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  • 1
    Online Resource
    Online Resource
    Progress of Seismology of the Sun and Stars : Proceedings of the Oji International Seminar Held at Hakone, Japan, 11-14 December 1989 (1990)
    Berlin, Heidelberg : Springer Berlin Heidelberg
    Details Availability
    Keywords: Physics ; Geophysics ; Observations, Astronomical ; Astronomy Observations ; Astrophysics ; Physics ; Physical geography ; Astronomy ; Astrophysics ; Physics ; Astronomy ; Geophysics/Geodesy ; Astrophysics
    Description / Table of Contents: Helio- and asteroseismology are fast- developing new fields of research that probe the internal structure of stars. The complicated multi-periodic oscillations are studied from both theoretical and observational points of view. Nine articles review the state of the art, including modeling the sun, excitations of oscillations, inverse problems, and the observations of seismic phenomena. One section is devoted to the seismology of stars, a field of research still in its very early development. In addition the reader will find about forty research papers on these subjects
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (XIII, 467 p. 49 illus., 1 illus. in color)
    Edition: Springer eBook Collection. Physics and Astronomy
    ISBN: 9783540466451 , 9783540530916
    Series Statement: Lecture Notes in Physics 367
    URL: http://dx.doi.org/10.1007/3-540-53091-6
    URL: https://swbplus.bsz-bw.de/bsz749206276cov.jpg
    DOI: 10.1007/3-540-53091-6
    Language: English
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  • 2
    Electronic Resource
    Electronic Resource
    Theoretical eigenfrequencies of solar oscillations of low harmonic degree l in five-minute range (1983)
    Springer
    Solar physics 82 (1983), S. 231-232 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have calculated eigenfrequencies of radial and nonradial p-mode oscillations with low harmonic index l (l = 0, 1, 2, 3, and 4) for a standard solar model with normal composition and appoximately the correct age. It is found that theoretical eigenfrequencies calculated for our standard model agree approximately with observed peaks in the power spectra for the full-disk five minute oscillation of the Sun (Claverie et al., 1980; Grec et al., 1983; Scherrer et al., 1983) in agreement with other recent works (Christensen-Dalsgaard and Gough, 1980; Scuflaire et al., 1981). However, there still remains a slight discrepancy between theory and observations in such a sense that the theoretical eigenfrequencies are slightly lower than observations (see Figure 1). Eigenfrequencies of nonradial p-modes with high degree l for the same model are also calculated for comparison with observations of the conventional five-minute oscillation with shorter horizontal wavelength. It is found that theoretical eigenfrequencies lie slightly above the observed ridges in the diagnostic (k, ω)-diagram, which is in accord with Ulrich and Rhodes (1977) and Berthomieu et al. (1980) because our standard model has a slightly shallow convective zone. It remains to be seen whether improvement in the equilibrium model can remove this small discrepancy, concurrently with a better agreement between theory and observation for the whole-disk oscillations of low degree. A full account of this work will be found in Shibahashi and Osaki (1981).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00145563
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Dynamics of a cloud of fast electrons travelling through the plasma (1976)
    Springer
    Solar physics 46 (1976), S. 323-346 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Numerical analysis has been carried out on the one-dimensional quasi-linear relaxation of a group of fast electrons travelling through the plasma. It is demonstrated that the electron velocity distribution of fast electrons tends to be a plateau form exciting the electron plasma waves and that the plasma waves are almost completely reabsorbed later by electrons arriving later. Both the velocity range and time interval in which quasi-plateau distribution is formed increase with distance from the origin of the fast electrons. There is no net energy loss of the electron cloud during the travel through the plasma if we neglect both the collisional losses and the scattering of plasma waves. Although the present computation is preliminary and limited to rather low beam density, we can see that the characteristics of both the electron beam and the plasma waves tend, with distance, to those of the analytical solution given by Ryutov and Sagdeev; though a modification to set a low velocity cutoff on the plasma waves due to the thermal electrons is necessary.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00149860
