Japan National Astronomical Observatory

Tokyo, Japan

Japan National Astronomical Observatory

Tokyo, Japan
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Sotani H.,Japan National Astronomical Observatory
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

We consider the structure of the magnetic fields inside the neutron stars in Eddington-inspired Born-Infeld (EiBI) gravity. In order to construct the magnetic fields, we derive the relativistic Grad-Shafranov equation in EiBI and numerically determine the magnetic distribution in such a way that the interior magnetic fields should be connected to the exterior distribution. Then, we find that the magnetic distribution inside the neutron stars in EiBI is qualitatively similar to that in general relativity, where the deviation of magnetic distribution in EiBI from that in general relativity is almost comparable to uncertainty due to the equation of state for the neutron star matter. However, we also find that the magnetic fields in the crust region are almost independent of the coupling constant in EiBI, which suggests a possibility of obtaining the information about the crust equation of state independent from the gravitational theory via the observations of the phenomena associated with the crust region. In any case, since the imprint of EiBI gravity on the magnetic fields is weak, the magnetic fields could be a poor probe of gravitational theories, considering the many magnetic uncertainties. © 2015 American Physical Society.


Fukushima T.,Japan National Astronomical Observatory
Journal of Geodesy | Year: 2013

A recursive method is developed to compute the ratios of the oblate spheroidal harmonics of the second kind and their first-, second-, and third-order derivatives. The recurrence formulas consist of three kinds: (1) fixed-degree increasing-order, (2) mixed-degree increasing-order, and (3) fixed-order decreasing-degree. The three seed values are evaluated by rapidly convergent series. The derivatives of the ratios are recursively obtained from the values and lower-order derivatives of the same harmonic order and of the same or higher degrees. The new method precisely and quickly computes the ratios and their low-order derivatives. It provides 13 correct digits of the ratios of degree as high as 262,000 and runs 20-100 times faster than the existing methods. © 2012 Springer-Verlag Berlin Heidelberg.


Fukushima T.,Japan National Astronomical Observatory
Journal of Geodesy | Year: 2012

By extending the exponent of floating point numbers with an additional integer as the power index of a large radix, we compute fully normalized associated Legendre functions (ALF) by recursion without underflow problem. The new method enables us to evaluate ALFs of extremely high degree as 2 32 = 4,294,967,296, which corresponds to around 1 cm resolution on the Earth's surface. By limiting the application of exponent extension to a few working variables in the recursion, choosing a suitable large power of 2 as the radix, and embedding the contents of the basic arithmetic procedure of floating point numbers with the exponent extension directly in the program computing the recurrence formulas, we achieve the evaluation of ALFs in the double-precision environment at the cost of around 10% increase in computational time per single ALF. This formulation realizes meaningful execution of the spherical harmonic synthesis and/or analysis of arbitrary degree and order. © 2011 Springer-Verlag.


We confirm that the first-, second-, and third-order derivatives of fully-normalized Legendre polynomial (LP) and associated Legendre function (ALF) of arbitrary degree and order can be correctly evaluated by means of non-singular fixed-degree formulas (Bosch in Phys Chem Earth 25:655-659, 2000) in the ordinary IEEE754 arithmetic when the values of fully-normalized LP and ALF are obtained without underflow problems, for e. g., using the extended range arithmetic we recently developed (Fukushima in J Geod 86:271-285, 2012). Also, we notice the same correctness for the popular but singular fixed-order formulas unless (1) the order of differentiation is greater than the order of harmonics and (2) the point of evaluation is close to the poles. The new formulation using the fixed-order formulas runs at a negligible extra computational time, i. e., 3-5 % increase in computational time per single ALF when compared with the standard algorithm without the exponent extension. This enables a practical computation of low-order derivatives of spherical harmonics of arbitrary degree and order. © 2012 Springer-Verlag.


Niino Y.,Japan National Astronomical Observatory
Astrophysical Journal | Year: 2012

We investigate the relation between stellar mass (M*), star formation rate (SFR), and metallicity (Z) of galaxies, the so-called fundamental metallicity relation, in the galaxy sample of the Sloan Digital Sky Survey Data Release 7. We separate the galaxies into narrow redshift bins and compare the relation at different redshifts and find statistically significant (>99%) evolution. We test various observational effects that might cause seeming Z evolution and find it difficult to explain the evolution of the relation only by the observational effects. In the current sample of low-redshift galaxies, galaxies with different M* and SFR are sampled from different redshifts, and there is degeneracy between M */SFR and redshift. Hence, it is not straightforward to distinguish a relation between Z and SFR from a relation between Z and redshift. The separation of the intrinsic relation from the redshift evolution effect is a crucial issue in the understanding of the evolution of galaxies. © 2012. The American Astronomical Society. All rights reserved..


