Modification of Angular Velocity by Inhomogeneous MRI Growth in Protoplanetary Disks

We have investigated evolution of magneto-rotational instability (MRI) in protoplanetary disks that have radially non-uniform magnetic field such that stable and unstable regions coexist initially, and found that a zone in which the disk gas rotates with a super-Keplerian velocity emerges as a result of the non-uniformly growing MRI turbulence. We have carried out two-dimensional resistive MHD simulations with a shearing box model. We found that if the spatially averaged magnetic Reynolds number, which is determined by widths of the stable and unstable regions in the initial conditions and values of the resistivity, is smaller than unity, the original Keplerian shear flow is transformed to the quasi-steady flow such that more flattened (rigid-rotation in extreme cases) velocity profile emerges locally and the outer part of the profile tends to be super-Keplerian. Angular momentum and mass transfer due to temporally generated MRI turbulence in the initially unstable region is responsible for the transformation. In the local super-Keplerian region, migrations due to aerodynamic gas drag and tidal interaction with disk gas are reversed. The simulation setting corresponds to the regions near the outer and inner edges of a global MRI dead zone in a disk. Therefore, the outer edge of dead zone, as well as the inner edge, would be a favorable site to accumulate dust particles to form planetesimals and retain planetary embryos against type I migration.Comment: 28 pages, 11figures, 1 table, accepted by Ap

Higgsing the stringy higher spin symmetry

It has recently been argued that the symmetric orbifold theory of T4 is dual to string theory on AdS3 x S3 x T4 at the tensionless point. At this point in moduli space, the theory possesses a very large symmetry algebra that includes, in particular, a $W_\infty$ algebra capturing the gauge fields of a dual higher spin theory. Using conformal perturbation theory, we study the behaviour of the symmetry generators of the symmetric orbifold theory under the deformation that corresponds to switching on the string tension. We show that the generators fall nicely into Regge trajectories, with the higher spin fields corresponding to the leading Regge trajectory. We also estimate the form of the Regge trajectories for large spin, and find evidence for the familiar logarithmic behaviour, thereby suggesting that the symmetric orbifold theory is dual to an AdS background with pure RR flux.Comment: 27 pages, 1 figure, note added in version

Scalar field perturbation on six-dimensional ultra-spinning black holes

We have studied the scalar field perturbations on six-dimensional ultra-spinning black holes. We have numerically calculated the quasinormal modes of rotating black holes. Our results suggest that such perturbations are stable.Comment: 8 pages, 6 figures; v2:typo corrected; v3:ref. corrected; v4:revise

Fast accretion of small planetesimals by protoplanetary cores

We explore the dynamics of small planetesimals coexisting with massive protoplanetary cores in a gaseous nebula. Gas drag strongly affects the motion of small bodies leading to the decay of their eccentricities and inclinations, which are excited by the gravity of protoplanetary cores. Drag acting on larger ($\gtrsim 1$ km), high velocity planetesimals causes a mere reduction of their average random velocity. By contrast, drag qualitatively changes the dynamics of smaller ($\lesssim 0.1-1$ km), low velocity objects: (1) small planetesimals sediment towards the midplane of the nebula forming vertically thin subdisk; (2) their random velocities rapidly decay between successive passages of the cores and, as a result, encounters with cores typically occur at the minimum relative velocity allowed by the shear in the disk. This leads to a drastic increase in the accretion rate of small planetesimals by the protoplanetary cores, allowing cores to grow faster than expected in the simple oligarchic picture, provided that the population of small planetesimals contains more than roughly 1% of the solid mass in the nebula. Fragmentation of larger planetesimals ($\gtrsim 1$ km) in energetic collisions triggered by the gravitational scattering by cores can easily channel this amount of material into small bodies on reasonable timescales ($< 1$ Myr in the outer Solar System), providing a means for the rapid growth (within several Myr at 30 AU) of rather massive protoplanetary cores. Effects of inelastic collisions between planetesimals and presence of multiple protoplanetary cores are discussed.Comment: 17 pages, 8 figures, additional clarifications, 1 more figure and table adde

The growth of planetary embryos: orderly, runaway, or oligarchic?

We consider the growth of a protoplanetary embryo embedded in a planetesimal disk. We take into account the dynamical evolution of the disk caused by (1) planetesimal-planetesimal interactions, which increase random motions and smooth gradients in the disk, and (2) gravitational scattering of planetesimals by the embryo, which tends to heat up the disk locally and repels planetesimals away. The embryo's growth is self-consistently coupled to the planetesimal disk dynamics. We demonstrate that details of the evolution depend on only two dimensionless parameters incorporating all the physical characteristics of the problem: the ratio of the physical radius to the Hill radius of any solid body in the disk and the number of planetesimals inside the annulus of the disk with width equal to the planetesimal Hill radius. The results of exploration in the framework of our model of several situations typical for protosolar nebula can be summarized as follows: initially, the planetesimal disk dynamics is not affected by the presence of the embryo and the growth of the embryo's mass proceeds very rapidly in the runaway regime. Later on, when the embryo starts being dynamically important, its accretion slows down similar to the ``oligarchic'' growth picture. The scenario of orderly growth suggested by Safronov (1972) is never realized in our calculations; scenario of runaway growth suggested by Wetherill & Stewart (1989) is only realized for a limited range in mass. Slow character of the planetesimal accretion on the oligarchic stage of the embryo's accumulation leads to a considerable increase of the protoplanetary formation timescale compared to that following from a simple runaway accretion picture valid in the homogeneous planetesimal disks.Comment: 42 pages, 13 figures, submitted to A

