Keywords

We study the dynamics of a vertically thin, dispersion-dominated disk of planetesimals with eccentricities $\tilde{e}$ and inclinations $\tilde{i}$ (normalized in Hill units) satisfying $\tilde{e}\gg 1$, $\tilde{i}\ll \tilde{e}^{-2}\ll 1$. This situation may be typical (even if only temporarily) for, e.g., a population of protoplanetary cores in the end of the oligarchic phase of planet formation. In this regime of orbital parameters, planetesimal scattering has an anisotropic character and strongly differs from scattering in thick ($\tilde{i}\sim \tilde{e}$) disks. We derive analytical expressions for the planetesimal scattering coefficients and compare them with numerical calculations. We find significant discrepancies in the inclination scattering coefficients obtained by the two approaches and ascribe this difference to the effects not accounted for in the analytical calculation: multiple scattering events (temporary captures, which may be relevant for the production of distant planetary satellites outside the Hill sphere) and distant interaction of planetesimals prior to their close encounter. Our calculations show that the inclination of a thin, dispersion-dominated planetesimal disk grows exponentially on a very short timescale implying that (1) such disks must be very short-lived and (2) planetesimal accretion in this dynamical phase is insignificant. Our results are also applicable to the dynamics of shear-dominated disks switching to the dispersion-dominated regime.

The discovery of decay products of a short-lived radioisotope (SLRI) in the Allende meteorite led to the hypothesis that a supernova shock wave transported freshly synthesized SLRI to the presolar dense cloud core, triggered its self-gravitational collapse, and injected the SLRI into the core. Previous multidimensional numerical calculations of the shock–cloud collision process showed that this hypothesis is plausible when the shock wave and dense cloud core are assumed to remain isothermal at ∼10 K, but not when compressional heating to ∼1000 K is assumed. Our two-dimensional models with the FLASH2.5 adaptive mesh refinement hydrodynamics code have shown that a 20 km s⁻¹ shock front can simultaneously trigger collapse of a 1 M_☉ core and inject shock wave material, provided that cooling by molecular species such as H₂O, CO, and H₂ is included. Here, we present the results for similar calculations with shock speeds ranging from 1 km s⁻¹ to 100 km s⁻¹. We find that shock speeds in the range from 5 km s⁻¹ to 70 km s⁻¹ are able to trigger the collapse of a 2.2 M_☉ cloud while simultaneously injecting shock wave material: lower speed shocks do not achieve injection, while higher speed shocks do not trigger sustained collapse. The calculations continue to support the shock-wave trigger hypothesis for the formation of the solar system, though the injection efficiencies in the present models are lower than desired.

We apply ionization balance and magnetohydrodynamical (MHD) calculations to investigate whether magnetic activity moderated by recombination on dust grains can account for the mass accretion rates and the mid-infrared spectra and variability of protostellar disks. The MHD calculations use the stratified shearing-box approach and include grain settling and the feedback from the changing dust abundance on the resistivity of the gas. The two-decade spread in accretion rates among solar-mass T Tauri stars is too large to result solely from variations in the grain size and stellar X-ray luminosity, but can plausibly be produced by varying these parameters together with the disk magnetic flux. The diverse shapes and strengths of the mid-infrared silicate bands can come from the coupling of grain settling to the distribution of the magnetorotational turbulence, through the following three effects. First, recombination on grains 1 μm or smaller yields a magnetically inactive dead zone extending more than two scale heights from the midplane, while turbulent motions in the magnetically active disk atmosphere overshoot the dead zone boundary by only about one scale height. Second, grains deep in the dead zone oscillate vertically in wave motions driven by the turbulent layer above, but on average settle at the rates found in laminar flow, so that the interior of the dead zone is a particle sink and the disk atmosphere will become dust-depleted unless resupplied from elsewhere. Third, with sufficient depletion, the dead zone is thinner and mixing dredges grains off the midplane. The last of these processes enables evolutionary signatures such as the degree of settling to sometimes decrease with age. The MHD results also show that the magnetic activity intermittently lifts clouds of small grains into the atmosphere. Consequently the photosphere height changes by up to one-third over timescales of a few orbits, while the extinction along lines of sight grazing the disk surface varies by factors of 2 over times down to a tenth of an orbit. We suggest that the changing shadows cast by the dust clouds on the outer disk are a cause of the daily to monthly mid-infrared variability found in many young stars.