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  • 4
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    Hot DAVs: a probable new class of pulsating white dwarf stars (2013)
    Oxford University Press
    Details
    Publication Date: 2013-05-26
    Description: We have discovered a pulsating DA white dwarf at the lower end of the temperature range 45 000–30 000 K where a few helium atmosphere white dwarfs are known. There are now three such pulsators known, suggesting that a new class of theoretically predicted pulsating white dwarf stars exists. We name them the hot DAV stars. From high-speed photometric observations with the ULTRACAM photometer on the 4.2-m William Herschel Telescope, we show that the hydrogen atmosphere white dwarf star WD1017–138 pulsates in at least one mode with a frequency of 1.62 mHz (a period of 624 s). The amplitude of that mode was near 1 mmag at a 10 confidence level on one night of observation and an 8.4 confidence level on a second night. The combined data have a confidence level of 11.8. This supports the two other detections of hot DAV stars previously reported. From three Very Large Telescope Ultraviolet and Visual Echelle Spectrograph spectra we confirm also that WD1017–138 is a hydrogen atmosphere white dwarf with no trace of helium or metals with T eff  = 32 600 K, log g  = 7.8 (cgs) and M  = 0.55 M . The existence of pulsations in these DA white dwarfs at the cool edge of the 45 000–30 000 K temperature range supports the thin hydrogen layer model for the deficit of helium atmosphere white dwarfs in this range. DA white dwarfs with thick hydrogen layers do not have the superadiabatic, chemically inhomogeneous (μ-gradient) zone that drives pulsation in this temperature range. The potential for higher amplitude hot DAV stars exists; their discovery would open the possibility of a direct test of the explanation for the deficit of helium atmosphere white dwarfs at these temperatures by asteroseismic probing of the atmospheric layers of the hot DAV stars. A search for pulsation in a further 22 candidates with ULTRACAM on the European Southern Observatory New Technology Telescope gave null results for pulsation at precisions in the range 0.5–3 mmag, suggesting that the pulsation amplitudes in such stars are relatively low, hence near the detection limit with the ground-based telescopes used in the survey.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Nearly uniform internal rotation of solar-like main-sequence stars revealed by space-based asteroseismology and spectroscopic measurements (2015)
    Oxford University Press
    Details
    Publication Date: 2015-07-27
    Description: The rotation rates in the deep interior and at the surface of 22 main-sequence stars with masses between 1.0 and 1.6 M are constrained by combining asteroseismological analysis with spectroscopic measurements. The asteroseismic data of each star are taken by the Kepler or CoRoT space mission. It is found that the difference between the surface rotation rate and the average rotation rate (excluding the convective core) of most of stars is small enough to suggest that an efficient process of angular momentum transport operates during and/or before the main-sequence stage of stars. If each of the surface convective zone and the underlying radiative zone, for individual stars, is assumed to rotate uniformly, the difference in the rotation rate between the two zones turns out to be no more than a factor of 2 in most of the stars independently of their ages.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Shape of a slowly rotating star measured by asteroseismology (2016)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2016-11-19
    Description: Stars are not perfectly spherically symmetric. They are deformed by rotation and magnetic fields. Until now, the study of stellar shapes has only been possible with optical interferometry for a few of the fastest-rotating nearby stars. We report an asteroseismic measurement, with much better precision than interferometry, of the asphericity of an A-type star with a rotation period of 100 days. Using the fact that different modes of oscillation probe different stellar latitudes, we infer a tiny but significant flattening of the star’s shape of R / R = (1.8 ± 0.6) x 10 –6 . For a stellar radius R that is 2.24 times the solar radius, the difference in radius between the equator and the poles is R = 3 ± 1 km. Because the observed R / R is only one-third of the expected rotational oblateness, we conjecture the presence of a weak magnetic field on a star that does not have an extended convective envelope. This calls to question the origin of the magnetic field.
