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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The isotopic composition of Ag in meteorites and the presence of Pd-107 in protoplanets

Results are presented on the isotopic composition of Ag and the concentrations of Pd and Ag in metal and sulfide phases in iron meteorites Gibeon, Derrick Peak, and Mundrabilla and in schereibersite in Derrick Peak. It was found that almost all iron meteorite samples with a ratio of Pd-108/Ag-109 greater than about 400 had an excess of Ag-107. The results, in conjunction with the data of Chen and Wasserburg (1983) on IIIA-IIIB meteorites, demonstrate the widespread occurrence of excess Ag-107 in diverse types of small early planetary bodies. The excess Ag-107 is believed to be produced by the decay of Pd-107.

Chen, J. H.↗
NASA NTRS1998

A Candidate Protoplanet in the Taurus Star Forming Region

A long narrow filament seems to connect the protobinary to the faint companion TMR-1C, suggesting a physical association. If the sources are physically related then we hypothesize that TMR-1C has been ejected by the protobinary.

HST/NICMOS faint companion protostar filament prot↗
NASA NTRS2018

Dawn Mission's Search for Satellites of Ceres: Intact Protoplanets Don't Have Satellites

Upon its approach to orbit the dwarf planet Ceres in early 2015, optical navigation and dedicated satellite search images were acquired with the Dawn mission's framing camera 2. A team of searchers individually processed and examined the images for evidence of objects moving with Ceres. Completeness of search with respect to the space searched was calculated as a function of distance to Ceres and found to be complete down to 15 Ceres radii (Ceres' mean radius is 470 kilometers). Upper limits of detectable magnitude were determined for each observed set of images and an upper limit in size was calculated assuming for the putative objects, Ceres' geometric albedo of 0.11. Nothing was found associated with Ceres down to a radius of 12 meters for the most sensitive search, and down to a radius of 323m for the least sensitive search circumstances. Examination of the physical properties of the 41 largest and most massive main belt asteroids suggests that large asteroids without satellites are intact and their interiors have internal strength. This is consistent with results from the Dawn mission at both Vesta and Ceres. Ceres' volatile-rich composition also is a likely contributor to both the absence of satellites at Ceres and of Ceres meteorites at Earth. These results suggest that collisional disruption creating rubble pile structure is a necessary condition for formation of satellites around main belt asteroids.

McFadden, Lucy A.↗
DOE OSTI2021

Accretion of Gas Giants Constrained by the Tidal Barrier

After protoplanets have acquired sufficient mass to open partial gaps in their natal protostellar disks, residual gas continues to diffuse onto horseshoe streamlines under the effect of viscous dissipation, and to meander in and out of the planets' Hill sphere. Within the Hill sphere, the horseshoe streamlines intercept gas flow in circumplanetary disks. The host star's tidal perturbation induces a barrier across the converging streamlines' interface. Viscous transfer of angular momentum across this tidal barrier determines the rate of mass diffusion from the horseshoe streamlines onto the circumplanetary disks, and eventually the accretion rate onto the protoplanets. Here, we carry out a series of numerical simulations to test the influence of this tidal barrier on superthermal planets. In weakly viscous disks, the protoplanets' accretion rate steeply decreases with their masses above the thermal limit. As their growth timescale exceeds the gas depletion timescale, their masses reach asymptotic values comparable to that of Jupiter. In relatively thick and strongly viscous disks, protoplanets' asymptotic masses exceed several times that of Jupiter. Two-dimensional numerical simulations show that such massive protoplanets strongly excite the eccentricity of nearby horseshoe streamlines, destabilize orderly flow, substantially enhance the diffusion rate across the tidal barrier, and elevate their growth rate until their natal disk is severely depleted. In contrast, eccentric streamlines remain stable in three-dimensional simulations. Based on the upper falloff in the observed mass distribution of known exoplanets, we suggest that their natal disks had relatively low viscosity (α ~ 10 –3 ) and modest thickness (H/R ~ 0.03–0.05).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