Pyroxenes and olivines are the dominant crystalline silicates observed in protoplanetary disks. Recent spectral observations from the Spitzer Space Telescope indicate that the abundance of olivine, generally associated with silica polymorphs, relative to pyroxene is higher in the outer cold part of the disk than in the inner warmer part. The interpretation of these unexpected results requires a comprehensive knowledge of the thermal processing of Mg-rich silicate dust. In this respect, amorphous analogs were thermally annealed to identify microscopic crystallization mechanisms. We show that pyroxenes are not produced in significant proportions below the glass transition temperature of the amorphous precursor. The annealing of amorphous enstatite leads to a mineralogical assemblage dominated by forsterite, with only minute amounts of pyroxenes at temperatures as high as the glass transition temperature of enstatite (1050 K). The decoupling of cation mobility in amorphous silicates, favors the crystallization of the most Mg-enriched silicates. These results are consistent with Spitzer observations of silicate dust and also with the documented mineralogy of presolar silicates, making the low-temperature annealing a likely formation process for these objects. Based on these laboratory experiments and Spitzer observations, it appears that the reported zoned mineralogy may directly records and calibrates the thermal gradient at the scale of protoplanetary disks.

We present the results of a search for occultation events by objects at distances between 100 and 1000 AU in light curves from the Taiwanese–American Occultation Survey. We searched for consecutive, shallow flux reductions in the stellar light curves obtained by our survey between 2005 February 7 and 2006 December 31 with a total of ∼4.5 × 10⁹ three-telescope simultaneous photometric measurements. No events were detected, allowing us to set upper limits on the number density as a function of size and distance of objects in Sedna-like orbits, using simple models.

Recent observation of the microlensing technique reveals two giant planets at 2.3 AU and 4.6 AU around the star OGLE-06-109L. The eccentricity of the outer planet (e_c) is estimated to be 0.11^+0.17_−0.04, comparable to that of Saturn (0.01–0.09). The similarities between the OGLE-06-109L system and the solar system indicate that they may have passed through similar histories during their formation stage. In this paper, we investigate the dynamics and formation of the orbital architecture in the OGLE-06-109L system. For the present two planets with their nominal locations, the secular motions are stable as long as their eccentricities (e_b, e_c) fulfill e²_b + e²_c ⩽ 0.3². Earth-size bodies might be formed and are stable in the habitable zone (0.25–0.36 AU) of the system. Three possible scenarios may be accounted for the formation of e_b and e_c: (1) convergent migration of two planets and the 3:1 mean motion resonance (MMR) trapping; (2) planetary scattering; and (3) divergent migration and the 3:1 MMR crossing. As we showed that the probability for the two giant planets in 3:1 MMR is low (∼3%), scenario (1) is less likely. According to models (2) and (3), the final eccentricity of inner planet (e_b) may oscillate between [0–0.06], comparable to that of Jupiter (0.03–0.06). An inspection of e_b, e_c's secular motion may be helpful to understand which model is really responsible for the eccentricity formation.

Using the Infrared Spectrograph aboard the Spitzer Space Telescope, we observed multiple epochs of 11 actively accreting T Tauri stars in the nearby Taurus–Auriga star-forming region. In total, 88 low-resolution mid-infrared spectra were collected over 1.5 years in Cycles 2 and 3. The results of this multi-epoch survey show that the 10 μm silicate complex in the spectra of two sources—DG Tau and XZ Tau—undergoes significant variations with the silicate feature growing both weaker and stronger over month- and year-long timescales. Shorter timescale variations on day- to week-long timescales were not detected within the measured flux errors. The time resolution coverage of this data set is inadequate for determining if the variations are periodic. Pure emission compositional models of the silicate complex in each epoch of the DG Tau and XZ Tau spectra provide poor fits to the observed silicate features. These results agree with those of previous groups that attempted to fit only single-epoch observations of these sources. Simple two-temperature, two-slab models with similar compositions successfully reproduce the observed variations in the silicate features. These models hint at a self-absorption origin of the diminution of the silicate complex instead of a compositional change in the population of emitting dust grains. We discuss several scenarios for producing such variability including disk shadowing, vertical mixing, variations in disk heating, and disk wind events associated with accretion outbursts.

