arXiv subjects

Hiromoto Shibahashi

Publications and source records attributed to Hiromoto Shibahashi.

33 records · Page 2Linked to original sources

arXiv2014

Asteroseismic measurement of surface-to-core rotation in a main sequence A star, KIC 11145123

We have discovered rotationally split core g-mode triplets and surface p-mode triplets and quintuplets in a terminal age main sequence A star, KIC 11145123, that shows both $δ$ Sct p-mode pulsations and $γ$ Dor g-mode pulsations. This gives the first robust determination of the rotation of the deep core and surface of a main sequence star, essentially model-independently. We find its rotation to be nearly uniform with a period near 100 d, but we show with high confidence that the surface rotates slightly faster than the core. A strong angular momentum transfer mechanism must be operating to produce the nearly rigid rotation, and a mechanism other than viscosity must be operating to produce a more rapidly rotating surface than core. Our asteroseismic result, along with previous asteroseismic constraints on internal rotation in some B stars, and measurements of internal rotation in some subgiant, giant and white dwarf stars, has made angular momentum transport in stars throughout their lifetimes an observational science.

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arXiv2014

Pulsations of Pre-White Dwarfs with Hydrogen-dominated Atmospheres

We carried out a fully non-adiabatic analysis for nonradial oscillations of pre-white dwarfs evolved from the post-Asymptotic Giant Branch (AGB) with hydrogen-dominated envelopes. It is shown that nuclear reactions in the hydrogen burning-shell excite low-degree g-modes in the period range of about 40-200 s for the pre-white dwarf models with Teff=40,000 K - 300,000 K. It is also shown that the amount of hydrogen {has} a significant influence on the instability domain of such pre-white dwarfs in the Hertzsprung-Russel (H-R) diagram. Thus, the thickness of hydrogen-dominated envelopes may be well constrained by observing the presence of the g-mode oscillations.

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arXiv2014

Finding binaries among Kepler pulsating stars from phase modulation of their pulsations

We present a method for finding binaries among pulsating stars that were observed by the Kepler Mission. We use entire four-year light curves to accurately measure the frequencies of the strongest pulsation modes, then track the pulsation phases at those frequencies in 10-d segments. This produces a series of time-delay measurements in which binarity is apparent as a periodic modulation whose amplitude gives the projected light travel time across the orbit. Fourier analysis of this time-delay curve provides the parameters of the orbit, including the period, eccentricity, angle of ascending node and time of periastron passage. Differentiating the time-delay curve yields the full radial-velocity curve directly from the Kepler photometry, without the need for spectroscopy. We show examples with $δ$ Scuti stars having large numbers of pulsation modes, including one system in which both components of the binary are pulsating. The method is straightforward to automate, thus radial velocity curves can be derived for hundreds of non-eclipsing binary stars from Kepler photometry alone.

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arXiv2014

Stability Analysis of Strange-Modes in Hot Massive Stars with Time-Dependent Convection

We carry out a nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection (TDC). In envelopes of such stars, convective luminosity is not so dominant as that in envelopes of stars in the redder side of the classical instability strip. Around the Fe opacity bump, however, convection non-negligibly contributes to energy transfer. Indeed, instability of modes excited at the Fe bump is likely to be suppressed with TDC compared with the case of adopting the frozen-in convection approximation. But we make sure that unstable strange-modes certainly appear in hot massive stars even by taking into account TDC. We also examine properties of the strange-mode instability, which is related to destabilization of strange-modes without adiabatic counterparts. In this type of instability, the phase lag between density and pressure varies from 0 to $180^{\circ}$ in an excitation zone unlike the case of the $κ$-mechanism. In addition, we confirm by comparing models with $Z=0$ and $Z=0.02$ that dominance of radiation pressure is important for this type of instability.

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arXiv2013

Analysis of strange-mode instability with time-dependent convection in hot massive stars

We carry out nonadiabatic analysis of strange-modes in hot massive stars with time-dependent convection (TDC) for the first time. Although convective luminosity in envelopes of hot massive stars is not as dominative as in stars near the red edge of the classical Cepheid instability strip in the Hertzsprung-Russell (H-R) diagram, we have found that the strange-mode instability can be affected by the treatment of convection. However, existence of the instability around and over the Humphreys-Davidson (H-D) limit is independent of the treatment. This implies that the strange-mode instability could be responsible for the lack of observed stars over the H-D limit regardless of uncertainties on convection theories.

