As planets form and grow within gaseous protoplanetary disks, the mutual gravitational interaction between the disk and planet leads to the exchange of angular momentum and migration of the planet. ...We review current understanding of disk-planet interactions, focusing in particular on physical processes that determine the speed and direction of migration. We describe the evolution of low-mass planets embedded in protoplanetary disks and examine the influence of Lindblad and corotation torques as a function of the disk properties. The role of the disk in causing the evolution of eccentricities and inclinations is also discussed. We describe the rapid migration of intermediate-mass planets that may occur as a runaway process and examine the transition to gap formation and slower migration driven by the viscous evolution of the disk for massive planets. The roles and influence of disk self-gravity and magnetohydrodynamic turbulence are discussed in detail, as a function of the planet mass, as is the evolution of multiple planet systems. Finally, we address the question of how well global models of planetary formation that include migration are able to match observations of extrasolar planets.
Context. Theory predicts that low-mass protoplanets in a protostellar disc migrate into the central star on a time scale that is short compared with the disc lifetime or the giant planet formation ...time scale. Protoplanet eccentricities of $e\ga H/r$ can slow or reverse migration, but previous 2D studies of multiple protoplanets embedded in a protoplanetary disc have shown that gravitational scattering cannot maintain significant planet eccentricities against disc-induced damping. The eventual fate of these systems was migration into the central star. Aims. Here we simulate the evolution of low-mass protoplanetary swarms in three dimensions. The aim is to examine both protoplanet survival rates and the dynamical structure of the resulting planetary systems, and to compare them with 2D simulations. Methods. We present results from a 3D hydrodynamic simulation of eight protoplanets embedded in a protoplanetary disc. We also present a suite of simulations performed using an N-body code, modified to include prescriptions for planetary migration and for eccentricity and inclination damping. These prescriptions were obtained by fitting analytic formulae to hydrodynamic simulations of planets embedded in discs with initially eccentric and/or inclined orbits. Results. As was found in two dimensions, differential migration produces groups of protoplanets in stable, multiple mean-motion resonances that migrate in lockstep, preventing prolonged periods of gravitational scattering. In almost all simulations, this leads to large-scale migration of the protoplanet swarm into the central star in the absence of a viable stopping mechanism. The evolution involves mutual collisions, occasional instances of large-scale scattering, and the frequent formation of the long-lived, co-orbital planet systems that arise in >30% of all runs. Conclusions. Disc-induced damping overwhelms eccentricity and inclination growth due to planet-planet interactions, leading to large-scale migration of protoplanet swarms. Co-orbital planets are a natural outcome of dynamical relaxation in a strongly dissipative environment, and if observed in nature would imply that such a period of evolution commonly arises during planetary formation.
We make publicly available a catalog of calibrated environmental measures for galaxies in the five 3D-Hubble Space Telescope (HST)/CANDELS deep fields. Leveraging the spectroscopic and grism ...redshifts from the 3D-HST survey, multiwavelength photometry from CANDELS, and wider field public data for edge corrections, we derive densities in fixed apertures to characterize the environment of galaxies brighter than mag in the redshift range . By linking observed galaxies to a mock sample, selected to reproduce the 3D-HST sample selection and redshift accuracy, each 3D-HST galaxy is assigned a probability density function of the host halo mass, and a probability that it is a central or a satellite galaxy. The same procedure is applied to a z = 0 sample selected from Sloan Digital Sky Survey. We compute the fraction of passive central and satellite galaxies as a function of stellar and halo mass, and redshift, and then derive the fraction of galaxies that were quenched by environment specific processes. Using the mock sample, we estimate that the timescale for satellite quenching is it is longer at lower stellar mass or lower redshift, but remarkably independent of halo mass. This indicates that, in the range of environments commonly found within the 3D-HST sample ( ), satellites are quenched by exhaustion of their gas reservoir in the absence of cosmological accretion. We find that the quenching times can be separated into a delay phase, during which satellite galaxies behave similarly to centrals at fixed stellar mass, and a phase where the star formation rate drops rapidly ( Gyr), as shown previously at z = 0. We conclude that this scenario requires satellite galaxies to retain a large reservoir of multi-phase gas upon accretion, even at high redshift, and that this gas sustains star formation for the long quenching times observed.
