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Hideyuki Saio

Publications and source records attributed to Hideyuki Saio.

At least 73 records · Page 4Linked to original sources
arXiv2013

The puzzle of the CNO abundances of α Cygni variables resolved by the Ledoux criterion?

Recent stellar evolution computations show that the blue supergiant (BSG) stars could come from two distinct populations: a first group arising from massive stars that just left the main sequence (MS) and are crossing the Hertzsprung-Russell diagram (HRD) towards the red supergiant (RSG) branch, and a second group coming from stars that have lost considerable amount of mass during the RSG stage and are crossing the HRD for a second time towards the blue region. Due to very different luminosity-to- mass ratio, only stars from the second group are expected to have excited pulsations observable at the surface. In a previous work, we have shown that our models were able to reproduce the pulsational properties of BSGs. However, these models failed to reproduce the surface chemical composition of stars evolving back from a RSG phase. In this paper, we show how the use of the Ledoux criterion instead of the Schwarzschild one for convection allows to significantly improve the agreement with the observed chemical composition, while keeping the agreement with the pulsation periods. This gives some support to the Ledoux criterion.

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arXiv2013

Pulsation of magnetic stars

Some Ap stars with strong magnetic fields pulsate in high order p-modes; they are called roAp (rapidly oscillating Ap) stars. The p-mode frequencies are modified by the magnetic fields. Although the large frequency separations are hardly affected, small separations are modified considerably. The magnetic field also affects the latitudinal amplitude distribution on the surface. We discuss the property of axisymmetric p-mode oscillations in roAp stars.

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arXiv2013

A- and B-type star pulsations in the Kepler and CoRoT era: theoretical considerations

Among A-type main-sequence variables, pulsations of $δ$ Sct and $γ$ Dor variables are driven in the He II ionization zone, while H-ionization zone and strong magnetic fields seem to play roles in the excitation of high-order p-modes in rapidly oscillating Ap (roAp) stars. Pulsations in B-type variables, $β$ Cephei and slowly pulsating B (SPB) stars are excited by the $κ$-mechanism at the Fe-opacity bump at $T\approx 2\times10^5$K. In addition, the strange-mode instability seems responsible for the excitation of pulsations in luminous AB-supergiants ($α$ Cygni variables). We discuss excitation mechanisms for pulsations in A- and B-type variables stars.

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arXiv2013

Four open questions in massive star evolution

We discuss four questions dealing with massive star evolution. The first one is about the origin of slowly rotating, non-evolved, nitrogen rich stars. We propose that these stars may originate from initially fast rotating stars whose surface has been braked down. The second question is about the evolutionary status of alpha-Cygni variables. According to their pulsation properties, these stars should be post red supergiant stars. However, some stars at least present surface abundances indicating that they should be pre red supergiant stars. How to reconcile these two contradictory requirements? The third one concerns the various supernova types which are the end point of the evolution of stars with initial masses between 18 and 30 Msun, i.e. the most massive stars which go through a red supergiant phase during their lifetime. Do they produce types IIP, IIL, IIn, IIb or Ib supernovae or do they end without producing any SN event? Finally, we shall discuss reasons why so few progenitors of type Ibc supernovae have yet been detected?

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arXiv2013

Strange mode instability for micro-variations in Luminous Blue Variables

If a massive star has lost significant mass during its red-supergiant stage, it would return to blue region in the HR diagram and spend a part of the core-He burning stage as a blue supergiant having a luminosity to mass ratio (L/M) considerably larger than about 10^4 (in solar units); the duration depends on the degree of internal mixing and on the metallicity. Then, various stellar pulsations are excited by enhanced κ-mechanism and strange mode instability. Assuming these pulsations to be responsible for (at least some of) the quasi-periodic light and radial-velocity variations in αCygni variables including luminous blue variables (LBVs; or S Dor variables), we can predict masses and surface compositions for these variables, and compare them with observed ones to constrain the evolutionary models. We discuss radial pulsations excited in evolutionary models of an initial mass of 40 M_\odot with solar metallicity of Z=0.014, and compare them to micro-variations in the two Galactic LBVs, HR Car and HD 160529. We have found that these stars should has lost more than half of the initial mass and their surface CNO abundances should be significantly modified from the original ones showing partial H-burning products.

