Apparent Stellar Wobble by a Planet in a Circumstellar Disk: Limitations on Planet Detection by Astrometry

Astrometric detection of a stellar wobble on the plane of the sky will provide us a next breakthrough in searching extrasolar planets. The Space Interferometry Mission (SIM) is expected to achieve a high-precision astrometry as accurate as 1 micro-as, which is precise enough to discover a new-born Jupiter mass planet around a pre-main-sequence (PMS) star in the Taurus-Auriga star forming region. PMS stars, however, have circum-stellar disks that may be obstacles to the precise measurement of the stellar position. We present results on disk influences to the stellar wobble. The density waves excited by a planet move both of the disk's mass center and the photo-center. The motion of the disk mass center induces an additional wobble of the stellar position, and the motion of the disk photo-center causes a contamination in the measurement of the stellar position. We show that the additional stellar motion dynamically caused by the disk's gravity is always negligible, but that the contamination of the disk light can interfere with the precise measurement of the stellar position, if the planet's mass is smaller than ~10 Jupiter mass. The motion of the disk photo-center is sensitive to a slight change in the wave pattern and the disk properties. Measurements by interferometers are generally insensitive to extended sources such as disks. Because of this property SIM will not suffer significant contaminations of the disk light, even if the planet's mass is as small as 1 Jupiter mass.Comment: 22 pages, 14 figures, accepted by Ap

Full QED+QCD Low-Energy Constants through Reweighting

The effect of sea quark electromagnetic charge on meson masses is investigated, and first results for full QED+QCD low-energy constants are presented. The electromagnetic charge for sea quarks is incorporated in quenched QED+full QCD lattice simulations by a reweighting method. The reweighting factor, which connects quenched and unquenched QED, is estimated using a stochastic method on 2+1 flavor dynamical domain-wall quark ensembles.Comment: 5 pages, 9 figures, REVTeX 4.1, v2: published versio

Application of an integrated approach to evaluate health risks for toxic chemicals by linking multimedia environmental and PBPK models

The paradigm of health risk assessment may consist of two main pillars, i.e., the exposure and dose-response assessments. Human exposure to chemicals via multiple pathways can be estimated by environmental multimedia models, which calculate the distribution of chemicals in the component media, i.e., air, water, soil, plants, and animal media. Combined with the information about human behaviors such as dietary habits, time spent outside, and etc, the multimedia models can provide an estimation of the daily chemical intake by inhalation or ingestion by humans. Physiologically based pharmacokinetic (PBPK) models are used to estimate the body burden of toxic chemicals throughout the entire human lifespan, integrating the evolution of the physiology and anatomy from childhood to advanced aged. The use of such PBPK models overcomes the limitations that dose-response modelling holds, e.g., it simply determines the relationship between the dose and the probability of an effect. The European project 2-FUN (Full-chain and UNcertainty Approaches for Assessing Health Risks in FUture ENvironmental Scenarios) aims at improving the approaches currently used in exposure and dose-response assessments. According to the aim of that project, an environmental multimedia model and a generic PBPK model are coupled as an integrated tool (2-FUN tool) and built up on a platform system, Ecolego. This study presents here the first application of the integrated tool to perform the full-chain risk assessment of a chemical for human health, considering multiple exposure pathways of chemical via inhalation of out-door air, and ingestion of water and foods. For this application of the tool, a case study was designed based on the information available in a region situated on the Seine river watershed, downstream of the Paris megacity and Benzo(a)pyrene (B(a)P) was selected as a target chemical substance. This study focuses especially on the propagation of uncertainty and inter-individual variability along the modelling chain. A probabilistic simulation was then performed to identify the input parameters and exposure pathways sensitive to model outputs (e.g., internal effective concentrations in organs)

Vector form factor in K_l3 semileptonic decay with two flavors of dynamical domain-wall quarks

We calculate the vector form factor in K \to \pi l \nu semileptonic decays at zero momentum transfer f_+(0) from numerical simulations of two-flavor QCD on the lattice. Our simulations are carried out on 16^3 \times 32 at a lattice spacing of a \simeq 0.12 fm using a combination of the DBW2 gauge and the domain-wall quark actions, which possesses excellent chiral symmetry even at finite lattice spacings. The size of fifth dimension is set to L_s=12, which leads to a residual quark mass of a few MeV. Through a set of double ratios of correlation functions, the form factor calculated on the lattice is accurately interpolated to zero momentum transfer, and then is extrapolated to the physical quark mass. We obtain f_+(0)=0.968(9)(6), where the first error is statistical and the second is the systematic error due to the chiral extrapolation. Previous estimates based on a phenomenological model and chiral perturbation theory are consistent with our result. Combining with an average of the decay rate from recent experiments, our estimate of f_+(0) leads to the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |V_{us}|=0.2245(27), which is consistent with CKM unitarity. These estimates of f_+(0) and |V_{us}| are subject to systematic uncertainties due to the finite lattice spacing and quenching of strange quarks, though nice consistency in f_+(0) with previous lattice calculations suggests that these errors are not large.Comment: 23 pages, 11 figures, 7 tables, RevTeX4; v3: one table added, results and conclusions unchanged, final version to appear in Phys.Rev.

