Roman R. Rafikov

Solar System Objects Observed in the Sloan Digital Sky Survey Commissioning Data

We discuss measurements of the properties of D13,000 asteroids detected in 500 deg2 of sky in the Sloan Digital Sky Survey (SDSS) commissioning data. The moving objects are detected in the magnitude range 14 \ r* \ 21.5, with a baseline of D5 minutes, resulting in typical velocity errors of D3%. Extensive tests show that the sample is at least 98% complete, with a contamination rate of less than 3%. We —nd that the size distribution of asteroids resembles a broken power law, independent of the heliocentric distance: D~2.3 for 0.4 km, and D~4 for 5

Can giant planets form by direct gravitational instability

Gravitational instability has been invoked as a possible mechanism of the giant planet production in protoplanetary disks. Here we critically revise its viability by noting that to form planets directly, it is not enough for protoplanetary disks to be gravitationally unstable. They must also be able to cool efficiently (on a timescale comparable to the local disk orbital period) to allow the formation of the bound clumps by fragmentation. A combination of the dynamical and thermal constraints puts very stringent lower limits on the properties of disks capable of fragmenting into the self-gravitating objects: for the gravitational instability to form giant planets at 10 AU in the disk cooled by the radiation transfer, the gas temperature must exceed 103 K with a minimum disk mass of 0.7 M☉ and a luminosity of 40 L☉. Although these requirements are relaxed in the more distant parts of the disk, masses of the bound objects formed as a result of instability are too large even at 100 AU (~10MJ) to explain the characteristics of known extrasolar giant planets. Such protoplanetary disks (and planets formed in them) have very unusual observational properties, and this severely constrains the possibility of giant planet formation by direct gravitational instability.

Discovery and spectroscopy of the young jovian planet 51 Eri b with the Gemini Planet Imager

An exoplanet extracted from the bright Direct imaging of Jupiter-like exoplanets around young stars provides a glimpse into how our solar system formed. The brightness of young stars requires the use of next-generation devices such as the Gemini Planet Imager (GPI). Using the GPI, Macintosh et al. discovered a Jupiter-like planet orbiting a young star, 51 Eridani (see the Perspective by Mawet). The planet, 51 Eri b, has a methane signature and is probably the smallest exoplanet that has been directly imaged. These findings open the door to understanding solar system origins and herald the dawn of a new era in next-generation planetary imaging. Science, this issue p. 64; see also p. 39 The Gemini Planet Imager detects a Jupiter-like exoplanet orbiting the young star 51 Eridani. [Also see Perspective by Mawet] Directly detecting thermal emission from young extrasolar planets allows measurement of their atmospheric compositions and luminosities, which are influenced by their formation mechanisms. Using the Gemini Planet Imager, we discovered a planet orbiting the ~20-million-year-old star 51 Eridani at a projected separation of 13 astronomical units. Near-infrared observations show a spectrum with strong methane and water-vapor absorption. Modeling of the spectra and photometry yields a luminosity (normalized by the luminosity of the Sun) of 1.6 to 4.0 × 10−6 and an effective temperature of 600 to 750 kelvin. For this age and luminosity, “hot-start” formation models indicate a mass twice that of Jupiter. This planet also has a sufficiently low luminosity to be consistent with the “cold-start” core-accretion process that may have formed Jupiter.

Planetary Torques as the Viscosity of Protoplanetary Disks

We revisit the idea that density wave wakes of planets drive accretion in protostellar disks. The effects of many small planets can be represented as a viscosity if the wakes damp locally but the viscosity is proportional to the damping length. Damping occurs mainly because of shocks even for Earth-mass planets. The excitation of the wake follows from standard linear theory including the torque cutoff. We use this as input to an approximate but quantitative nonlinear theory based on Burgers equation for the subsequent propagation and shock. Shock damping is indeed local, but weakly so. If all metals in a minimum-mass solar nebula are invested in planets of a few Earth masses each, dimensionless viscosities (α) of the order of -4 dex to -3 dex result. We compare this with observational constraints. Such small planets would have escaped detection in radial velocity surveys and could be ubiquitous. If so, then the similarity of the observed lifetime of T Tauri disks to the theoretical timescale for assembling a rocky planet may be fate rather than coincidence.

Planet Migration and Gap Formation by Tidally Induced Shocks

Gap formation in a gas disk triggered by disk-planet tidal interaction is considered. Density waves launched by the planet are assumed to be damped as a result of their nonlinear evolution leading to shock formation and its subsequent dissipation. As a consequence, wave angular momentum is transferred to the disk, leading to evolution of its surface density. Planetary migration is an important ingredient of the theory; effects of the planet-induced surface density perturbations on the migration speed are considered. A gap is assumed to form when a stationary solution for the surface density profile is no longer possible in the frame of reference migrating with the planet. An analytical limit on the planetary mass necessary to open a gap in an inviscid disk is derived. The critical mass turns out to be smaller than the mass M1 for which the planetary Hill radius equals the disk scale height by a factor of at least Q5/7 (Q is the Toomre stability parameter), depending on the strength of the migration feedback. In viscous disks the critical planetary mass could vary from ~0.2M1 to M1, depending on the disk viscosity. This implies that a gap could be formed by a planet with mass of 2-15 M⊕, depending on the disk aspect ratio, viscosity, and the planets location in the nebula.

