grains on one side of the disk of the star Oph IRS 48 in the 0.44 millimeter (685 GHz) continuum emission (Fig. 1). We argue that this can be understood in the framework of dust trapping in a large anticyclonic vortex in the disk.The young A-type star Oph IRS 48 (distance ~ 120 pc, 1 pc =3.1·1013 km) has a well studied disk with a large inner cavity (deficit of dust in the inner disk region), a so-called transition disk. Mid-infrared imaging at 18.7 μm reveals a disk ring in the small dust grain (size ~50 μm) emission at an inclination of ~50°, peaking at 55 AU radius (1 AU = 1.5·10 8 km = distance from Earth to the Sun) or 0.46 arcseconds from the star (11). Spatially resolved obs...

Infrared $5-35 m spectra for 40 solar mass T Tauri stars and 7 intermediate-mass Herbig Ae stars with circumstellar disks were obtained using the Spitzer Space Telescope as part of the c2d IRS survey. This work complements prior spectroscopic studies of silicate infrared emission from disks, which were focused on intermediate-mass stars, with observations of solar mass stars limited primarily to the 10 m region. The observed 10 and 20 m silicate feature strengths/shapes are consistent with source-to-source variations in grain size. A large fraction of the features are weak and flat, consistent with micron-sized grains indicating fast grain growth (from 0.1 to 1.0 m in radius). In addition, approximately half of the T Tauri star spectra show crystalline silicate features near 28 and 33 m, indicating significant processing when compared to interstellar grains. A few sources show large 10-to-20 m ratios and require even larger grains emitting at 20 m than at 10 m. This size difference may arise from the difference in the depth into the disk probed by the two silicate emission bands in disks where dust settling has occurred. The 10 m feature strength versus shape trend is not correlated with age or H equivalent width, suggesting that some amount of turbulent mixing and regeneration of small grains is occurring. The strength versus shape trend is related to spectral type, however, with M stars showing significantly flatter 10 m features ( larger grain sizes) than A / B stars. The connection between spectral type and grain size is interpreted in terms of the variation in the silicate emission radius as a function of stellar luminosity, but could also be indicative of other spectral-type-dependent factors (e.g., X-rays, UV radiation, and stellar/disk winds).

We have identified four circumstellar disks with a deficit of dust emission from their inner 15-50 AU. All four stars have F-G spectral type and were uncovered as part of the Spitzer Space Telescope "Cores to Disks" Legacy Program Infrared Spectrograph (IRS) first-look survey of ∼100 pre-main-sequence stars. Modeling of the spectral energy distributions indicates a reduction in dust density by factors of 100-1000 from disk radii between ∼0.4 and 15-50 AU but with massive gas-rich disks at larger radii. This large contrast between the inner and outer disk has led us to use the term "cold disks" to distinguish these unusual systems. However, hot dust [(0.02-0.2) ] is still present close to the central M moon star ( AU). We introduce the 30 mm/13 mm flux density ratio as a new diagnostic for identifying cold disks. The R ≤ 0.8 mechanisms for dust clearing over such large gaps are discussed. Although rare, cold disks are likely in transition from an optically thick to an optically thin state and so offer excellent laboratories for the study of planet formation.