    Electronic ISSN: 2375-2548
    Topics: Natural Sciences in General
    Published by American Association for the Advancement of Science (AAAS)
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  • 7
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    Finding binaries from phase modulation of pulsating stars with Kepler - IV. Detection limits and radial velocity verification (2016)
    Oxford University Press
    Details
    Publication Date: 2016-07-29
    Description: We explore the detection limits of the phase modulation (PM) method of finding binary systems among multiperiodic pulsating stars. The method is an attractive way of finding non-transiting planets in the habitable zones of intermediate-mass stars, whose rapid rotation inhibits detections via the radial velocity (RV) method. While oscillation amplitudes of a few mmag are required to find planets, many  Scuti stars have these amplitudes. In suboptimal cases where the signal to noise of the oscillations is lower, low-mass brown dwarfs (~13 M Jup ) are detectable at orbital periods longer than about 1 yr, and the lowest mass main-sequence stars (0.1–0.2 M ) are detectable at all orbital periods where the PM method can be applied. We use purpose-written Markov chain Monte Carlo (MCMC) software for the calculation of the PM orbits, which offers robust uncertainties for comparison with RV solutions. Using Kepler data and ground-based RVs, we verify that these two methods are in agreement, even at short orbital periods where the PM method undersamples the orbit. We develop new theory to account for the undersampling of the time delays, which is also necessary for the inclusion of RVs as observational data in the MCMC software. We show that combining RVs with time delays substantially refines the orbits because of the complementarity of working in both the spatial (PM) and velocity (RV) domains simultaneously. Software outputs were tested through an extensive hare-and-hounds exercise, covering a wide range of orbital configurations including binaries containing two pulsators.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Super-Nyquist asteroseismology with the Kepler Space Telescope (2013)
    Oxford University Press
    Details
    Publication Date: 2013-03-23
    Description: 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, i.e. 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  Sct, 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.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Deriving the orbital properties of pulsators in binary systems through their light arrival time delays (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-24
    Description: We present the latest developments to the phase modulation method for finding binaries among pulsating stars. We demonstrate how the orbital elements of a pulsating binary star can be obtained analytically, that is, without converting time delays to radial velocities by numerical differentiation. Using the time delays directly offers greater precision, and allows the parameters of much smaller orbits to be derived. The method is applied to KIC 9651065, KIC 10990452 and KIC 8264492, and a set of the orbital parameters is obtained for each system. Radial velocity curves for these stars are deduced from the orbital elements thus obtained.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    A unifying explanation of complex frequency spectra of {gamma} Dor, SPB and Be stars: combination frequencies and highly non-sinusoidal light curves (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-13
    Description: There are many Slowly Pulsating B (SPB) stars and Dor stars in the Kepler mission data set. The light curves of these pulsating stars have been classified phenomenologically into stars with symmetric light curves and with asymmetric light curves. In the same effective temperature ranges as the Dor and SPB stars, there are variable stars with downward light curves that have been conjectured to be caused by spots. Among these phenomenological classes of stars, some show ‘frequency groups’ in their amplitude spectra that have not previously been understood. While it has been recognized that non-linear pulsation gives rise to combination frequencies in a Fourier description of the light curves of these stars, such combination frequencies have been considered to be a only a minor constituent of the amplitude spectra. In this paper, we unify the Fourier description of the light curves of these groups of stars, showing that many of them can be understood in terms of only a few base frequencies, which we attribute to g-mode pulsations, and combination frequencies, where sometimes a very large number of combination frequencies dominate the amplitude spectra. The frequency groups seen in these stars are thus tremendously simplified. We show observationally that the combination frequencies can have amplitudes greater than the base frequency amplitudes, and we show theoretically how this arises. Thus for some Dor and SPB stars, combination frequencies can have the highest observed amplitudes. Among the B stars are pulsating Be stars that show emission lines in their spectra from occasional ejection of material into a circumstellar disc. Our analysis gives strong support to the understanding of these pulsating Be stars as rapidly rotating SPB stars, explained entirely by g-mode pulsations.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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