Origin of the Different Architectures of the Jovian and Saturnian Satellite Systems

The Jovian regular satellite system mainly consists of four Galilean satellites that have similar masses and are trapped in mutual mean motion resonances except for the outer satellite, Callisto. On the other hand, the Saturnian regular satellite system has only one big icy body, Titan, and a population of much smaller icy moons. We have investigated the origin of these major differences between the Jovian and Saturnian satellite systems by semi-analytically simulating the growth and orbital migration of proto-satellites in an accreting proto-satellite disk. We set up two different disk evolution/structure models that correspond to Jovian and Saturnian systems, by building upon previously developed models of an actively-supplied proto-satellite disk, the formation of gas giants, and observations of young stars. Our simulations extend previous models by including the (1) different termination timescales of gas infall onto the proto-satellite disk and (2) different evolution of a cavity in the disk, between the Jovian and Saturnian systems. We have performed Monte Carlo simulations and show that in the case of the Jovian systems, four to five similar-mass satellites are likely to remain trapped in mean motion resonances. This orbital configuration is formed by type I migration, temporal stopping of the migration near the disk inner edge, and quick truncation of gas infall caused by Jupiter opening a gap in the Solar nebula. The Saturnian systems tend to end up with one dominant body in the outer regions caused by the slower decay of gas infall associated with global depletion of the Solar nebula. The total mass and compositional zoning of the predicted Jovian and Saturnian satellite systems are consistent with the observed satellite systems.Comment: Accepted to ApJ, 33pages, 6figures, 2table

The Giant Impact Simulations with Density Independent Smoothed Particle Hydrodynamics

At present, the giant impact (GI) is the most widely accepted model for the origin of the Moon. Most of the numerical simulations of GI have been carried out with the smoothed particle hydrodynamics (SPH) method. Recently, however, it has been pointed out that standard formulation of SPH (SSPH) has difficulties in the treatment of a contact discontinuity such as a core-mantle boundary and a free surface such as a planetary surface. This difficulty comes from the assumption of differentiability of density in SSPH. We have developed an alternative formulation of SPH, density independent SPH (DISPH), which is based on differentiability of pressure instead of density to solve the problem of a contact discontinuity. In this paper, we report the results of the GI simulations with DISPH and compare them with those obtained with SSPH. We found that the disk properties, such as mass and angular momentum produced by DISPH is different from that of SSPH. In general, the disks formed by DISPH are more compact: while formation of a smaller mass moon for low-oblique impacts is expected with DISPH, inhibition of ejection would promote formation of a larger mass moon for high-oblique impacts. Since only the improvement of core-mantle boundary significantly affects the properties of circumplanetary disks generated by GI and DISPH has not been significantly improved from SSPH for a free surface, we should be very careful when some conclusions are drawn from the numerical simulations for GI. And it is necessary to develop the numerical hydrodynamical scheme for GI that can properly treat the free surface as well as the contact discontinuity.Comment: Accepted for publication in Icaru

Technology Transfer as Social Process -A Sociological Perspective

Technology transfer is the process by which important scientific and technological advance is translated into sociallydefined benefits. Seen in this perspective, technology transfer may be regarded as the way the United States invests in the future, its own and that of other nations. With the President\u27s current scientific priority list heavily studded with space-derived items, and the White House Fact Sheet on Space Policy\u27s strong emphasis on application, the mandate is unmistakable. How it can best be implemented is not so certain. Even viewed in retrospect, most known innovations travel a tortuous road. In prospect, the path is almost completely unpredictable. What is clear is that there must be explicit recognition that technology transfer is in essence a social process, that it does not take place by itself, and that it occurs in a social environment, in which success, however defined, depends on a complicated web of synergistic factors only tangentially related to the technology itself. The notion of technology transfer is at least as old as fire and certainly as commonplace as the adoption of the wheel. This familiarity with the concept has probably contributed to the tendency toward underestimating its complexity. NASA\u27s considerable experience with technological innovation and the dynamics of transferring space-derived knowledge and knowhow into terrestrial and perhaps more pedestrian channels serves as the basis for this paper. With Landsat the primary, but not the only, example, we analyze from the sociological perspective the factors implementing and impeding technology transfer