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  • 1
    Electronic Resource
    Electronic Resource
    Rapidly Oscillating Ap Stars (1990)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Astronomy and Astrophysics 28 (1990), S. 607-655 
    Details
    ISSN: 0066-4146
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.aa.28.090190.003135
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  • 2
    Electronic Resource
    Electronic Resource
    Annulation of pyridinium rings onto nitrogen heterocycles (1977)
    s.l. : American Chemical Society
    The @journal of organic chemistry 42 (1977), S. 2474-2480 
    Details
    ISSN: 1520-6904
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/jo00434a028
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  • 3
    Electronic Resource
    Electronic Resource
    Observations of pulsating Ap stars in South Africa (1995)
    Springer
    Astrophysics and space science 230 (1995), S. 29-39 
    Details
    ISSN: 1572-946X
    Keywords: Stars: Pulsation ; Stars: Peculiar A
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The rapidly oscillating Ap (roAp) stars currently represent the only main sequence stars other than the Sun which exhibit non-radial acoustic pulsations of high overtone. This makes them excellent subjects for asteroseismology, an approach which promises to yield accurate knowledge of the interior structures of stars. Of the 27 known roAp stars, 24 were discovered in Sutherland despite extensive searches conducted elsewhere. This paper reviews the discovery of the roAp phenomenon and describes the factors that contribute to the high discovery rate for these stars at Sutherland. Two long-term observational projects in progress at Sutherland are discussed,viz. the Cape roAp Star Survey and long-term monitoring of frequency variations in roAp stars.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00658165
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  • 4
    Electronic Resource
    Electronic Resource
    Radial and nonradial oscillations in Delta Scuti stars (1980)
    Springer
    Space science reviews 27 (1980), S. 377-380 
    Details
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Mode identifications from the phase shift between the B-V color curves and the V light curves of Delta Scuti stars are discussed. For five Delta Scuti stars the frequency of highest amplitude is due to pulsation in a radial mode with one or more frequencies due to nonradial modes lying nearby. Thus there may be a resonance between the frequencies of radial and nonradial modes. This behavior has only been found in subgiant and giant stars so far, but this could easily be a selection effect. Theoretical predictions of frequencies for different 1-modes of the same overtone for main sequence, subgiant, and giant stars are needed to compare with these observations. Theoretical predictions of the strength of resonance coupling between radial and nonradial modes as a function of frequency separation are also needed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00168323
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  • 5
    Electronic Resource
    Electronic Resource
    Nonlinear, nonradial pulsation in rapidly oscillating Ap stars (1993)
    Springer
    Astrophysics and space science 210 (1993), S. 207-214 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The rapidly oscillating Ap stars pulsate in high-overtone, low degreep-modes with their pulsation axes aligned with their oblique magnetic axes. They show non-linearity in their pulsation in three ways: 1) The harmonics of the basic pulsation frequency are detectable. 2) The pulsation phase seems to vary stochastically on a time scale of days to years depending on the star. 3) The form of the nonradial surface distortion is not constant with time. These three effects are illustrated with HR 3831, the best studied of the roAp stars. HR 3831 pulsates in distorted dipole mode which can be modelled as a linear sum of axisymmetricl = 0, 1, 2, and 3 spherical harmonics aligned with the magnetic axis. This gives rise to a 7-frequency multiplet split by exactly the rotation frequency. The form of the distortion shows small changes on a time-scale of years. HR 3831 shows a 5-frequency rotationally split first harmonic multiplet, a 3-frequency rotationally split second harmonic multiplet, and a single third harmonic frequency has probably been detected at an amplitude of 0.065 mmag. The first harmonic has changed its form significantly over the last 10 years. A technique for decomposing the fundamental frequency septuplet into its component spherical harmonics is used to fit the pulsation phase as a function of rotation phase. This allows a unique O-C to be defined for any length of light curve. The long term behaviour of the O-C diagram cannot be modelled adequately with a combination of periodic (Doppler shift) and quadratic (evolution) terms; there seems to be a significant stochastic component. The direction of the pulsation phase reversal at rotational phase 0.747 is indeterminate; sometimes it is a positive-going reversal, sometimes negative-going. At present it is not known whether this is a numerical artifact, or a physical effect in the star. If it is a physical effect, it means that small non-periodic differences in pulsation amplitude between the bipolar hemispheres have been detected.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00657901