ALMA HINTS AT THE PRESENCE OF TWO COMPANIONS IN THE DISK AROUND HD 100546

HD 100546 is a well-studied Herbig Be star-disk system that likely hosts a close-in companion with compelling observational evidence for an embedded protoplanet at 68 AU. We present Atacama Large Millimeter/Submillimeter Array observations of the HD 100546 disk which resolve the gas and dust structure at (sub)millimeter wavelengths. The CO emission (at 345.795 GHz) originates from an extensive molecular disk (390 ± 20 AU in radius) whereas the continuum emission is more compact (230 ± 20 AU in radius), suggesting radial drift of the millimeter-sized grains. The CO emission is similar in extent to scattered light images indicating well-mixed gas and micrometer-sized grains in the disk atmosphere. Assuming azimuthal symmetry, a single-component power-law model cannot reproduce the continuum visibilities. The visibilities and images are better reproduced by a double-component model: a compact ring with a width of 21 AU centered at 26 AU and an outer ring with a width of 75 ± 3 AU centered at 190 ± 3 AU. The influence of a companion and protoplanet on the dust evolution is investigated. The companion at 10 AU facilitates the accumulation of millimeter-sized grains within a compact ring, ≈20-30 AU, by ≈10 Myr. The injection of a protoplanet at 1 Myr hastens the ring formation (≈1.2 Myr) and also triggers the development of an outer ring (≈100-200 AU). These observations provide additional evidence for the presence of a close-in companion and hint at dynamical clearing by a protoplanet in the outer disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

RESOLVING THE HD 100546 PROTOPLANETARY SYSTEM WITH THE GEMINI PLANET IMAGER: EVIDENCE FOR MULTIPLE FORMING, ACCRETING PLANETS

We report Gemini Planet Imager H-band high-contrast imaging/integral field spectroscopy and polarimetry of the HD 100546, a 10 Myr old early-type star recently confirmed to host a thermal infrared (IR) bright (super-)Jovian protoplanet at wide separation, HD 100546 b. We resolve the inner disk cavity in polarized light, recover the thermal IR-bright arm, and identify one additional spiral arm. We easily recover HD 100546 b and show that much of its emission plausibly originates from an unresolved point source. The point-source component of HD 100546 b has extremely red IR colors compared to field brown dwarfs, qualitatively similar to young cloudy super-Jovian planets; however, these colors may instead indicate that HD 100546 b is still accreting material from a circumplanetary disk. Additionally, we identify a second point-source-like peak at r{sub proj} ∼ 14 AU, located just interior to or at the inner disk wall consistent with being a <10–20 M{sub J} candidate second protoplanet—“HD 100546 c”—and lying within a weakly polarized region of the disk but along an extension of the thermal IR-bright spiral arm. Alternatively, it is equally plausible that this feature is a weakly polarized but locally bright region of the inner disk wall. Astrometric monitoring of this feature over the next 2 years and emission line measurements could confirm its status as a protoplanet, rotating disk hot spot that is possibly a signpost of a protoplanet, or a stationary emission source from within the disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

DM ORI: A YOUNG STAR OCCULTED BY A DISTURBANCE IN ITS PROTOPLANETARY DISK

In some planet formation theories, protoplanets grow gravitationally within a young star’s protoplanetary disk, a signature of which may be a localized disturbance in the disk’s radial and/or vertical structure. Using time-series photometric observations by the Kilodegree Extremely Little Telescope South project and the All-Sky Automated Survey for SuperNovae, combined with archival observations, we present the discovery of two extended dimming events of the young star, DM Ori. This young system faded by ∼1.5 mag from 2000 March to 2002 August and then again in 2013 January until 2014 September (depth ∼1.7 mag). We constrain the duration of the 2000–2002 dimming to be < 860 days, and the event in 2013–2014 to be < 585 days, separated by ∼12.5 years. A model of the spectral energy distribution indicates a large infrared excess consistent with an extensive circumstellar disk. Using basic kinematic arguments, we propose that DM Ori is likely being periodically occulted by a feature (possibly a warp or perturbation) in its circumstellar disk. In this scenario, the occulting feature is located >6 au from the host star, moving at ∼14.6 km s{sup −1} and is ∼4.9 au in width. This localized structure may indicate a disturbance such as that which may be caused by a protoplanet early in its formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Constraining Accretion Signatures of Exoplanets in the TW Hya Transitional Disk