Context. Several circumbinary planets have been detected by the Kepler mission. Recent work has emphasized the difficulty of forming these planets at their observed locations due to perturbations by ...the binary. It has been suggested that these planets formed further out in their discs in more quiescent environments and migrated in to locations where they are observed. Aims. We examine the orbital evolution of planets embedded in circumbinary disc models for the three systems Kepler-16, Kepler-34 and Kepler-35. The aims are: to explore the plausibility of a formation scenario in which cores form at large distances from the binaries and undergo inward migration and gas accretion as the gas disc disperses; to determine which sets of disc parameters lead to planets whose final orbits provide reasonable fits to the observed systems. Methods. Using a grid-based hydrodynamics code we performed simulations of a close binary system interacting with circumbinary discs with differing aspect ratios, h, and viscous stress parameters α. Once the binary+disc system reaches quasi-equilibrium we embed a planet in the disc and examine its evolution under the action of binary and disc forces. We consider fully-formed planets with masses equal to those inferred from Kepler data, and low-mass cores that migrate and accrete gas while the gas disc is being dispersed. Results. A typical outcome for all systems is stalling of inward migration as the planet enters the tidally-truncated inner cavity formed by the binary system. The circumbinary disc becomes eccentric through interaction with the binary, and the disc eccentricity forces the planet into a non-circular orbit. For each of the Kepler-16b, Kepler-34b and Kepler-35b systems we obtain planets whose parameters agree reasonably well with the observational data, but none of our simulations are able to produce highly accurate fits for all orbital parameters. Conclusions. The final orbital configuration of a circumbinary planet is determined by a delicate interplay between the detailed stucture of the circumbinary disc and the orbital parameters of the planet as it migrates into the inner disc cavity. Simplified simulations such as those presented here provide support for a formation scenario in which a core forms, migrates inward and accretes gas, but accurate fitting of the observed Kepler systems is likely to require disc models that are significantly more sophisticated in terms of their input physics.
Gas-giant planets, like Jupiter and Saturn, acquire massive gaseous envelopes during the approximately 3 Myr-long lifetimes of protoplanetary discs. In the core accretion scenario, the formation of a ...solid core of around ten Earth masses triggers a phase of rapid gas accretion. Previous 3D grid-based hydrodynamical simulations found that runaway gas accretion rates correspond to approximately 10 to 100 Jupiter masses per Myr. Such high accretion rates would result in all planets with larger than ten Earth-mass cores to form Jupiter-like planets, which is in clear contrast to the ice giants in the Solar System and the observed exoplanet population. In this work, we used 3D hydrodynamical simulations, that include radiative transfer, to model the growth of the envelope on planets with different masses. We find that gas flows rapidly through the outer part of the envelope, but this flow does not drive accretion. Instead, gas accretion is the result of quasi-static contraction of the inner envelope, which can be orders of magnitude smaller than the mass flow through the outer atmosphere. For planets smaller than Saturn, we measured moderate gas accretion rates that are below one Jupiter mass per Myr. Higher mass planets, however, accrete up to ten times faster and do not reveal a self-driven mechanism that can halt gas accretion. Therefore, the reason for the final masses of Saturn and Jupiter remains difficult to understand, unless their completion coincided with the dissipation of the solar nebula.
Over the past few years, MRI has become an indispensable tool for diagnosing multiple sclerosis (MS). However, the current MRI criteria for MS diagnosis have imperfect sensitivity and specificity. ...The central vein sign (CVS) has recently been proposed as a novel MRI biomarker to improve the accuracy and speed of MS diagnosis. Evidence indicates that the presence of the CVS in individual lesions can accurately differentiate MS from other diseases that mimic this condition. However, the predictive value of the CVS for the development of clinical MS in patients with suspected demyelinating disease is still unknown. Moreover, the lack of standardization for the definition and imaging of the CVS currently limits its clinical implementation and validation. On the basis of a thorough review of the existing literature on the CVS and the consensus opinion of the members of the North American Imaging in Multiple Sclerosis (NAIMS) Cooperative, this article provides statements and recommendations aimed at helping radiologists and neurologists to better understand, refine, standardize and evaluate the CVS in the diagnosis of MS.
A nearby multiplanet system
Exoplanets can interact gravitationally with other objects orbiting the same star, affecting their evolution and stability. Studying these effects requires locating ...systems with multiple planets. Monitoring the nearby red dwarf star GJ 887, Jeffers
et al.
detected periodic radial velocity signals, indicating the presence of two planets on orbits with periods of about 9 and 22 days and a further candidate planet (see the Perspective by Davies). The inclinations of the orbits are unknown, so only minimum masses could be determined, but those were consistent with both planets being super-Earths—more massive than Earth but less than Neptune. This system is only 3.3 parsecs from the Sun, which should facilitate follow-up with other techniques.
Science
, this issue p.
1477
; see also p.
1432
Two super-Earth exoplanets orbit GJ 887, a red dwarf star located only 3.3 parsecs from the Sun.
The closet exoplanets to the Sun provide opportunities for detailed characterization of planets outside the Solar System. We report the discovery, using radial velocity measurements, of a compact multiplanet system of super-Earth exoplanets orbiting the nearby red dwarf star GJ 887. The two planets have orbital periods of 9.3 and 21.8 days. Assuming an Earth-like albedo, the equilibrium temperature of the 21.8-day planet is ~350 kelvin. The planets are interior to, but close to the inner edge of, the liquid-water habitable zone. We also detect an unconfirmed signal with a period of ~50 days, which could correspond to a third super-Earth in a more temperate orbit. Our observations show that GJ 887 has photometric variability below 500 parts per million, which is unusually quiet for a red dwarf.
