Taishi Nakamoto

Supercritical Accretion Flows around Black Holes: Two-dimensional, Radiation Pressure-dominated Disks with Photon Trapping

The quasi-steady structure of supercritical accretion flows around a black hole is studied based on two-dimensional radiation-hydrodynamic (2D-RHD) simulations. The supercritical flow is composed of two parts: the disk region and the outflow regions above and below the disk. Within the disk region the circular motion and the patchy density structure are observed, which is caused by Kelvin-Helmholtz instability and probably by convection. The mass accretion rate decreases inward, roughly in proportion to the radius, and the remaining part of the disk material leaves the disk to form the outflow because of the strong radiation pressure force. We confirm that photon trapping plays an important role within the disk. Thus, matter can fall onto the black hole at a rate exceeding the Eddington rate. The emission is highly anisotropic and moderately collimated so that the apparent luminosity can exceed the Eddington luminosity by a factor of a few in the face-on view. The mass accretion rate onto the black hole increases with the absorption opacity (metallicity) of the accreting matter. This implies that the black hole tends to grow faster in metal-rich regions, such as in starburst galaxies or star-forming regions.

Cosmological radiative transfer codes comparison project - I. The static density field tests

Radiative transfer (RT) simulations are now at the forefront of numerical astrophysics. They are becoming crucial for an increasing number of astrophysical and cosmological problems; at the same time their computational cost has come within reach of currently available computational power. Further progress is retarded by the considerable number of different algorithms (including various flavours of ray tracing and moment schemes) developed, which makes the selection of the most suitable technique for a given problem a non-trivial task. Assessing the validity ranges, accuracy and performances of these schemes is the main aim of this paper, for which we have compared 11 independent RT codes on five test problems: (0) basic physics; (1) isothermal H II region expansion; (2) H II region expansion with evolving temperature; (3) I-front trapping and shadowing by a dense clump and (4) multiple sources in a cosmological density field. The outputs of these tests have been compared and differences analysed. The agreement between the various codes is satisfactory although not perfect. The main source of discrepancy appears to reside in the multifrequency treatment approach, resulting in different thicknesses of the ionized-neutral transition regions and the temperature structure. The present results and tests represent the most complete benchmark available for the development of new codes and improvement of existing ones. To further this aim all test inputs and outputs are made publicly available in digital form.

Formation, early evolution, and gravitational stability of protoplanetary disks

The formation, viscous evolution, and gravitational stability of protoplanetary disks are investigated. The formation process is parameterized by the angular velocity of the molecular cloud core omega, while the viscous evolution is parameterized by the viscosity parameter alpha in the disk; in this study we consider a range of (0.4-6) x 10(exp -14)/s for omega and from 10(exp -5) to 10(exp -1) for alpha. The axisymmetric gravitational stabilities of the disks are checked using Toomres criterion. The resulting disk surface temperature distribution, (d log T(sub s)/d log R) approximately = -0.6 (R is the cylindrical radius), can be attributed to two heating sources: the viscous heating dominant in the inner disk region, and the accretion shock heating dominant in the outer disk region. This surface temperature distribution matches that observed in many disks around young stellar objects. During the infall stage, disks with alpha less than 10(exp -1.5) become gravitationally unstable independent of omega. The gravitational instabilities occur at radii ranging from 5 to 40 AU. The ratio of the disk mass to the central star mass ranges from 0.2 to 0.5 at the times of instability, about 4 x 10(exp -5) x (omega/10(exp -14)/s)(exp -0.67) yr. Most disks with low alpha and high omega become gravitationally unstable during their formation phase.

Evolution of Snow Line in Optically Thick Protoplanetary Disks: Effects of Water Ice Opacity and Dust Grain Size

Evolution of a snow line in an optically thick protoplanetary disk is investigated with numerical simulations. The ice-condensing region in the disk is obtained by calculating the temperature and the density with the 1+1D approach. The snow line migrates as the mass accretion rate () in the disk decreases with time. Calculations are carried out from an early phase with high disk accretion rates ( yr–1) to a later phase with low disk accretion rates ( yr–1) using the same numerical method. It is found that the snow line moves inward for yr–1, while it gradually moves outward in the later evolution phase with yr–1. In addition to the silicate opacity, the ice opacity is taken into consideration. In the inward migration phase, the additional ice opacity increases the distance of the snow line from the central star by a factor of 1.3 for dust grains 10 μm in size and of 1.6 for 100 μm. It is inevitable that the snow line comes inside Earths orbit in the course of the disk evolution if the viscosity parameter α is in the range 0.001-0.1, the dust-to-gas mass ratio is higher than a tenth of the solar abundance value, and the dust grains are smaller than 1 mm. The formation of water-devoid planetesimals in the terrestrial planet region seems to be difficult throughout the disk evolution, which imposes a new challenge to planet formation theory.

The escape of ionizing photons from supernova-dominated primordial galaxies

In order to assess the contribution of Lyman break galaxies (LBGs) and Lyman α emitters (LAEs) at redshifts 3 6. That implies that additional ionization sources may be required at z > 6.

MASS-LOSS EVOLUTION OF CLOSE-IN EXOPLANETS: EVAPORATION OF HOT JUPITERS AND THE EFFECT ON POPULATION

During their evolution, short-period exoplanets may lose envelope mass through atmospheric escape owing to intense X-ray and extreme ultraviolet (XUV) radiation from their host stars. Roche-lobe overflow induced by orbital evolution or intense atmospheric escape can also contribute to mass loss. To study the effects of mass loss on inner planet populations, we calculate the evolution of hot Jupiters considering mass loss of their envelopes and thermal contraction. Mass loss is assumed to occur through XUV-driven atmospheric escape and the following Roche-lobe overflow. The runaway effect of mass loss results in a dichotomy of populations: hot Jupiters that retain their envelopes and super Earths whose envelopes are completely lost. Evolution primarily depends on the core masses of planets and only slightly on migration history. In hot Jupiters with small cores ( 10 Earth masses), runaway atmospheric escape followed by Roche-lobe overflow may create sub-Jupiter deserts, as observed in both mass and radius distributions of planetary populations. Comparing our results with formation scenarios and observed exoplanets populations, we propose that populations of closely orbiting exoplanets are formed by capturing planets at/inside the inner edges of protoplanetary disks and subsequent evaporation of sub-Jupiters.

THEORETICAL EMISSION SPECTRA OF ATMOSPHERES OF HOT ROCKY SUPER-EARTHS

Motivated by recent detection of transiting high-density super-Earths, we explore the detectability of hot rocky super-Earths orbiting very close to their host stars. In the environment hot enough for their rocky surfaces to be molten, they would have the atmosphere composed of gas species from the magma oceans. In this study, we investigate the radiative properties of the atmosphere that is in the gas/melt equilibrium with the underlying magma ocean. Our equilibrium calculations yield Na, K, Fe, Si, SiO, O, and O

Observational Possibility of the "Snow Line" on the Surface of Circumstellar Disks with the Scattered Light

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Shock-wave heating model for chondrule formation : Hydrodynamic simulation of molten droplets exposed to gas flows

as the major atmospheric species. We compile the radiative-absorption line data of those species available in literature, and calculate their absorption opacities in the wavelength region of 0.1--100~

Deformation and internal flow of a chondrule-precursor molten sphere in a shocked nebular gas

\mathrm{\mu m}