Close-in Region Research Articles

Formation of close-in super-Earths in evolving protoplanetary disks due to disk winds

Context. Planets with masses larger than about 0.1 M⊕ undergo rapid inward migration (type I migration) in a standard protoplanetary disk. Recent magnetohydrodynamical simulations revealed the presence of magnetically driven disk winds, which would alter the disk profile and the type I migration in the close-in region. Aims. We investigate orbital evolution of planetary embryos in disks that viscously evolve under the effects of disk winds. The aim is to discuss effects of altered disk profiles on type I migration. In addition, we aim to examine whether observed distributions of close-in super-Earths can be reproduced by simulations that include effects of disk winds. Methods. We perform N-body simulations of super-Earth formation from planetary embryos, in which a recent model for disk evolution is used. We explore a wide range of parameters and draw general trends. We also carry out N-body simulations of close-in super-Earth formation from embryos in such disks under various conditions. Results. We find that the type I migration is significantly suppressed in many cases. Even in cases in which inward migration occurs, the migration timescale is lengthened to 1 Myr, which mitigates the type I migration problem. This is because the gas surface density is decreased and has a flatter profile in the close-in region due to disk winds. We find that when the type I migration is significantly suppressed, planets undergo late orbital instability during the gas depletion, leading to a non-resonant configuration. We also find that observed distributions of close-in super-Earths (e.g., period ratio, mass ratio) can be reproduced. In addition, we show that in some results of simulations, systems with a chain of resonant planets, like the TRAPPIST-1 system, form.

Formation of the terrestrial planets in the solar system around 1 au via radial concentration of planetesimals

Context. No planets exist inside the orbit of Mercury and the terrestrial planets of the solar system exhibit a localized configuration. According to thermal structure calculation of protoplanetary disks, a silicate condensation line (~1300 K) is located around 0.1 au from the Sun except for the early phase of disk evolution, and planetesimals could have formed inside the orbit of Mercury. A recent study of disk evolution that includes magnetically driven disk winds showed that the gas disk obtains a positive surface density slope inside ~1 au from the central star. In a region with positive midplane pressure gradient, planetesimals undergo outward radial drift. Aims. We investigate the radial drift of planetesimals and type I migration of planetary embryos in a disk that viscously evolves with magnetically driven disk winds. We show a case in which no planets remain in the close-in region. Methods. Radial drifts of planetesimals are simulated using a recent disk evolution model that includes effects of disk winds. The late stage of planet formation is also examined by performing N-body simulations of planetary embryos. Results. We demonstrate that in the middle stage of disk evolution, planetesimals can undergo convergent radial drift in a magnetorotational instability (MRI)-inactive disk, in which the pressure maximum is created, and accumulate in a narrow ring-like region with an inner edge at ~0.7 au from the Sun. We also show that planetary embryos that may grow from the narrow planetesimal ring do not exhibit significant type I migration in the late stage of disk evolution. Conclusions. The origin of the localized configuration of the terrestrial planets of the solar system, in particular the deficit of close-in planets, can be explained by the convergent radial drift of planetesimals in disks with a positive pressure gradient in the close-in region.

Improving the Side-mode Suppression Ratio and Reducing the Frequency Drift in an Opto-Electronic Oscillator With a Feedback Control Loop and Additional Phase Modulation

We present two stabilization methods for an opto-electronic oscillator. By using a frequency discriminator with a feedback control loop the long-term stabilization was improved, and a frequency drift of 0.05 ppm/K was achieved without any fiber thermal stabilization. In addition, the phase-noise degradation as a result of the discriminator control loop was investigated. To the best of our knowledge, this has not been done before for this type of method. Our study revealed that a properly designed control loop does not increase the phase noise. The second stabilization method was used to increase the side-mode suppression ratio. This was achieved with an additional control loop in the form of an oscillator-loop phase modulation. In this way, a 5-dB increase in the side-mode suppression ratio was recorded. The phase-noise degradation was less than 1 dB in the close-in region. The two methods were designed to operate simultaneously. This design provides both simplicity and cost effectiveness for the stabilized opto-electronic oscillator.

