Centimeter-sized Grains Research Articles

Centimeter-sized Grains in the Compact Dust Ring around Very-low-mass Star CIDA 1

We examined the grain size in the dust ring encircling the 0.19 M ☉ T Tauri star CIDA 1 using the Karl G. Jansky Very Large Array at multiple centimeter wavelengths, with a spatial resolution of 0.″2–0.″9. We detected distinct partial-ring structures at these wavelengths around CIDA 1. Based on spatial distributions and spectral indices, we determined that these centimeter emissions originated from dust, rather than free–free or synchrotron emissions. To estimate the maximum grain size () within the ring, we compared the observed spectral energy distribution (SED) with SEDs calculated for different values using radiative transfer calculations. Our findings indicate an value of approximately 2.5 cm in the ring, assuming that the dust opacity can be approximated by the DSHARP models. These results suggest that grain growth took place within the CIDA 1 ring, potentially facilitating more efficient planet formation through pebble accretion scenarios involving centimeter-sized pebbles.

A new microstructural refinement mechanism of ultra-large β grains in near-β titanium alloys during primary hot-working

In this work, the multi-step hot-working of a near-β titanium alloy, Ti-55511 alloy with centimeter-sized grains, was performed at 1170 °C. It was confirmed that the static/metadynamic recrystallization during interpass annealing was the primary mechanism responsible for the microstructural refinement. Meanwhile, a new breakdown mechanism of the ultra-large β grains with 〈001〉 preferential orientation was observed during deformation, which was essentially different from the traditional discontinuous or continuous dynamic recrystallization. That is, new grain boundaries and even boundary networks were produced interconnecting the corners/protrusions of the 〈001〉 -orientated grains through a low-angle grain boundary merging mechanism, which progressively severed the large β grains into smaller ones. Such a breakdown mechanism may be an important complement for microstructural refinement during primary hot-working, which allowed the breakdown of the ultra-large β grains under fairly low strain level.

Resilience of Planetesimal Formation in Weakly Reinforced Pressure Bumps

Abstract The discovery that axisymmetric dust rings are ubiquitous in protoplanetary disks has provoked a flurry of research on the role of pressure bumps in planet formation. High-resolution simulations by our group have shown that even a modest bump can collect enough dust to trigger planetesimal formation by the streaming instability. In this work, we probe the limits of planetesimal formation when the external source of pressure bump reinforcement is extremely weak. We conduct simulations of radially elongated shearing boxes to capture the entire bump, which is generated and maintained over some timescale t reinf by a Newtonian relaxation scheme. We find that planetesimal formation is extremely resilient for centimeter-sized grains. We reduced the strength of reinforcement by up to a factor of 100 and the location and initial masses of planetesimals were essentially unaffected. However, we do find that strong reinforcement causes much faster pebble drift compared to the standard pebble drift rates. The resulting larger pebble flux enhances the planetesimal growth rate by pebble accretion. We hypothesize that, to sustain the bump, our code has to extract angular momentum (the strength of this negative torque depends on t reinf ), and some of this torque is transferred to the particles, causing them to drift faster for a stronger torque (i.e., smaller t reinf ). Since any physical process that sustains a pressure bump must do so by torquing the gas, we conjecture that the effect on pebble drift is a real phenomenon, motivating further work with physically realistic sources to generate the bump.

The shape and surface environment of 2016 HO[formula omitted

Asteroid 2016 HO3 is a fast-rotating Near-Earth asteroid (NEA), and a potential target for future exploration. Studying the surface of this asteroid and whether it retains any regolith are primary science objectives, which can provide key clues to the formation and evolution of this small body. In order to support such future explorations, the stability condition for and distribution of regolith on a non-spherical fast-spinning small body are explored in this paper using derived models of 2016 HO3. First, a tri-axial ellipsoid shape model of 2016 HO3 is established based on light curve data. The possible shape configurations are presented. Then, the ambient environment accelerations are analyzed and compared. The stability conditions to retain regolith on the surface of the ellipsoid model is derived and evaluated numerically. The influence of shape, density, cohesion and the angle of friction on the distribution is discussed. Finally, the motion of failed regolith is investigated. The results show that 2016 HO3 is likely an elongated asteroid, which has a length to width ratio smaller than 0.48. A layer of millimeter-sized to centimeter-sized grains can exist on the surface of asteroid 2016 HO3, preferentially kept near the polar and the short axis regions. The required cohesion to bind such particles to the surface is less than 0.2 Pa. Meanwhile, any regolith disturbed from the surface will undergo periods of sliding and bouncing before escaping. This research can provide a reference for preliminary mission design for future missions to asteroid 2016 HO3.