Surface Outflow in Optically Thick Dust Disks by the Radiation Pressure

We study the outflow of dust particles on the surface layers of optically thick disks. At the surface of disks around young stars, small dust particles (size < 10 micron) experience stellar radiation pressure support and orbit more slowly than the surrounding gas. The resulting tail-wind imparts energy and angular momentum to the dust particles, moving them outward. This outflow occurs in the thin surface layer of the disk that is exposed to starlight, and the outward mass flux is carried primarily by particles of size ~0.1 micron. Beneath the irradiated surface layer, dust particles experience a head-wind, which drives them inward. For the specific case of a minimum-mass-solar-nebula, less than a thousandth of the dust mass experiences outward flow. If the stellar luminosity is 15 times brighter than the sun, however, or if the gas disk mass is as small as ~100 M_earth, then the surface outflow can dominate the inward flux in certain radial ranges, leading to the formation of rings or gaps in the dust disks.Comment: 14 pages, 7 figures, ApJ accepte

Radial Flow of Dust Particles in Accretion Disks

We study the radial migration of dust particles in accreting protostellar disks analogous to the primordial solar nebula. This study takes account of the two dimensional (radial and normal) structure of the disk gas, including the effects of the variation in the gas velocity as a function of distance from the midplane. It is shown that the dust component of disks accretes slower than the gas component. At high altitude from the disk midplane, the gas rotates faster than particles because of the inward pressure gradient force, and its drag force causes particles to move outward in the radial direction. Viscous torque induces the gas within a scale height from the disk midplane to flow outward, carrying small (size < 100 micron at 10 AU) particles with it. Only particles at intermediate altitude or with sufficiently large sizes (> 1 mm at 10 AU) move inward. When the particles' radial velocities are averaged over the entire vertical direction, particles have a net inward flux. At large distances from the central star, particles migrate inward with a velocity much faster than the gas accretion velocity. However, their inward velocity is reduced below that of the gas in the inner regions of the disk. The rate of velocity decrease is a function of the particles' size. While larger particles retain fast accretion velocity until they approach closer to the star, 10 micron particles have slower velocity than the gas in the most part of the disk (r < 100 AU). This differential migration of particles causes the size fractionation. Dust disks composed mostly of small particles (size < 10 micron) accrete slower than gas disks, resulting in the increase in the dust-gas ratio during the gas accretion phase.Comment: ApJ, accepted, 17 pages, 14 figure

Attenuation of Millimeter Emission from Circumstellar Disks Induced by the Rapid Dust Accretion

From millimeter observations of classical T Tauri stars, it is suggested that dust grains in circumstellar disks have grown to millimeter size or larger. However, gas drag on such large grains induces rapid accretion of the dust. We examine the evolution of dust disks composed of millimeter sized grains, and show that rapid accretion of the dust disk causes attenuation of millimeter continuum emission. If a dust disk is composed mainly of grains of 1 cm to 1 m, its millimeter emission goes off within 10^6 yr. Hence, grains in this size range cannot be a main population of the dust. Considering our results together with grain growth suggested by the millimeter continuum observations, we expect that the millimeter continuum emission of disks comes mainly from grains in a narrow size range of [1 mm -1 cm]. This suggests either that growth of millimeter sized grains to centimeter size takes more than 10^6 yr, or that millimeter sized grains are continuously replenished. In the former case, planet formation is probably difficult, especially in the outer disks. In the latter case, reservoirs of millimeter grains are possibly large (> 10 m) bodies, which can reside in the disk more than 10^6 yr. Constraints on the grain growth time-scale are discussed for the above two cases.Comment: Accepted by ApJ, 17 pages, 15 figure

The Differential Lifetimes of Protostellar Gas and Dust Disks

We construct a protostellar disk model that takes into account the combined effect of viscous evolution, photoevaporation and the differential radial motion of dust grains and gas. For T Tauri disks, the lifetimes of dust disks that are mainly composed of millimeter sized grains are always shorter than the gas disks' lifetimes, and become similar only when the grains are fluffy (density < 0.1 g cm^{-3}). If grain growth during the classical T Tauri phase produces plenty of millimeter sized grains, such grains completely accrete onto the star in 10^7 yr, before photoevaporation begins to drain the inner gas disk and the star evolves to the weak line T Tauri phase. In the weak line phase, only dust-poor gas disks remain at large radii (> 10 AU), without strong signs of gas accretion nor of millimeter thermal emission from the dust. For Herbig AeBe stars, the strong photoevaporation clears the inner disks in 10^6 yr, before the dust grains in the outer disk migrate to the inner region. In this case, the grains left behind in the outer gas disk accumulate at the disk inner edge (at 10-100 AU from the star). The dust grains remain there even after the entire gas disk has been photoevaporated, and form a gas-poor dust ring similar to that observed around HR 4796A. Hence, depending on the strength of the stellar ionizing flux, our model predicts opposite types of products around young stars. For low mass stars with a low photoevaporation rate, dust-poor gas disks with an inner hole would form, whereas for high mass stars with a high photoevaporation rate, gas-poor dust rings would form. This prediction should be examined by observations of gas and dust around weak line T Tauri stars and evolved Herbig AeBe stars.Comment: Accepted by ApJ, 11 pages, 4 figure

Domain wall fermions with improved gauge actions

金沢大学理学部We study the chiral properties of quenched domain wall fermions with several gauge actions. We demonstrate that the residual chiral symmetry breaking, which is present for a finite number of lattice sites in the fifth dimension (Ls), can be substantially suppressed using improved gauge actions. In particular the Symanzik action, the Iwasaki action, and a renormalization group improved gauge action, called the doubly blocked Wilson ~DBW2! action, are studied and compared to the Wilson action. All improved gauge actions studied show a reduction in the additive residual quark mass mres . Remarkably, in the DBW2 case mres is roughly two orders of magnitude smaller than the Wilson gauge action at a2152 GeV and Ls516. Significant reduction in mres is also realized at stronger gauge coupling corresponding to a2151.3 GeV. As our numerical investigation indicates, this reduction is achieved by reducing the number of topological lattice dislocations present in the gauge field configurations. We also present detailed results for the quenched light hadron spectrum and the pion decay constant using the DBW2 gauge action