Suppression of type I migration by disk winds

Planets less massive than Saturn tend to rapidly migrate inward in protoplanetary disks. This is the so-called type I migration. Simulations attempting to reproduce the observed properties of exoplanets show that type I migration needs to be significantly reduced over a wide region of the disk for a long time. However, the mechanism capable of suppressing type I migration over a wide region has remained elusive. The recently found turbulence-driven disk winds offer new possibilities. We investigate the effects of disk winds on the disk profile and type I migration for a range of parameters that describe the strength of disk winds. We also examine the in situ formation of close-in super-Earths in disks that evolve through disk winds. The disk profile, which is regulated by viscous diffusion and disk winds, was derived by solving the diffusion equation. We carried out a number of simulations and plot here migration maps that indicate the type I migration rate. We also performed N-body simulations of the formation of close-in super-Earths from a population of planetesimals and planetary embryos. We define a key parameter, Kw, which determines the ratio of strengths between the viscous diffusion and disk winds. For a wide range of Kw, the type I migration rate is presented in migration maps. These maps show that type I migration is suppressed over the whole close-in region when the effects of disk winds are relatively strong (Kw < 100). From the results of N-body simulations, we see that type I migration is significantly slowed down assuming Kw = 40. We also show that the results of N-body simulations match statistical orbital distributions of close-in super-Earths.

N-BODY SIMULATIONS OF TERRESTRIAL PLANET FORMATION UNDER THE INFLUENCE OF A HOT JUPITER

We investigate the formation of multiple-planet systems in the presence of a hot Jupiter using extended N-body simulations that are performed simultaneously with semi-analytic calculations. Our primary aims are to describe the planet formation process starting from planetesimals using high-resolution simulations, and to examine the dependences of the architecture of planetary systems on input parameters (e.g., disk mass, disk viscosity). We observe that protoplanets that arise from oligarchic growth and undergo type I migration stop migrating when they join a chain of resonant planets outside the orbit of a hot Jupiter. The formation of a resonant chain is almost independent of our model parameters, and is thus a robust process. At the end of our simulations, several terrestrial planets remain at around 0.1 AU. The formed planets are not equal-mass; the largest planet constitutes more than 50 percent of the total mass in the close-in region, which is also less dependent on parameters. In the previous work of this paper (Ogihara et al. 2013), we have found a new physical mechanism of induced migration of the hot Jupiter, which is called a crowding-out. If the hot Jupiter opens up a wide gap in the disk (e.g., owing to low disk viscosity), crowding-out becomes less efficient and the hot Jupiter remains. We also discuss angular momentum transfer between the planets and disk.

Spherical shock development near laser-heated microshell targets

Spherical expansion of the luminous front around a laser-heated microshell target in 20 Torr of helium is compared with theory in the mass-dependent region. Microshell targets from 74- to 7970-ng initial mass irradiated with up to 0.13 TW (Terawatt) of 1.06-μm light were studied. Effects of the initial mass and changing γ on the front history are discussed. It is concluded that extraction of expansion energy from the front motion is not easily accomplished in this close-in region.

The Effect of Nuclear Stimulation on Formation Permeability and Gas Recovery At Project Gasbuggy