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arXiv2012

Super-Nyquist asteroseismology with the Kepler Space Telescope

Barycentric corrections made to the timing of Kepler observations, necessitated by variations in light arrival time at the satellite, break the regular time-sampling of the data -- the time stamps are periodically modulated. A consequence is that Nyquist aliases are split into multiplets that can be identified by their shape. Real pulsation frequencies are distinguishable from these aliases and their frequencies are completely recoverable, even in the super-Nyquist regime, that is, when the sampling interval is longer than half the pulsation period. We provide an analytical derivation of the phenomenon, alongside demonstrations with simulated and real Kepler data for δSct, roAp, and sdBV stars. For Kepler data sets spanning more than one Kepler orbital period (372.5 d), there are no Nyquist ambiguities on the determination of pulsation frequencies, which are the fundamental data of asteroseismology.

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arXiv2012

Dipole low-order g mode instability of metal-poor low-mass main-sequence stars due to the $\varepsilon$-mechanism

We analyzed vibrational stability of metal-poor low-mass main-sequence stars due to the $\varepsilon$-mechanism. Since outer convection zones of the metal-poor stars is limited only to the very outer layers, the uncertainty in treatment of convection does not affect the result significantly. We found that the dipole g$_1$- and g$_2$-modes certainly become unstable due to the $\varepsilon$-mechanism for $Z\la 6\times 10^{-4}$. Besides that, we found that as the metallicity decreases the mass range of the $\varepsilon$-mechanism instability extends toward higher mass.

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arXiv2012

Vibrational Instability of Metal-Poor Low-Mass Main-Sequence Stars

We find that low-degree low-order g-modes become unstable in metal-poor low-mass stars due to the $\varepsilon$-mechanism of the pp-chain. Since the outer convection zone of these stars is limited only to the very outer layers, the uncertainty in the treatment of convection does not affect the result significantly. The decrease in metallicity leads to decrease in opacity and hence increase in luminosity of a star. This makes the star compact and results in decrease in the density contrast, which is favorable to the $\varepsilon$-mechanism instability. We find also instability for high order g-modes of metal-poor low-mass stars by the convective blocking mechanism. Since the effective temperature and the luminosity of metal-poor stars are significantly higher than those of Pop I stars, the stars showing $γ$ Dor-type pulsation are substantially less massive than in the case of Pop I stars. We demonstrate that those modes are unstable for about $1\,M_\odot$ stars in the metal-poor case.

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arXiv2012

Numerical simulations of line-profile variation beyond a single-surface approximation for oscillations in roAp stars

Prior to the last decade, most observations of roAp stars have concerned the light variations. Recently some new, striking results of spectroscopic observations with high time resolution, high spectral dispersion, and a high signal-to-noise ratio became available. Since the oscillations found in roAp stars are high overtones, the vertical wavelengths of the oscillations are so short that the amplitude and phase of the variation of each spectroscopic line are highly dependent on the level of the line profile. Hence, analyses of the variation of the spectroscopic lines of roAp stars potentially provide us with new information about the vertical structure of the atmosphere of these stars. In order to extract such information, a numerical simulation of the line-profile variation beyond a single-surface approximation is necessary. We carried out a numerical simulation of line-profile variation by taking account of the finite thickness of the line forming layer. We demonstrated how effective this treatment is, by comparing the simulation with the observed line profiles.

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arXiv2012

Fully Nonadiabatic Analysis of Vibrational Instability of Population III Stars due to the $\varepsilon$-Mechanism

A linear nonadiabatic analysis of the vibrational stability of population III main-sequence stars was carried out. It was demonstrated that, in the case of massive stars with $M \gtrsim 5\Mo$, helium burning (triple alpha reaction) starts during the main-sequence stage and produces $^{12}{\rm C}$, leading to the activation of a part of the CNO-cycle. It was found that, despite of that, those stars with $M\lesssim 13\Mo$ become unstable against the dipole g$_1$- and g$_2$-modes due to the $\varepsilon$-mechanism, during the early evolutionary phase at which the pp-chain is still the dominant nuclear energy source. The instability due to the $\varepsilon$-mechanism occurs against g-modes having a large amplitude in the off-centered $^3$He accumulation shell in the deep interior, and the growth time is much shorter than the evolutionary timescale. This instability is therefore likely to induce mixing in the stellar interior, and have a significant influence on the evolution of these stars.

