Search Results - (Author, Cooperation:D. W. Kurtz)

2011-09-16

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

1572-946X

Stars: Pulsation ; Stars: Peculiar A

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource

1572-946X

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource

1572-946X

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource

1572-946X

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource

1572-9672

Springer Online Journal Archives 1860-2000

Physics

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.

Electronic Resource