Oxygen is one of the major rock-forming elements in the solar system and the third most abundant element of the Sun. Oxygen isotopic composition of the Sun, however, is not known due to a poor resolution of astronomical spectroscopic measurements. Several Δ¹⁷O values have been proposed for the composition of the Sun based on (1) the oxygen isotopic measurements of the solar wind implanted into metallic particles in lunar soil (< −20‰ by Hashizume & Chaussidon and ∼ +26‰ by Ireland et al.), (2) the solar wind returned by the Genesis spacecraft (−27‰ ± 6‰ by McKeegan et al.), and (3) the mineralogically pristine calcium–aluminum-rich inclusions (CAIs) (−23.3‰ ± 1.9‰ by Makide et al. and −35‰ by Gounelle et al.). CAIs are the oldest solar system solids, and are believed to have formed by evaporation, condensation, and melting processes in hot nebular region(s) when the Sun was infalling (Class 0) or evolved (Class 1) protostar. Corundum (Al₂O₃) is thermodynamically the first condensate from a cooling gas of solar composition. Corundum-bearing CAIs, however, are exceptionally rare, suggesting either continuous reaction of the corundum condensates with a cooling nebular gas and their replacement by hibonite (CaAl₁₂O₁₉) or their destruction by melting together with less refractory condensates during formation of igneous CAIs. In contrast to the corundum-bearing CAIs, isolated micrometer-sized corundum grains are common in the acid-resistant residues from unmetamorphosed chondrites. These grains could have avoided multistage reprocessing during CAI formation and, therefore, can potentially provide constraints on the initial oxygen isotopic composition of the solar nebula, and, hence, of the Sun. Here we report oxygen isotopic compositions of ∼60 micrometer-sized corundum grains in the acid-resistant residues from unequilibrated ordinary chondrites (Semarkona (LL3.0), Bishunpur (LL3.1), Roosevelt County 075 (H3.2)) and unmetamorphosed carbonaceous chondrites (Orgueil (CI1), Murray (CM2), and Alan Hills A77307 (CO3.0)) measured with a Cameca ims-1280 ion microprobe. All corundum grains, except two, are ¹⁶O-rich (Δ¹⁷O = −22.7‰ ± 8.5‰, 2σ), and compositionally similar to the mineralogically pristine CAIs from the CR carbonaceous chondrites (−23.3‰ ± 1.9‰, 2σ), and solar wind returned by the Genesis spacecraft (−27‰ ± 6‰, 2σ). One corundum grain is highly ¹⁷O-enriched (δ¹⁷O ∼ +60‰, δ¹⁸O ∼ −40‰) and is probably of the presolar origin; the origin of another ¹⁷O-rich grain (δ¹⁷O ∼ −15‰, δ¹⁸O ∼ −35‰) is unclear. We conclude that the ¹⁶O-rich corundum grains in the acid-resistant residues from unequilibrated ordinary and unmetamorphosed carbonaceous chondrites recorded initial oxygen isotopic composition of the solar nebula, and, hence, of the Sun. Our inferred oxygen isotopic composition of the Sun is inconsistent with the more extreme ¹⁶O-rich value (Δ¹⁷O ∼ −35‰) proposed by Gounelle et al. on the basis of two extremely ¹⁶O-rich CAIs from the CH/CB-like chondrite Isheyevo and with the ¹⁶O-poor value observed as a component of the solar wind implanted into the metallic particles in lunar soil (Ireland et al.).

The ¹⁸O/¹⁷O ratio of the solar system is 5.2 while that of the interstellar medium (ISM) and young stellar objects is ∼4. This difference cannot be explained by pollution of the Sun's natal molecular cloud by ¹⁸O-rich supernova ejecta because (1) the necessary B-star progenitors live longer than the duration of star formation in molecular clouds, (2) the delivery of ejecta gas is too inefficient and the amount of dust in supernova ejecta is too small compared to the required pollution (2% of total mass or ∼20% of oxygen), and (3) the predicted amounts of concomitant short-lived radionuclides (SLRs) conflicts with the abundances of ²⁶Al and ⁴¹Ca in the early solar system. Proposals for the introduction of ¹⁸O-rich material must also be consistent with any explanation for the origin of the observed slope-one relationship between ¹⁷O/¹⁶O and ¹⁸O/¹⁶O in the high-temperature components of primitive meteorites. The difference in ¹⁸O/¹⁷O ratios can be explained by enrichment of the ISM by the ¹⁷O-rich winds of asymptotic giant branch (AGB) stars, the sequestration of comparatively ¹⁸O-rich gas from star-forming regions into long-lived, low-mass stars, and a monotonic decrease in the ¹⁸O/¹⁷O ratio of interstellar gas. At plausible rates of star formation and gas infall, Galactic chemical evolution does not follow a slope-one line in a three-isotope plot, but instead moves along a steeper trajectory toward an ¹⁷O-rich state. Evolution of the ISM and star-forming gas by AGB winds also explains the difference in the carbon isotope ratios of the solar system and ISM.

We describe an analytic model for the evolution of a protoplanetary disk heated by viscous accretion and radiation from the central star. The disk is assumed to be flared and viscosity is assumed to follow an "alpha" model, where viscosity is proportional to the local sound speed and scale height. In the inner disk, the midplane temperature is mainly determined by the energy released by the viscous accretion of material through the disk and onto the star. In the outer disk, stellar irradiation is the dominant heat source. A third regime is present in the innermost part of the disk, where viscous heating dominates but the opacity declines rapidly with increasing temperature due to dust grain sublimation. Changes in the protostellar radius and luminosity over time are readily incorporated into the model, although these have a relatively minor effect on the disk evolution. The model yields the surface density and midplane temperature at any point in space and time during the lifetime of the disk. It is especially suited to studies of planet formation that require a self-consistent model of disk evolution with minimal computational expense.