At Project Gasbuggy, permeability was affected for some 220 feet beyond the chimney. However, irrespective of the permeability of the close-in region and of chimney size, gas eventually must flow into the chimney from the outer reaches of the drainage area of the well. Thus the preshot formation permeability is a highly critical factor in calculating anticipated recovery from nuclear stimulated reservoirs. Introduction Project Gasbuggy is a joint experiment by the Atomic Project Gasbuggy is a joint experiment by the Atomic Energy Commission, the Dept. of the Interior, and the El Paso Natural Gas Co. to investigate the feasibility of using underground nuclear explosives to stimulate production and increase ultimate recovery of natural gas from a low-permeability gas-bearing formation. The low-permeability Pictured Cliffs formation found in the San Juan basin of the Four Corners area of New Mexico, Arizona, Utah, and Colorado was selected for the experiment. The actual test site is about 55 miles east of Farmington, N. M., in the southwest quarter of Sec. 36, T29N, R4W, Rio Arriba County (Fig. 1). To supplement reservoir and production data available from the eight original field wells in the Gasbuggy test area, two preshot test wells, GB-1 and GB-2 (Fig. 2), were drilled in 1967. Extensive production tests were conducted on GB-1. The Project production tests were conducted on GB-1. The Project Gasbuggy nuclear explosive of 29-kiloton yield was detonated on Dec. 10, 1967, at a depth of 4,240 ft, approximately 40 ft below the base of the Pictured Cliffs formation (Fig. 3). Postshot re-entry of the emplacement hole, designated as GB-ER, began on Dec. 12, 1967, and terminated at a total depth of 3,916 ft on Jan. 10, 1968. Initial computations indicated a collapsed chimney about 160 ft in diameter extending from the 4,240-ft detonation depth to about 3,900 ft, the top of the 300-ft-thick Pictured Cliffs gas sand. In addition to GB-ER, preshot well GB-2 was reentered as GB-2RS, and a new well, GB-3, was drilled in 1969 at a surface distance of 250 ft from GB-ER. GB-ER has undergone extensive testing covering a period of 3 years. During this period, pressures were monitored in GB-2RS and GB-3. Presented in this paper is an analysis of the effectiveness of nuclear stimulation on increasing the formation permeability and the resulting gas recovery at Project Gasbuggy as determined from a flow model. Effect on Reservoir Rock Studies of a number of contained underground nuclear explosions have yielded a rather consistent representation of the geometric features produced. The items of primary interest related to the breakage and displacement of rock resulting from a nuclear stimulation include (1) chimney configuration, (2) chimney void volume, (3) chimney permeability, and (4) permeability of the rock outside the chimney. Boardman has discussed these as a function of explosive yield, depth of burst, and rock type. The chimney configuration and chimney void volume as reported by Boardman and by Atkinson et al. are in close agreement. As to the permeability in the chimney and the permeability outside the chimney, Boardman reported no quantitative answer. For the nuclear stimulation of hydrocarbon reservoirs, as for other stimulation processes, the quantitative value of the permeability increase outside the chimney is vital to the prediction of the ultimate recovery of hydrocarbons. JPT P. 1199

New Theoretical Approach to Nuclear Heavy-Ion Scattering

Some recent calculations based on the statistical model have employed the adiabatic assumption inconsistently. In particular, the high central repulsive core in ion-ion collisions is due to a trial density function which leads to unphysically large densities in the close-in region.

Project Sterling: Subsurface phenomenology measurements near a decoupled nuclear event

The purpose of the Sterling event, a 380-ton nuclear explosion detonated in a spherical cavity, was to test the decoupling theory for nuclear explosives detonated in explosively generated cavities. Measurements of stress history, cavity pressure, and cavity collapse were attempted in the very close-in region between 17 and 52 meters from the cavity center; free-field measurements of particle velocity and acceleration were made in the immediate region between 52 and 660 meters as measured from the cavity center. Peak accelerations measured over this interval ranged from 150 to 0.2 g; peak velocities ranged from 0.13 to 0.01 m/sec. The assumption of cube-root scaling was used in the analysis of the reduced displacement potentials and their Fourier transforms to obtain a free-field decoupling ratio of about 100 for a frequency range near 10 Hz. Above 10 Hz, the decoupling ratio drops rapidly to about 20. These results are in reasonably good agreement with the decoupling theory.

Particle-in-Cell Method in Shock Propagation Problems

The paper considers a particle-in-cell approach for the analysis of high velocity impact problems in which large motions preclude the practical use of either a purely Lagrangian or purely Eulerian computational scheme for integrating the equations of motion. An application is made to the analysis of the extreme close-in region of a nuclear explosion on a rocklike medium. Numerical results for the pressure distribution in the medium and particle position plots are presented for a particular case which had been considered previously by other investigators. A comparison of the pressure curves is given.

Fine Structure of theKX-Ray Absorption Edge of Germanium

A double-crystal spectrometer, with a proportional counting system for intensity measurement, has been employed for the investigation of the x-ray absorption structure on the high-energy side of the Ge $K$ edge, using 38% polarized x rays and a thin single crystal of germanium in the transmission method. The structure was studied up to 185 ev from the main edge and several new absorption structure features, not reported hitherto, were obtained. These features are satisfactorily explained by Hayasi's theory in the close-in region and by Kronig's theory in the extended region. A significant shift, without any intensity variation, was noted in the extended fine structure on changing the orientation of the single-crystal absorber. A quantitative correlation has been made between the observed structures and the theoretically predicted values with a view to testing different theories on absorption fine structure.