Type I Planet Migration in Nearly Laminar Disks

We describe 2D hydrodynamic simulations of the migration of low-mass planets ($\leq 30 M_{\oplus}$) in nearly laminar disks (viscosity parameter $\alpha < 10^{-3}$) over timescales of several thousand orbit periods. We consider disk masses of 1, 2, and 5 times the minimum mass solar nebula, disk thickness parameters of $H/r = 0.035$ and 0.05, and a variety of $\alpha$ values and planet masses. Disk self-gravity is fully included. Previous analytic work has suggested that Type I planet migration can be halted in disks of sufficiently low turbulent viscosity, for $\alpha \sim 10^{-4}$. The halting is due to a feedback effect of breaking density waves that results in a slight mass redistribution and consequently an increased outward torque contribution. The simulations confirm the existence of a critical mass ($M_{cr} \sim 10 M_{\oplus}$) beyond which migration halts in nearly laminar disks. For \alpha \ga 10^{-3}, density feedback effects are washed out and Type I migration persists. The critical masses are in good agreement with the analytic model of Rafikov (2002). In addition, for \alpha \la 10^{-4} steep density gradients produce a vortex instability, resulting in a small time-varying eccentricity in the planet's orbit and a slight outward migration. Migration in nearly laminar disks may be sufficiently slow to reconcile the timescales of migration theory with those of giant planet formation in the core accretion model.Comment: 3 figures, accepted to ApJ

Dust Size Growth and Settling in a Protoplanetary Disk

We have studied dust evolution in a quiescent or turbulent protoplanetary disk by numerically solving coagulation equation for settling dust particles, using the minimum mass solar nebular model. As a result, if we assume an ideally quiescent disk, the dust particles settle toward the disk midplane to form a gravitationally unstable layer within 2x10^3 - 4x10^4 yr at 1 - 30 AU, which is in good agreement with an analytic calculation by Nakagawa, Sekiya, & Hayashi (1986) although they did not take into account the particle size distribution explicitly. In an opposite extreme case of a globally turbulent disk, on the other hand, the dust particles fluctuate owing to turbulent motion of the gas and most particles become large enough to move inward very rapidly within 70 - 3x10^4 yr at 1 - 30 AU, depending on the strength of turbulence. Our result suggests that global turbulent motion should cease for the planetesimal formation in protoplanetary disks.Comment: 27 pages, 8 figures, accepted for publication in the Ap

The Eastern Arm of M83 Revisited: High-Resolution Mapping of 12CO 1-0 Emission

We have used the Owens Valley Millimeter Array to map 12CO (J=1-0) along a 3.5 kpc segment of M83's eastern spiral arm at resolutions of 6.5"x3.5", 10", and 16". The CO emission in most of this segment lies along the sharp dust lane demarking the inner edge of the spiral arm, but beyond a certain point along the arm the emission shifts downstream from the dust lane to become better aligned with the young stars seen in blue and H-beta images. This morphology resembles that of the western arm of M100. Three possibilities, none of which is wholly satisfactory, are considered to explain the deviation of the CO arm from the dust lane: heating of the CO by UV radiation from young stars, heating by low-energy cosmic rays, and a molecular medium consisting of two (diffuse and dense) components which react differently to the density wave. Regardless, the question of what CO emission traces along this spiral arm is a complicated one. Strong tangential streaming is observed where the arm crosses the kinematic major axis of the galaxy, implying that the shear becomes locally prograde in the arms. Inferred from the streaming is a very high gas surface density of about 230 solar masses/pc**2 and an arm-interarm contrast greater than 2.3 in the part of the arm near the major axis. Using two different criteria, we find that the gas at this location is well above the threshold for gravitational instability -- much more clearly so than in either M51 or M100.Comment: Accepted for publication in ApJ. 25 pages, 5 figures. Manuscript in LaTeX, figures in pdf. Fig 3 in colo

Bar-driven Transport of Molecular Gas to Galactic Centers and Its Consequences

We study the characteristics of molecular gas in the central regions of spiral galaxies on the basis of our CO(J=1-0) imaging survey of 20 nearby spiral galaxies using the NRO and OVRO millimeter arrays. Condensations of molecular gas at galactic centers with sizescales < 1 kpc and CO-derived masses M_gas(R<500pc) = 10^8 - 10^9 M_sun are found to be prevalent in the gas-rich L^* galaxies. Moreover, the degree of gas concentration to the central kpc is found to be higher in barred systems than in unbarred galaxies. This is the first statistical evidence for the higher central concentration of molecular gas in barred galaxies, and it strongly supports the theory of bar-driven gas transport. It is most likely that more than half of molecular gas within the central kpc of a barred galaxy was transported there from outside by the bar. The supply of gas has exceeded the consumption of gas by star formation in the central kpc, resulting in the excess gas in the centers of barred systems. The mean rate of gas inflow is statistically estimated to be larger than 0.1 - 1 M_sun/yr. The correlation between gas properties in the central kpc and the type of nuclear spectrum (HII, LINER, or Seyfert) is investigated. A correlation is found in which galaxies with larger gas-to-dynamical mass ratios tend to have HII nuclear spectra, while galaxies with smaller ratios show spectra indicating AGN. Also, the theoretical prediction of bar-dissolution by condensation of gas to galactic centers is observationally tested. It is suggested that the timescale for bar dissolution is larger than 10^8 - 10^10 yr, or a bar in a L^* galaxy is not destroyed by a condensation of 10^8 - 10^9 M_sun gas in the central kpc.Comment: AASTeX, 20 pages, 8 eps figs, ApJ in press (10 Nov. 1999 issue

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 the radiative cooling. The crucial factor is the rate at which energy can be radiated from the disk 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 which 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.Comment: 8 pages, submitted to Ap

Super-Earths: A New Class of Planetary Bodies

Super-Earths, a class of planetary bodies with masses ranging from a few Earth-masses to slightly smaller than Uranus, have recently found a special place in the exoplanetary science. Being slightly larger than a typical terrestrial planet, super-Earths may have physical and dynamical characteristics similar to those of Earth whereas unlike terrestrial planets, they are relatively easier to detect. Because of their sizes, super-Earths can maintain moderate atmospheres and possibly dynamic interiors with plate tectonics. They also seem to be more common around low-mass stars where the habitable zone is in closer distances. This article presents a review of the current state of research on super-Earths, and discusses the models of the formation, dynamical evolution, and possible habitability of these objects. Given the recent advances in detection techniques, the detectability of super-Earths is also discussed, and a review of the prospects of their detection in the habitable zones of low-mass stars is presented.Comment: A (non-technical) review of the literature on the current state ofresearch on super-Earths. The topics include observation, formation, dynamical evolution, habitability, composition, interior dynamics, magnetic field, atmosphere, and propsect of detection. The article has 44 pages, 27 figures, and 203 references. It has been accepted for publication in the journal Contemporary Physics (2011