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arXiv2013

Evolution of blue supergiants and αCygni variables; Puzzling CNO surface abundances

A massive star can enter the blue supergiant region either evolving directly from the main-sequence, or evolving from a previous red supergiant stage. The fractions of the blue supergiants having different histories depend on the internal mixing and mass-loss during the red supergiant stage. We study the possibility to use diagnostics based on stellar pulsation to discriminate blue supergiants having different evolution histories. For this purpose we have studied the pulsation property of massive star models calculated with the Geneva stellar evolution code for initial masses ranging from 8 to 50 M$_\odot$ with a solar metallicity of $Z=0.014$. We have found that radial pulsations are excited in the blue-supergiant region only in the models that had been red-supergiants before. This would provide us with a useful mean to diagnose the history of evolution of each blue-supergiant. At a given effective temperature, much more nonradial pulsations are excited in the model after the red-supergiant stage than in the model evolving towards the red-supergiant. The properties of radial and nonradial pulsations in blue supergiants are discussed. Predicted periods are compared with period ranges observed in some α-Cygni variables in the Galaxy and NGC 300. We have found that blue supergiant models after the red-supergiant stage roughly agree with observed period ranges in most cases. However, we are left with the puzzle that the predicted surface N/C and N/O ratios seem to be too high compared with those of Deneb and Rigel.

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arXiv2013

On the pulsation modes of OGLE small amplitude red giant variables in the LMC

We discuss the properties of pulsations in the OGLE Small Amplitude Red Giants (OSARGs) in the Large Magellanic Cloud (LMC). We consider stars below the red-giant tip in this paper. They are multi-periodic and form three sequences in the period-luminosity plane. Comparing the periods and period ratios with our theoretical models, we have found that these sequences correspond to radial first to third overtones, and nonradial dipole p$_4$ and quadrupole p$_2$ modes. The red-giant branch stars of OSARGs consist of stars have initial masses of $\sim0.9 - 1.4M_\odot$ which corresponds to a luminosity range of $\log L/L_\odot \simeq 2.8 - 3.4$. With these parameters, the scaled optimal frequency $ν_{\rm max}$ for solar-like oscillations goes through roughly the middle of the three sequences in the period-luminosity plane, suggesting the stochastic excitation is likely the cause of the pulsations in OSARGs.

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arXiv2013

How the mass-loss rates of red-supergiants determine the fate of massive stars ?

Mass-loss rates are one of the most relevant parameters determining the evolution of massive stars. In particular, the rates at which the star loses mass during the red-supergiant (RSG) phase is the least constrained by the observations or theory. In this paper, we show how the mass loss during the RSG phase affects the later evolution of the star, as well as the final type of supernova towards which it leads. We also discuss some possibilities to discriminate between blue stars that went through a RSG phase and those which remained in the blue part of the Hertzsprung-Russell diagram.

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arXiv2013

Prospects for asteroseismology of rapidly rotating B-type stars

In rapidly rotating stars Coriolis forces and centrifugal deformations modify the properties of oscillations; the Coriolis force is important for low-frequency modes, while the centrifugal deformation affects mainly p-modes. Here, we discuss properties of g- and r-mode oscillations in rotating stars. Predicted frequency spectra of high-order g-modes (and r-modes) excited in rapidly rotating stars show frequency groupings associated with azimuthal order $m$. We compare such properties with observations in rapidly rotating Be stars, and discuss what is learnt from such comparisons.