Hints on the origins of particle traps in protoplanetary disks given by the Mdust – M⋆ relation

Context. Demographic surveys of protoplanetary disks, carried out mainly with the Atacama Large Millimeter/submillimete Array, have provided access to a large range of disk dust masses (Mdust) around stars with different stellar types and in different star-forming regions. These surveys found a power-law relation between Mdust and M⋆ that steepens in time, but which is also flatter for transition disks (TDs). Aims. We aim to study the effect of dust evolution in the Mdust−M⋆ relation. In particular, we are interested in investigating the effect of particle traps on this relation. Methods. We performed dust evolution models, which included perturbations to the gas surface density with different amplitudes to investigate the effect of particle trapping on the Mdust−M⋆ relation. These perturbations were aimed at mimicking pressure bumps that originated from planets. We focused on the effect caused by different stellar and disk masses based on exoplanet statistics that demonstrate a dependence of planet mass on stellar mass and metallicity. Results. Models of dust evolution can reproduce the observed Mdust−M⋆ relation in different star-forming regions when strong pressure bumps are included and when the disk mass scales with stellar mass (case of Mdisk = 0.05 M⋆ in our models). This result arises from dust trapping and dust growth beyond centimeter-sized grains inside pressure bumps. However, the flatter relation of Mdust − M⋆ for TDs and disks with substructures cannot be reproduced by the models unless the formation of boulders is inhibited inside pressure bumps. Conclusions. In the context of pressure bumps originating from planets, our results agree with current exoplanet statistics on giant planet occurrence increasing with stellar mass, but we cannot draw a conclusion about the type of planets needed in the case of low-mass stars. This is attributed to the fact that for M⋆ < 1 M⊙, the observed Mdust obtained from models is very low due to the efficient growth of dust particles beyond centimeter-sizes inside pressure bumps.

A CONCENTRATION OF CENTIMETER-SIZED GRAINS IN THE OPHIUCHUS IRS 48 DUST TRAP

Azimuthally asymmetric dust distributions observed with ALMA in transition disks have been interpreted as dust traps. We present VLA Ka band (34 GHz or 0.9 cm) and ALMA Cycle 2 Band 9 (680 GHz or 0.45 mm) observations at 0.2" resolution of the Oph IRS 48 disk, which suggest that larger particles could be more azimuthally concentrated than smaller dust grains, assuming an axisymmetric temperature field or optically thin 680 GHz emission. Fitting an intensity model to both data demonstrates that the azimuthal extent of the millimeter emission is 2.3 $\pm0.9$ times as wide as the centimeter emission, marginally consistent with the particle trapping mechanism under the above assumptions. The 34 GHz continuum image also reveals evidence for ionized gas emission from the star. Both the morphology and the spectral index variations are consistent with an increase of large particles in the center of the trap, but uncertainties remain due to the continuum optical depth at 680 GHz. Particle trapping has been proposed in planet formation models to allow dust particles to grow beyond millimeter sizes in the outer regions of protoplanetary disks. The new observations in the Oph IRS 48 disk provide support for the dust trapping mechanism for centimeter-sized grains, although additional data is required for definitive confirmation.

Giant secondary grain growth in Cu films on sapphire

Single crystal metal films on insulating substrates are attractive for microelectronics and other applications, but they are difficult to achieve on macroscopic length scales. The conventional approach to obtaining such films is epitaxial growth at high temperature using slow deposition in ultrahigh vacuum conditions. Here we describe a different approach that is both simpler to implement and produces superior results: sputter deposition at modest temperatures followed by annealing to induce secondary grain growth. We show that polycrystalline as-deposited Cu on α-Al2O3(0001) can be transformed into Cu(111) with centimeter-sized grains. Employing optical microscopy, x-ray diffraction, and electron backscatter diffraction to characterize the films before and after annealing, we find a particular as-deposited grain structure that promotes the growth of giant grains upon annealing. To demonstrate one potential application of such films, we grow graphene by chemical vapor deposition on wafers of annealed Cu and obtain epitaxial graphene grains of 0.2 mm diameter.

CONSTRAINTS ON THE RADIAL VARIATION OF GRAIN GROWTH IN THE AS 209 CIRCUMSTELLAR DISK

We present dust continuum observations of the protoplanetary disk surrounding the pre-main sequence star AS 209, spanning more than an order of magnitude in wavelength from 0.88 to 9.8 mm. The disk was observed with sub-arcsecond angular resolution (0.2"-0.5") to investigate radial variations in its dust properties. At longer wavelengths, the disk emission structure is notably more compact, providing model-independent evidence for changes in the grain properties across the disk. We find that physical models which reproduce the disk emission require a radial dependence of the dust opacity \kappa_{\nu}. Assuming that the observed wavelength-dependent structure can be attributed to radial variations in the dust opacity spectral index (\beta), we find that \beta(R) increases from \beta<0.5 at \sim20 AU to \beta>1.5 for R>80 AU, inconsistent with a constant value of \beta\ across the disk (at the 10\sigma\ level). Furthermore, if radial variations of \kappa_{\nu} are caused by particle growth, we find that the maximum size of the particle-size distribution (a_{max}) increases from sub-millimeter-sized grains in the outer disk (R>70 AU) to millimeter and centimeter-sized grains in the inner disk regions (R< 70 AU). We compare our observational constraint on a_{max}(R) with predictions from physical models of dust evolution in proto-planetary disks. For the dust composition and particle-size distribution investigated here, our observational constraints on a_{max}(R) are consistent with models where the maximum grain size is limited by radial drift.