Martinache F.,Japan National Astronomical Observatory
Publications of the Astronomical Society of the Pacific | Year: 2013

This article introduces a novel wavefront sensing approach that relies on the Fourier analysis of a single conventional direct image. In the high Strehl ratio regime, the relation between the phase measured in the Fourier plane and the wavefront errors in the pupil can be linearized, as was shown in a previous work that introduced the notion of generalized closure-phase, or kernel-phase. The technique, to be usable as presented requires two conditions to be met: (1) the wavefront errors must be kept small (of the order of one radian or less), and (2) the pupil must include some asymmetry, which can be introduced with a mask, for the problem to become solvable. Simulations show that this asymmetric pupil Fourier wavefront sensing or APF-WFS technique can improve the Strehl ratio from 50% to over 90% in just a few iterations, with excellent photon noise sensitivity properties, suggesting that on-sky close loop APF-WFS is possible with an extreme adaptive optics system.


Yamazaki D.G.,Japan National Astronomical Observatory
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

We investigate the effects of the background primordial magnetic field (PMF) on the cosmic microwave background (CMB). The sound speed of the tightly coupled photon-baryon fluid is increased by the background PMF. The increased sound speed causes the odd peaks of the CMB temperature fluctuations to be suppressed and the CMB peak positions to be shifted to a larger scale. The background PMF causes a stronger decaying potential and increases the amplitude of the CMB. These two effects of the background PMF on a smaller scale cancel out, and the overall effects of the background PMF are the suppression of the CMB around the first peak and the shifting of peaks to a larger scale. We also discuss obtaining information about the PMF generation mechanisms, and we examine the nonlinear evolution of the PMF by the constraint on the maximum scale for the PMF distributions. Finally, we discuss degeneracies between the PMF parameters and the standard cosmological parameters. © 2014 American Physical Society.


Wanajo S.,Japan National Astronomical Observatory
Astrophysical Journal Letters | Year: 2013

We examine the r-process in the neutrino-driven proto-neutron-star (PNS) wind of core-collapse supernovae in light of the recent findings of massive neutron stars in binaries as well as of an indication of neutron-richness in the PNS ejecta because of the nucleon potential corrections on neutrino opacities. To this end, a spherically symmetric, general relativistic, steady-state wind model is applied for a wide range of PNS masses between 1.2 MOdot; and 2.4 MOdot; with the latter reaching the causality limit. Nucleosynthesis calculations with these PNS models are performed by assuming a time evolution of electron fraction with its minimal value of Y e = 0.4, which mimics recent hydrodynamical results. The fundamental nucleosynthetic aspect of the PNS wind is found to be the production of Sr, Y, and Zr in quasi-equilibrium and of the elements with A ≈ 90-110 by a weak r-process, which can be an explanation for the abundance signatures in r-process-poor Galactic halo stars. PNSs more massive than 2.0 MȮ; can eject heavy r-process elements, however, with substantially smaller amount than what is needed to account for the solar content. PNS winds can be thus the major origin of light trans-iron elements but no more than 10% of those heavier than A ∼ 110, although they may be the sources of the low-level abundances of Sr and Ba found in numerous metal-poor stars if the maximum mass of PNSs exceeds 2.0 MȮ. © 2013. The American Astronomical Society. All rights reserved.


Reid M.J.,Harvard - Smithsonian Center for Astrophysics | Honma M.,Japan National Astronomical Observatory
Annual Review of Astronomy and Astrophysics | Year: 2014

Astrometry provides the foundation for astrophysics. Accurate positions are required for the association of sources detected at different times or wavelengths, and distances are essential to estimate the size, luminosity, mass, and ages of most objects. Very long baseline interferometry at radio wavelengths, with diffraction-limited imaging at submilliarcsecond resolution, has long held the promise of microarcsecond astrometry. However, only in the past decade has this been routinely achieved. Currently, parallaxes for sources across the Milky Way are being measured with ∼10 μas accuracy, and proper motions of galaxies are being determined with accuracies of ∼1 μas year-1. The astrophysical applications of these measurements cover many fields, including star formation, evolved stars, stellar and supermassive black holes, Galactic structure, the history and fate of the Local Group, the Hubble constant, and tests of general relativity. This review summarizes the methods used and the astrophysical applications of microarcsecond radio astrometry. Copyright © 2014 by Annual Reviews.


Sotani H.,Japan National Astronomical Observatory
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

We show that the shear modes in the neutron star crust are quite sensitive to the existence of non-uniform nuclear structures, the so-called 'pasta'. Due to the existence of pasta phase, the frequencies of shear modes are reduced, where the dependence of fundamental frequency is different from that of overtones. Since the torsional shear frequencies depend strongly on the structure of pasta phase, through the observations of stellar oscillations, one can probe the pasta structure in the crust, although that is quite difficult via the other observations. Additionally, considering the effect of pasta phase, we show the possibility to explain all the observed frequencies in the SGR 1806-20 by using only crust torsional shear modes. © 2011 The Author. Monthly Notices of the Royal Astronomical Society © 2011 RAS.

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