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arXiv2012

FM stars: A Fourier view of pulsating binary stars, a new technique for measuring radial velocities photometrically

Some pulsating stars are good clocks. When they are found in binary stars, the frequencies of their luminosity variations are modulated by the Doppler effect caused by orbital motion. For each pulsation frequency this manifests itself as a multiplet separated by the orbital frequency in the Fourier transform of the light curve of the star. We derive the theoretical relations to exploit data from the Fourier transform to derive all the parameters of a binary system traditionally extracted from spectroscopic radial velocities, including the mass function which is easily derived from the amplitude ratio of the first orbital sidelobes to the central frequency for each pulsation frequency. This is a new technique that yields radial velocities from the Doppler shift of a pulsation frequency, thus eliminates the need to obtain spectra. For binary stars with pulsating components, an orbital solution can be obtained from the light curve alone. We give a complete derivation of this and demonstrate it both with artificial data, and with a case of a hierarchical eclipsing binary with {\it Kepler} mission data, KIC 4150611 (HD 181469). We show that it is possible to detect Jupiter-mass planets orbiting $δ$ Sct and other pulsating stars with our technique. We also show how to distinguish orbital frequency multiplets from potentially similar nonradial $m$-mode multiplets and from oblique pulsation multiplets.

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arXiv2007

Initial Helioseismic Observations by Hinode/SOT

Results from initial helioseismic observations by Solar Optical Telescope onboard Hinode are reported. It has been demonstrated that intensity oscillation data from Broadband Filter Imager can be used for various helioseismic analyses. The k-omega power spectra, as well as corresponding time-distance cross-correlation function that promises high-resolution time-distance analysis below 6-Mm travelling distance, were obtained for G-band and CaII-H data. Subsurface supergranular patterns have been observed from our first time-distance analysis. The results show that the solar oscillation spectrum is extended to much higher frequencies and wavenumbers, and the time-distance diagram is extended to much shorter travel distances and times than they were observed before, thus revealing great potential for high-resolution helioseismic observations from Hinode.

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arXiv2007

Observations of Sunspot Oscillations in G band and Ca II H line with Solar Optical Telescope on Hinode

Exploiting high-resolution observations made by the Solar Optical Telescope onboard Hinode, we investigate the spatial distribution of power spectral density of oscillatory signal in and around NOAA active region 10935. The G-band data show that in the umbra the oscillatory power is suppressed in all frequency ranges. On the other hand, in Ca II H intensity maps oscillations in the umbra, so-called umbral flashes, are clearly seen with the power peaking around 5.5 mHz. The Ca II H power distribution shows the enhanced elements with the spatial scale of the umbral flashes over most of the umbra but there is a region with suppressed power at the center of the umbra. The origin and property of this node-like feature remain unexplained.

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arXiv2001

Constraint on solar axions from seismic solar models

We reexamine the theoretical limit on the axion coupling to two photons, $g_{aγγ}$, in the light of a new solar model (seismic solar model) and the latest solar neutrino observations, the Super-Kamiokande and the Sudbury Neutrino Observatory (SNO). From the comparison of the theoretically expected and the measured neutrino fluxes, we set a limit $g_{aγγ} < 4.0\times 10^{-10} ~\mathrm{GeV} ^{-1}$. This limit based on a new procedure is about a factor of 3 improvement over the previous theoretical limit and a more severe limit than the solar axion experiments. Therefore this limit is the most stringent limit on solar axions.

hep-ph↗

arXiv2001

Solar Models with Helioseismic Constraints and the Solar Neutrino Problem

Imposing a constraint of the sound-speed profile determined from helioseismology and updating the microphysics, we have revised our seismic solar model, constructed with the assumption of a homogeneous metal abundance distribution, and have shown that the theoretically expected neutrino fluxes are still significantly more than the observations. With the same sound-speed profile constraint, we also constructed solar models with low metal abundance in the core, and evaluated the neutrino fluxes of these models to see if nonstandard solar models with a low metal core can solve the solar neutrino problem. Some of these models are in agreement with the Homestake data, the Super-Kamiokande data, and the sound-speed profile simultaneously, but none of these satisfy both the neutrino flux data, including GALLEX and SAGE, and the helioseismically determined density profile.

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