Incompatible live donor kidney transplantation (ILDKT) offers a survival advantage over dialysis to patients with anti‐HLA donor‐specific antibody (DSA). Program‐specific reports (PSRs) fail to ...account for ILDKT, placing this practice at regulatory risk. We collected DSA data, categorized as positive Luminex, negative flow crossmatch (PLNF) (n = 185), positive flow, negative cytotoxic crossmatch (PFNC) (n = 536) or positive cytotoxic crossmatch (PCC) (n = 304), from 22 centers. We tested associations between DSA, graft loss and mortality after adjusting for PSR model factors, using 9669 compatible patients as a comparison. PLNF patients had similar graft loss; however, PFNC (adjusted hazard ratio aHR = 1.64, 95% confidence interval CI: 1.15–2.23, p = 0.007) and PCC (aHR = 5.01, 95% CI: 3.71–6.77, p < 0.001) were associated with increased graft loss in the first year. PLNF patients had similar mortality; however, PFNC (aHR = 2.04; 95% CI: 1.28–3.26; p = 0.003) and PCC (aHR = 4.59; 95% CI: 2.98–7.07; p < 0.001) were associated with increased mortality. We simulated Centers for Medicare & Medicaid Services flagging to examine ILDKT's effect on the risk of being flagged. Compared to equal‐quality centers performing no ILDKT, centers performing 5%, 10% or 20% PFNC had a 1.19‐, 1.33‐ and 1.73‐fold higher odds of being flagged. Centers performing 5%, 10% or 20% PCC had a 2.22‐, 4.09‐ and 10.72‐fold higher odds. Failure to account for ILDKT's increased risk places centers providing this life‐saving treatment in jeopardy of regulatory intervention.
In this 22‐center study of HLA‐incompatible live donor kidney transplants (ILDKT), the authors demonstrate the increased risk of graft loss and death associated with increasing anti‐HLA donor‐specific antibody strength, and they quantify the significantly increased risk of flagging for regulatory scrutiny by the Centers for Medicare & Medicaid Studies that is incurred by centers that perform ILDKT. See editorial by Cole and Tinckam on page 1475.
SN 2014J in M82 is the closest detected Type Ia supernova (SN Ia) in at least 28 yr and perhaps in 410 yr. Despite its small distance of 3.3 Mpc, SN 2014J is surprisingly faint, peaking at ...V = 10.6 mag, and assuming a typical SN Ia luminosity, we infer an observed visual extinction of A
V
= 2.0 ± 0.1 mag. But this picture, with R
V
= 1.6 ± 0.2, is too simple to account for all observations. We combine 10 epochs (spanning a month) of HST/Space Telescope Imaging Spectrograph (STIS) ultraviolet through near-infrared spectroscopy with HST/Wide Field Camera 3 (WFC3), Katzman Automatic Imaging Telescope, and FanCam photometry from the optical to the infrared and nine epochs of high-resolution TRES (Tillinghast Reflection Echelle Spectrograph) spectroscopy to investigate the sources of extinction and reddening for SN 2014J. We argue that the wide range of observed properties for SN 2014J is caused by a combination of dust reddening, likely originating in the interstellar medium of M82, and scattering off circumstellar material. For this model, roughly half of the extinction is caused by reddening from typical dust (E(B − V) = 0.45 mag and R
V
= 2.6) and roughly half by scattering off Large Magellanic Cloud-like dust in the circumstellar environment of SN 2014J.
Context.Theory predicts that low mass protoplanets in a laminar protostellar disc will migrate into the central star prior to disc dispersal. It is known that protoplanets on orbits with eccentricity ...$e \ga H/r$, where H is the disc scale height and r is the radius, can halt or reverse their migration. Aims.We examine whether a system of interacting protoplanetary cores can excite and sustain significant eccentricity of the population, allowing some planetary cores to survive in the disc over its lifetime. Methods.We employ two distinct numerical schemes: an N-body code, adapted to include migration and eccentricity damping due to the gas disc via analytic prescriptions, and a hydrodynamics code that explicitly evolves a 2D protoplanetary disc model with embedded protoplanets. The former allows us to study the long term evolution, the latter to model the systems with greater fidelity but for shorter times. Results.After a brief period of chaotic interaction between the protoplanets that involves scattering, orbital exchange, collisions and the formation of co-orbital planets, we find that the system settles into a quiescent state of inward migration. Differential migration causes the protoplanets to form a series of mean motion resonances, such that a planet is often in resonance with both its interior and exterior neighbours. This helps prevent close encounters and leads to the protoplanetary swarm, or subgroups within it, migrating inward at a uniform rate. In about $2 \%$ of runs a single planet is scattered onto a distant orbit with significant eccentricity, allowing it to survive in the disc for ${\sim} 10^6$ years. Over $20 \%$ of runs produce co-orbital planets that survive for the duration of the simulation, occupying mutual horseshoe or tadpole orbits. Conclusions.Disc-induced damping overwhelms eccentricity growth through planet-planet interactions, such that a protoplanetary swarm migrates inward. We suggest co-orbital planets may be observed in future exoplanet searches.
PDF