Images of the Vega Dust Ring at 350 and 450 μm: New Clues to the Trappingof Multiple-Sized Dust Particles in Planetary Resonances

We have used the SHARC II camera at Caltech Submillimeter Observatory to make 350 and 450 μm images of the Vega dust disk at spatial resolutions (FWHM) of 9.7 and 11.1, respectively. The images show a ringlike morphology (radius ~100 AU) with inhomogeneous structure that is qualitatively different from that previously reported at 850 μm and longer wavelengths. We attribute the 350/450 μm emission to a grain population whose characteristic size (~1 mm) is intermediate between that of the centimeter-sized grains responsible for emission longward of 850 μm and the much smaller grains (≲18 μm) in the extensive halo, visible at 70 μm, discussed by Su et al. We have combined our submillimeter images with Spitzer data at 70 μm to produce two-dimensional of line-of-sight optical depth (relative column density). These maps suggest that the millimeter-sized grains are located preferentially in three symmetrically located concentrations. If so, then this structure could be understood in terms of the Wyatt model in which planetesimals are trapped in the mean motion resonances of a Neptune-mass planet at 65 AU, provided allowance is made for the spatial distribution of dust grains to differ from that of the parent planetesimals. The peaks of the tau are, in fact, located near the expected positions corresponding to the 4 : 3 resonance. If this identification is confirmed by future observations, it would resolve an ambiguity with regard to the location of the planet.

Touchdown of the Hayabusa Spacecraft at the Muses Sea on Itokawa

After global observations of asteroid 25143 Itokawa by the Hayabusa spacecraft, we selected the smooth terrain of the Muses Sea for two touchdowns carried out on 19 and 25 November 2005 UTC for the first asteroid sample collection with an impact sampling mechanism. Here, we report initial findings about geological features, surface condition, regolith grain size, compositional variation, and constraints on the physical properties of this site by using both scientific and housekeeping data during the descent sequence of the first touchdown. Close-up images revealed the first touchdown site as a regolith field densely filled with size-sorted, millimeter- to centimeter-sized grains.

Vortices in Protoplanetary Disks

We use a high-order accuracy spectral code to carry out two-dimensional, time-dependent numerical simulations of vortices in accretion disks. In particular, we examine the stability and the lifetime of vortices in circumstellar disks around young stellar objects. The results show that cyclonic vortices dissipate quickly, while anticyclonic vortices can survive in the flow for hundreds of orbits. When more than one vortex is present, the anticyclonic vortices interact through vorticity waves and merge together to form larger vortices. The exponential decay time τ of anticyclonic vortices is of the order of 30-60 orbital periods for a viscosity parameter α ≈ 10-4 (and it increases to τ ≈ 315 for α = 10-5), which is sufficiently long to allow heavy dust particles to rapidly concentrate in the core of anticyclonic vortices in protoplanetary disks. This dust concentration increases the local density of centimeter-sized grains, thereby favoring the formation of larger scale objects that are then capable of efficiently triggering a state of gravitational instability. The relatively long-lived vortices discussed here may therefore play a key role in the formation process of giant planets.

Growth and sedimentation of dust grains in the primordial solar nebula

Of the formation processes in the solar system, the process of growth and sedimentation of dust grains in the primordial solar nebula is investigated for a region near the Earth's orbit. The growth equation for dust grains, which are sinking as well as being in thermal motion, is solved numerically in the wide mass range between 10 −12 and 10 6 g. Any turbulent motions in the nebula are assumed to have already decayed when the sedimentation begins. The numerical simulation shows that the growth and sedimentation proceed faster than was found by Kusaka et al. (1970) but in accordance with the estimate of Safronov (1969) owing to a cooperative interaction of the growth and the sedimentation; that is, at about 3 × 10 3 years after the beginning of the growth and sedimentation a dust layer, composed of centimeter-sized grains, is formed at the equator of the solar nebula. Furthermore, the mass density of dust grains floating in the outer layers of the nebula is found to be of the order of 10 −5 after 10 5 years compared with that before the sedimentation. From these results, it can be estimated that at about 5 × 10 3 years after the beginning of sedimentation the dust layer breaks up owing to the onset of gravitational instability.