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arXiv2013

Models of rotating massive stars: impacts of various prescriptions

The rotation of stars has many interesting and important consequences for the photometric and chemical evolution of galaxies. Many of the predictions of models of stellar rotation are now compared with observations of surface abundances and velocities, with interferometric studies of fast rotating stars, with internal rotation profiles as they can be deduced by asteroseismology, to cite just a few observational constraints. In this paper, we investigate how the outputs of models depend on the prescriptions used for the diffusion coefficients included in the shellular rotating models. After recalling the various prescriptions found in the literature, we discuss their impacts on the evolutionary tracks and lifetimes of the Main-Sequence (MS) phase, the changes of the surface composition and velocities during the MS phase, the distribution of the core helium lifetime in the blue and the red part of the HR diagram, the extensions of the blue loops, the evolution of the angular momentum of the core, and the synthesis of primary nitrogen in fast-rotating metal-poor massive stars. While some of these outputs depend only slightly on the prescriptions used (for instance, the evolution of the surface velocities), most of them show a significant dependence. The models which best fit the changes of the surface abundances are those computed with the vertical shear diffusion coefficient of Maeder (1997) and the horizontal shear diffusion coefficient by Zahn (1992).

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arXiv2011

A Single Degenerate Progenitor Model for Type Ia Supernovae Highly Exceeding the Chandrasekhar Mass Limit

Recent observations of Type Ia supernovae (SNe Ia) suggest that some of the progenitor white dwarfs (WDs) had masses up to 2.4-2.8 M_sun, highly exceeding the Chandrasekhar mass limit. We present a new single degenerate (SD) model for SN Ia progenitors, in which the WD mass possibly reaches 2.3-2.7 M_sun. Three binary evolution processes are incorporated; optically thick winds from mass-accreting WDs, mass-stripping from the binary companion star by the WD winds, and WDs being supported by differential rotation. The WD mass can increase by accretion up to 2.3 (2.7) M_sun from the initial value of 1.1 (1.2) M_sun, being consistent with high luminosity SNe Ia such as SN 2003fg, SN 2006gz, SN 2007if, and SN 2009dc. There are three characteristic mass ranges of exploding WDs. In an extreme massive case, differentially rotating WDs explode as an SN Ia soon after the WD mass exceeds 2.4 M_sun because of a secular instability at T/|W|\sim 0.14. For a mid mass range of M_WD=1.5-2.4 M_sun, it takes some time (spinning-down time) until carbon is ignited to induce an SN Ia explosion after the WD mass has reached maximum, because it needs a loss or redistribution of angular momentum. For a lower mass case of rigidly rotating WDs, M_WD=1.38-1.5 M_sun, the spinning-down time depends on the timescale of angular momentum loss from the WD. The difference in the spinning-down time may produce the "prompt" and "tardy" components. We also suggest the very bright super-Chandrasekhar mass SNe Ia are born in a low metallicity environment.

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arXiv2011

Physics of stars understood/expected from asteroesismology

What can be learned about the physics of stellar interiors from studying stellar oscillations? This review address the potential to improve our understandings of convective core overshoot and of more general convection-related effects, microscopic diffusion, rotation and magnetic fields, and finally evolution-induced period changes.

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arXiv2011

Double white dwarf mergers and elemental surface abundances in extreme helium and R Coronae Borealis stars

The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB) stars are discussed in terms of the merger of a carbon-oxygen white dwarf with a helium white dwarf. The model is expressed as a linear mixture of the individual layers of both constituent white dwarfs, taking account of the specific evolution of each star. In developing this recipe from previous versions, particular attention has been given to the inter-shell abundances of the asymptotic giant branch star which evolved to become the carbon-oxygen white dwarf. Thus the surface composition of the merged star is estimated as a function of the initial mass and metallicity of its progenitor. The question of whether additional nucleosynthesis occurs during the white dwarf merger has been examined. The high observed abundances of carbon and oxygen must either originate by dredge-up from the core of the carbon-oxygen white dwarf during a cold merger or be generated directly by alpha-burning during a hot merger. The presence of large quantities of O18 may be consistent with both scenarios, since a significant O18 pocket develops at the carbon/helium boundary in a number of our post-AGB models. The production of fluorine, neon and phosphorus in the AGB intershell produces n overabundance at the surface of the merged stars, but generally not in sufficient quantity. However, the evidence for an AGB origin for these elements points to progenitor stars with initial masses in the range 1.9 - 3 solar masses. There is not yet sufficient information to discriminate the origin (fossil or prompt) of all the abundance anomalies observed in EHe and RCB stars. Further work is required on argon and s-process elements in the AGB intershell, and on the predicted yields of all elements from a hot merger.