We present a near-infrared direct imaging search for accretion signatures of possible protoplanets around the young stellar object (YSO) TW Hya, a multi-ring disk exhibiting evidence of planet formation. The Paβ line (1.282 μm) is an indication of accretion onto a protoplanet, and its intensity is much higher than that of blackbody radiation from the protoplanet. We focused on the Paβ line and performed Keck/OSIRIS spectroscopic observations. Although spectral differential imaging (SDI) reduction detected no accretion signatures, the results of the present study allowed us to set 5σ detection limits for Paβ emission of 5.8 × 10{sup −18} and 1.5 × 10{sup −18} erg{sup −1} s{sup −1} cm{sup −2} at 0.″4 and 1.″6, respectively. We considered the mass of potential planets using theoretical simulations of circumplanetary disks and hydrogen emission. The resulting masses were 1.45 ± 0.04 M {sub J} and 2.29{sub −0.04}{sup +0.03} M{sub J} at 25 and 95 au, respectively, which agree with the detection limits obtained from previous broadband imaging. The detection limits should allow for the identification of protoplanets as small as ∼1 M {sub J}, which may assist in direct imaging searches around faint YSOs for which extreme adaptive optics instruments are unavailable.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The Role of Early Giant-planet Instability in Terrestrial Planet Formation

The terrestrial planets are believed to have formed by violent collisions of tens of lunar- to Mars-size protoplanets at time t < 200 Myr after the protoplanetary gas disk dispersal (t {sub 0}). The solar system giant planets rapidly formed during the protoplanetary disk stage and, after t {sub 0}, radially migrated by interacting with outer disk planetesimals. An early (t < 100 Myr) dynamical instability is thought to have occurred with Jupiter having gravitational encounters with a planetary-size body, jumping inward by ∼0.2–0.5 au, and landing on its current, mildly eccentric orbit. Here we investigate how the giant-planet instability affected the formation of the terrestrial planets. We study several instability cases that were previously shown to match many solar system constraints. We find that resonances with giant planets help to remove solids available for accretion near ∼1.5 au, thus stalling the growth of Mars. It does not matter, however, whether the giant planets are placed on their current orbits at t {sub 0} or whether they realistically evolve in one of our instability models; the results are practically the same. The tight orbital spacing of Venus and Earth is difficult to reproduce in our simulations, including cases where bodies grow from a narrow annulus at 0.7–1 au, because protoplanets tend to spread radially during accretion. The best results are obtained in the narrow-annulus model when protoplanets emerging from the dispersing gas nebula are assumed to have (at least) the mass of Mars. This suggests efficient accretion of the terrestrial protoplanets during the first ∼10 Myr of the solar system.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1991

Collision probabilities in the presence of nebular gas drag

We are developing a model to determine what fraction of the planetesimals would have hit a protoplanet on their sunward journey as opposed to having a close approach and passing into an inferior orbit. The model involves direct numerical integration of restricted-three-body orbits using a predictor-corrector integrator. A simple gas drag law with a v(exp 2) dependence was also included in the equations of motion. Runs of 100 to 500 particles were already performed, while some future runs may require several times this number in order to get good impact statistics. All planetesimals start in superior orbits with semi-major axes 5 to 10 R(sub H) from the protoplanets, where R(sub H) is the protoplanet's Hill Sphere radius. The orbit is followed until the planetesimal passed into an inferior orbit at least 10 R(sub H) from the protoplanet. This process typically requires 10(exp 4) to 10(exp 5) orbits.

Kary, David M.↗
DOE OSTI2015

On shocks driven by high-mass planets in radiatively inefficient disks. I. Two-dimensional global disk simulations

Recent observations of gaps and non-axisymmetric features in the dust distributions of transition disks have been interpreted as evidence of embedded massive protoplanets. However, comparing the predictions of planet–disk interaction models to the observed features has shown far from perfect agreement. This may be due to the strong approximations used for the predictions. For example, spiral arm fitting typically uses results that are based on low-mass planets in an isothermal gas. In this work, we describe two-dimensional, global, hydrodynamical simulations of disks with embedded protoplanets, with and without the assumption of local isothermality, for a range of planet-to-star mass ratios 1–10 M{sub J} for a 1 M{sub ⊙} star. We use the Pencil Code in polar coordinates for our models. We find that the inner and outer spiral wakes of massive protoplanets (M≳5 M{sub J}) produce significant shock heating that can trigger buoyant instabilities. These drive sustained turbulence throughout the disk when they occur. The strength of this effect depends strongly on the mass of the planet and the thermal relaxation timescale; for a 10 M{sub J} planet embedded in a thin, purely adiabatic disk, the spirals, gaps, and vortices typically associated with planet–disk interactions are disrupted. We find that the effect is only weakly dependent on the initial radial temperature profile. The spirals that form in disks heated by the effects we have described may fit the spiral structures observed in transition disks better than the spirals predicted by linear isothermal theory.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