The local axisymmetric instability criterion in a thin, rotating, multicomponent disc

Purely gravitational perturbations are considered in a thin rotating disc composed of gas and several stellar components. The dispersion relation for the axisymmetric density waves propagating through the disc is found and the criterion for the local axisymmetric stability of the whole system is formulated. In the appropriate limit of two-component gas we confirm the findings of Jog & Solomon and extend consideration to the case when one component is collisionless. Gravitational stability of the Galactic disc in the solar neighbourhood based on the multicomponent instability condition is explored using recent measurements of the stellar composition and kinematics in the local Galactic disc obtained by the Hipparcos satellite.

Properties of Gravitoturbulent Accretion Disks

We explore the properties of cold gravitoturbulent accretion disks?non-fragmenting disks hovering on the verge of gravitational instability (GI)?using a realistic prescription for the effective viscosity caused by gravitational torques. This prescription is based on a direct relationship between the angular momentum transport in a thin accretion disk and the disk cooling in a steady state. Assuming that opacity is dominated by dust we are able to self-consistently derive disk properties for a given assuming marginal gravitational stability. We also allow external irradiation of the disk and account for a non-zero background viscosity, which can be due to the magneto-rotational instability. Spatial transitions between different co-existing disk states (e.g., between irradiated and self-luminous or between gravitoturbulent and viscous) are described and the location of the boundary at which the disk must fragment is determined in a variety of situations. We demonstrate in particular that at low enough external irradiation stabilizes the gravitoturbulent disk against fragmentation to very large distances thus providing means of steady mass transport to the central object. Implications of our results for the possibility of planet formation by GI in protoplanetary disks and star formation in the Galactic center and for the problem of feeding supermassive black holes in galactic nuclei are discussed.

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 (1 km), high-velocity planetesimals causes a mere reduction of their average random velocity. By contrast, drag qualitatively changes the dynamics of smaller (0.1–1 km), low-velocity objects: (1) small planetesimals sediment toward the midplane of the nebula, forming a 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 (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 million years at 30 AU) of rather massive protoplanetary cores. Effects of inelastic collisions between planetesimals and presence of multiple protoplanetary cores are discussed.

Convective Cooling and Fragmentation of Gravitationally Unstable Disks

Gravitationally unstable disks can fragment and form bound objects provided that their cooling time is short. In protoplanetary disks, radiative cooling is likely to be too slow to permit formation of planets by fragmentation within several tens of AU from the star. Recently, convection has been suggested as a faster means of heat loss from the disk, but here we demonstrate that it is only marginally more efficient than radiative cooling. The crucial factor is the rate at which energy can be radiated from the disks photosphere, which is robustly limited from above in the convective case by the adiabatic temperature gradient (given a certain midplane temperature). Thus, although vigorous convection is definitely possible in disks, the inefficiency of radiative loss from the photosphere may create a bottleneck, limiting the ability of the disk to form self-gravitating objects. Based on this argument, we derive a set of analytical constraints that diagnose the susceptibility of an unstable disk to fragmentation and show that the formation of giant planets by fragmentation of protoplanetary disks is unlikely to occur at distances of tens of AU. At the same time, these constraints do not preclude the possibility of fragmentation and star formation in accretion disks around supermassive black holes.

The Missing Cavities in the SEEDS Polarized Scattered Light Images of Transitional Protoplanetary Disks: A Generic Disk Model

Transitional circumstellar disks around young stellar objects have a distinctive infrared deficit around 10 μm in their spectral energy distributions, recently measured by the Spitzer Infrared Spectrograph (IRS), suggesting dust depletion in the inner regions. These disks have been confirmed to have giant central cavities by imaging of the submillimeter continuum emission using the Submillimeter Array (SMA). However, the polarized near-infrared scattered light images for most objects in a systematic IRS/SMA cross sample, obtained by HiCIAO on the Subaru telescope, show no evidence for the cavity, in clear contrast with SMA and Spitzer observations. Radiative transfer modeling indicates that many of these scattered light images are consistent with a smooth spatial distribution for μm-sized grains, with little discontinuity in the surface density of the μm-sized grains at the cavity edge. Here we present a generic disk model that can simultaneously account for the general features in IRS, SMA, and Subaru observations. Particularly, the scattered light images for this model are computed, which agree with the general trend seen in Subaru data. Decoupling between the spatial distributions of the μm-sized dust and mm-sized dust inside the cavity is suggested by the model, which, if confirmed, necessitates a mechanism, such as dust filtration, for differentiating the small and big dust in the cavity clearing process. Our model also suggests an inwardly increasing gas-to-dust ratio in the inner disk, and different spatial distributions for the small dust inside and outside the cavity, echoing the predictions in grain coagulation and growth models.