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  • 6
    Electronic Resource
    Electronic Resource
    A frequency analysis of a newδ Scuti star HR 7439 (1992)
    Springer
    Astrophysics and space science 196 (1992), S. 57-64 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract HR 7439 (HD 184705) is an F0V star which we have discovered to be a low amplitude δ Scuti star. This star is multi-periodic with frequencies in the range of 17 to 22 d−1 (periods in the range 65 to 85 min). These periods are relatively short for such a cool star. Calculation of the pulsation constant Q and comparison with models indicates pulsation in the fourth and fifth overtones. Interestingly, an appropriate model calculated by Stellingwerf shows highest growth rates for the fourth and fifth overtones.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00645241
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  • 7
    Electronic Resource
    Electronic Resource
    A search for rapid oscillations in FG Sge (1986)
    Springer
    Astrophysics and space science 125 (1986), S. 311-314 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract It has recently been noted that the spectrum of the peculiar variable star FG Sge is beginning to resemble that of HD 101065 (Przybylski's star). Since Przybylski's star is known to show light variations of 0.01 mag with a period of 12.14 min, a search for similar rapid oscillations in FG Sge was carried out. This paper presents 25 hr of high-speed photometry of FG Sge which show that it has no variations in the frequency-range 0.5 to 10.0 mHz larger than 1.2 mmag peak-to-peak.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00648029
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  • 8
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    Asteroseismology of KIC 11754974: a high-amplitude SX Phe pulsator in a 343-d binary system (2013)
    Oxford University Press
    Details
    Publication Date: 2013-06-09
    Description: The candidate SX Phe star KIC 11754974 shows a remarkably high number of combination frequencies in the Fourier amplitude spectrum: 123 of the 166 frequencies in our multifrequency fit are linear combinations of independent modes. Predictable patterns in frequency spacings are seen in the Fourier transform of the light curve. We present an analysis of 180 d of short-cadence Kepler photometry and of new spectroscopic data for this evolved, late A-type star. We infer from the 1150-d, long-cadence light curve, and in two different ways, that our target is the primary of a 343-d, non-eclipsing binary system. According to both methods, the mass function is similar, f ( M ) = 0.0207 ± 0.0003 M . The observed pulsations are modelled extensively, using separate, state-of-the-art, time-dependent convection (TDC) and rotating models. The models match the observed temperature and low metallicity, finding a mass of 1.50–1.56 M . The models suggest that the whole star is metal poor, and that the low metallicity is not just a surface abundance peculiarity. This is the best frequency analysis of an SX Phe star, and the only Kepler  Sct star to be modelled with both TDC and rotating models.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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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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  • 10
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    KIC 4768731: a bright long-period roAp star in the Kepler field (2015)
    Oxford University Press
    Details
    Publication Date: 2015-08-06
    Description: We report the identification of 61.45 d –1 (711.2 μHz) oscillations, with amplitudes of 62.6 μmag, in KIC 4768731 (HD 225914) using Kepler photometry. This relatively bright ( V  = 9.17) chemically peculiar star with spectral type A5 Vp SrCr(Eu) has previously been found to exhibit rotational modulation with a period of 5.21 d. Fourier analysis reveals a simple dipole pulsator with an amplitude that has remained stable over a 4-yr time span, but with a frequency that is variable. Analysis of high-resolution spectra yields stellar parameters of T eff  = 8100 ± 200 K, log g  = 4.0 ± 0.2, [Fe/H] = +0.31 ± 0.24 and v sin i  = 14.8 ± 1.6 km s –1 . Line profile variations caused by rotation are also evident. Lines of Sr, Cr, Eu, Mg and Si are strongest when the star is brightest, while Y and Ba vary in antiphase with the other elements. The abundances of rare earth elements are only modestly enhanced compared to other roAp stars of similar T eff and log g . Radial velocities in the literature suggest a significant change over the past 30 yr, but the radial velocities presented here show no significant change over a period of 4 yr.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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