XMM-Newton X-Ray Observations of LkCa 15: A T Tauri Star with a Formative Planetary System

High-resolution ground-based images of the T Tauri star LkCa 15 have revealed multiple companions that are thought to comprise a formative planetary system. The candidate protoplanets orbit at distances of ∼15–20 au within the dust-depleted inner region of the circumstellar disk. Because of its young age (∼1–4 Myr), LkCa 15 provides a benchmark system for testing planet-formation models. We detected LkCa 15 as a bright X-ray source in a short 10 ks Chandra observation in 2009. We report here new results obtained from a deeper 37 ks XMM-Newton observation in 2014. The new data provide better sampling in the time domain and improved sensitivity at low energies below 1 keV. Spectral fits with thermal emission models require at least two temperature components at kT {sub cool} ≈ 0.4 keV and kT {sub hot} ≈ 2.2 keV. The value of kT {sub hot} is about a factor of two less than inferred from Chandra, suggesting that the hot-component temperature is variable. The best-fit absorption column density is in good agreement with that expected from optical extinction estimates A{sub V} ≈ 1.3–1.7 mag. The intrinsic X-ray luminosity is L {sub x}(0.2–10 keV) = 3 × 10{sup 30} erg s{sup −1}. Estimates of the X-ray heating rate of the inner disk and protoplanets are sensitive to the assumed disk gas surface density for which recent ALMA observations give estimates Σ{sub 0,gas} ∼10{sup 2} g cm{sup −2} at 1 au from the star. At such densities, X-ray heating is confined mainly to the upper disk layers and X-ray penetration through the disk midplane to the protoplanets at r ≈ 15–20 au is negligible.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1979

On the origin of asteroids

A general scenario is described for the early history of the solar system. The primitive solar nebula is formed from the infall of gas from a collapsing interstellar cloud fragment. It becomes repeatedly unstable against collapse to form giant gaseous protoplanets. In the course of protoplanet evolution the center of the protoplanet enters a thermodynamic regime in which common rocky minerals become liquids; convection brings solids to the central region where a substantial fraction of them rain out to form a protoplanetary core. In the inner solar system protoplanetary envelopes are tidally stripped away, thus injecting into the solar nebula large equantities of chondrules and inclusions. Late in the development of the solar nebula, after most of the gas has disappeared, turbulence dies out and the small solids settle into a thin layer at midplane of the nebula. Gravitational instabilities in this layer form asteroidal and cometary bodies. Some further consequences of this scenario are discussed.

Cameron, A. G. W.↗
NASA NTRS1991

Bending waves and orbital inclinations

Disk tides may play an important role in the formation of a planetary system. Modification of protoplanet semi-major axes and eccentricities through density waves was suggested and calculations of linear and non-linear protoplanet-nebula interactions were carried out by a number of researchers. The possible significance of radial drift to the accretion process was discussed, while the evolution of orbital eccentricities was studied for a perturber orbiting external to a ring and for a perturber embedded in the ring. The possible importance of bending waves to the early evolution of protoplanet inclinations is described.

Ward, William R.↗
NASA NTRS1993

On the tidal interaction between protostellar disks and companions

Formation of protoplanets and binary stars in a protostellar disk modifies the structure of the disk. Through tidal interactions, energy and angular momentum are transferred between the disk and protostellar or protoplanetary companion. We summarize recent progress in theoretical investigations of the disk-companion tidal interaction. We show that low-mass protoplanets excite density waves at their Lindblad resonances and that these waves are likely to be dissipated locally. When a protoplanet acquires sufficient mass, its tidal torque induces the formation of a gap in the vicinity of its orbit. Gap formation leads to the termination of protoplanetary growth by accretion. For proto-Jupiter to attain its present mass, we require that (1) the primordial solar nebula is heated by viscous dissipation; (2) the viscous evolution time scale of the nebula is comparable to the age of typical T Tauri stars with circumstellar disks; and (3) the mass distribution in the nebula is comparable to that estimated from a minimum-mass nebula model.