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arXiv2010

Linear analyses for the stability of radial and nonradial oscillations of massive stars

In order to understand the periodic and semi-periodic variations of luminous O- B- A-type stars, linear nonadiabatic stability analyses for radial and nonradial oscillations have been performed for massive evolutionary models ($8M_\odot - 90M_\odot$). In addition to radial and nonradial oscillations excited by the kappa-mechanism and strange-mode instability, we discuss the importance of low-degree oscillatory convection (nonadiabatic g$^-$) modes. Although their kinetic energy is largely confined to the convection zone generated by the Fe opacity peak near $2\times10^5$K, the amplitude can emerge to the photosphere and should be observable in a certain effective temperature range. They have periods longer than those of the radial strange modes so that they seem to be responsible for some of the long-period microvariations of LBVs (S Dor variables) and $α$ Cyg variables. Moreover, monotonously unstable radial modes are found in some models whose initial masses are greater than or equal to $60M_\odot$ with $Z=0.02$. The monotonous instability probably corresponds to the presence of an optically thick wind. The instability boundary roughly coincides with the Humphreys-Davidson limit.

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arXiv2010

Stability of g-modes in rotating B-type stars

We have studied the stability of low degree $g$-modes in uniformly rotating B-type stars, taking into account the effects of the Coriolis force and the rotational deformation. From an analysis treating rotation frequency as a small parameter it is found that slow rotation tends to $destabilize$ high radial-order $retrograde$ $g$-modes, although the effect is very small or absent for relatively low order modes. Calculating eigenfrequencies at selected rotation rates, we find, on the other hand, that rapid rotation tends to $stabilize$ $retrograde$ $g$-modes. The stabilizing effect appears stronger for less massive B-type stars having low effective temperatures. If we change rotation rate continuously, the frequency of a $g$-mode belonging to ($l', m$) crosses frequencies of other $g$-modes belonging to ($l', m$). If the parity of the two encountering modes are the same, they interact each other and the stability (i.e., imaginary part of eigenfrequency) of each mode is modified. Using an asymptotic method we discuss the property of such mode crossings and couplings. For rapidly rotating stars mode couplings are important for the stability of low degree $g$-modes. In particular, we find that the stabilization of retrograde $g$-modes in rapidly rotating stars is due to many strong mode couplings, while %prograde sectoral% modes are exceptionally immune to the damping effects from the mode couplings.

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arXiv2010

Radial and nonradial oscillations of massive supergiants

Stability of radial and nonradial oscillations of massive supergiants is discussed. The kappa-mechanism and strange-mode instability exciteoscillations having various periods in wide ranges of the upper part of the HR diagram. In addition, in very luminous ($\log L/L_\odot \gtrsim 5.9$) models, monotonously unstable modes exist, which probably indicates the occurrence of optically thick winds. The instability boundary is not far from the Humphreys-Davidson limit. Furthermore, it is found that there exist low-degree($\ell = 1, 2$) oscillatory convection modes associated with the Fe-opacity peak convection zone, and they can emerge to the stellar surface so that they are very likely observable in a considerable range in the HR diagram. The convection modes have periods similar to g-modes, and their growth-times are comparable to the periods. Theoretical predictions are compared with some of the supergiant variables.

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arXiv2010

Kepler observations of a roAp star: delta Scuti and gamma Doradus pulsations in Ap stars

Observations of the A5p star KIC 8677585 obtained during the Kepler 10-d commissioning run with 1-min time resolution show that it is a roAp star with several frequencies with periods near 10 min. In addition, a low frequency at 3.142 cycles/day is also clearly present. Multiperiodic gamma Doradus and delta Scuti pulsations, never before seen in any Ap star, are present in Kepler observations of at least three other Ap stars. Since gamma Doradus pulsations are seen in Ap stars, it is likely that the low-frequency in KIC 8677585 is also a gamma Doradus pulsation. The simultaneous presence of both gamma Doradus and roAp pulsations and the unexpected detection of delta Scuti and gamma Doradus pulsations in Ap stars present new opportunities and challenges for the interpretation of these stars.

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