Lin, D. N. C.↗
NASA NTRS1993

Orbital Resonances in the Solar Nebula: Strengths and Weaknesses

A planetesimal moving in the Solar Nebula experiences an aerodynamic drag which causes its orbit to circularize and shrink. However, resonant perturbations from a protoplanet interior to the planetesimal's orbit can counteract both the orbital decay and the damping of the eccentricity: the planetesimal can be captured into an orbital resonance and its eccentricity pumped up to a modestly high equilibrium value. Thus, orbital resonances constitute (partial) barriers to the delivery of planetesimals into the feeding zone of the protoplanet. We have established the characteristics of the phenomenon of resonance capture by gas drag in the circular restricted three-body approximation. We have determined the strengths of the equilibrium resonant orbits with respect to impulsive velocity perturbations. We conclude that planetesimals captured in orbital resonances are quite vulnerable to being dislocated from these orbits by mutual planetesimal interactions, but that the resonances are effective in slowing down the rate of orbital decay of planetesimals. Only very small bodies, less than or equal to 100 m, are able to reach a approx. 1 Earth mass protoplanet without being slowed down by resonances.

Malhotra, Renu↗
NASA NTRS1993

Orbital Resonances in the Solar Nebula: Strengths and Weaknesses

A planetesimal moving in the Solar Nebula experiences an aero- dynamic drag which causes its orbit to circularize and shrink. However, resonant perturbations from a protoplanet interior to the planetesimal's orbit ran counteract both the orbital decay and the damping of the eccentricity: the planetesimal can be captured into an orbital resonance and its eccentricity pumped up to a modestly high equilibrium value. Thus, orbital resonances constitute (partial) barriers to the delivery of planetesimals into the feeding zone of the protoplanet. We have established the characteristics of the phenomenon of resonance capture by gas drag in the circular restricted three-body approximation. We have determined the strengths of the equilibrium resonant orbits with respect to impulsive velocity perturbations. We conclude that planetesimals captured in orbital resonances are quite vulnerable to being dislocated from these orbits by mutual planetesimal interactions, but that the resonances are effective in slowing down the rate of orbital decay of planetesimals. Only very small bodies, less or approx. equal to 100 m, are able to reach a approx. 1 mass of the earth protoplanet without being slowed down by resonances.

Malhotra, Renu↗
DOE OSTI2015

WARM DEBRIS DISKS PRODUCED BY GIANT IMPACTS DURING TERRESTRIAL PLANET FORMATION

In our solar system, Mars-sized protoplanets frequently collided with each other during the last stage of terrestrial planet formation, called the giant impact stage. Giant impacts eject a large amount of material from the colliding protoplanets into the terrestrial planet region, which may form debris disks with observable infrared excesses. Indeed, tens of warm debris disks around young solar-type stars have been observed. Here we quantitatively estimate the total mass of ejected materials during the giant impact stages. We found that ∼0.4 times the Earth’s mass is ejected in total throughout the giant impact stage. Ejected materials are ground down by collisional cascade until micron-sized grains are blown out by radiation pressure. The depletion timescale of these ejected materials is determined primarily by the mass of the largest body among them. We conducted high-resolution simulations of giant impacts to accurately obtain the mass of the largest ejected body. We then calculated the evolution of the debris disks produced by a series of giant impacts and depleted by collisional cascades to obtain the infrared excess evolution of the debris disks. We found that the infrared excess is almost always higher than the stellar infrared flux throughout the giant impact stage (∼100 Myr) and is sometimes ∼10 times higher immediately after a giant impact. Therefore, giant impact stages would explain the infrared excess from most observed warm debris disks. The observed fraction of stars with warm debris disks indicates that the formation probability of our solar-system-like terrestrial planets is approximately 10%.

79 ASTRONOMY AND ASTROPHYSICS↗