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ABSTRACT: Cyclic frequency variations have been detected in several rapidly-oscillating Ap stars. We describe our long-term monitoring
projects, entailing nightly, or weekly, runs of 1 hour or more of high-speed photometry on each star, which have now been
running for two years, and which will run for years to come. We place special emphasis on several advantages that the Sutherland
site of the South African Astronomical Observatory has over all other observatories for projects of this sort. We give a detailed
discussion of many pitfalls in high-speed photometry which, if unheeded, preclude many observatories from producing observations
of useful quality for studies of sub-mmag amplitude variations.
Keywordslong-term monitoring–period changes–rapidly-oscillating Ap stars
07/2011: pages 185-196;
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ABSTRACT: To search photometric variability in chemically peculiar A type stars in the northern hemisphere. High-speed photometric observations of Ap and Am star candidates have been carried out from ARIES (Manora Peak, Nainital) using a three-channel fast photometer attached to the ARIES 104-cm Sampurnanand telescope. This paper presents three new variables: HD 113878, HD 118660 and HD 207561. During the time span of the survey (1999 December to 2004 January) pulsations of the $\delta$ Sct type were also found for the two evolved Am stars HD 102480 and HD 98851, as reported in Joshi et al. (2002, 2003). Additionally, we present 140 null results of the survey for this time span. The star HD 113878 pulsates with a period of 2.31 hr, which is typical of $\delta$ Sct stars. HD 118660 exhibits multi-periodic variability with a prominent period of nearly 1 hr. These periods need to be investigated and make HD 118660 a particularly interesting target for further observations. For HD 207561, a star classified as Am, a probable pulsation with a period of 6 min was found in the light curves obtained on two consecutive nights. Both HD 102480 and HD 98851 exhibit unusual alternating high and low amplitude maxima, with a period ratio of 2:1. The analysis of the null results confirms the photometric quality of the Nainital Comment: 14 pages, 13 figures, Accepted for publication in A&A
05/2006;
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ABSTRACT: We present new high-speed, multisite photometric observations of the rapidly oscillating Ap star HD 119027 acquired over seven nights during 1996. A frequency analysis of these observations reveals the presence of oscillations at 1835, 1875, 1888, 1913, 1940, 1942 and (possibly) 1953 μHz. These frequencies are consistent with a spacing of either 13 or 26 μHz, depending on the reality of the oscillations at 1875 and 1953 μHz. The data in hand do not permit us to discriminate between the two possible spacings. If the smaller value of the spacing is correct, it suggests that HD 119027 is outside the main-sequence band. Two of the frequencies listed above are separated by only 1.95 μHz, suggesting that they are modes of (n,ℓ) and (n − 1, ℓ + 2), which in roAp stars is a quantity governed by the internal magnetic field.
Monthly Notices of the Royal Astronomical Society 04/2002; 300(1):188 - 192. · 4.90 Impact Factor
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Information Bulletin on Variable Stars. 01/2000; 4853:1.
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Peter Martinez,
D. W. Kurtz,
B. N. Ashoka,
U. S. Chaubey,
S. K. Gupta,
F. Leone,
G. Catanzaro,
R. Sagar,
E. Raj,
S. Seetha,
K. Kasturirangan
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ABSTRACT: Pulsation in cool main-sequence Am stars is a rare phenomenon observed in a few marginal Am stars and only one classical Am star. The chemically peculiar star HD 13079 is shown to be pulsating with a 78-min period and a peak-to-peak B amplitude of 0.02 mag. The Hipparcos parallax, uvbyβ photometry and pulsations together suggest that HD 13079 is an Am star near the zero-age main sequence, and that it is a fundamental-mode pulsator on the red edge of the instability strip.
Monthly Notices of the Royal Astronomical Society 11/1999; 309(4):871 - 874. · 4.90 Impact Factor
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Information Bulletin on Variable Stars. 02/1999; 4677:1.
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S. J. Kleinman,
R. E. Nather,
D. E. Winget,
J.C. Clemens,
P. A. Bradley,
A. Kanaan,
J. L. Provencal,
C. F. Claver,
T. K. Watson,
K. Yanagida, [......], Peter Martinez,
G. Vauclair,
N. Dolez,
M. Chevreton,
M. A. Barstow,
S. O. Kepler,
O. Giovannini,
T. Augusteijn,
C. J. Hansen,
S. D. Kawaler
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ABSTRACT: The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post main sequence evolution, along with their cooling rates, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously been explored through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning ten years, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modelling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modelling, thereby joining the rest of the known white dwarf pulsators. Comment: 29 pages including 5 figures To appear in ApJ 1 Mar 98
11/1997;
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ABSTRACT: New high-speed photometric observations of the rapidly oscillating Ap
star HD80316 suggest that its principal pulsation mode is an oblique
dipole mode with a frequency of either 2251.68 or 2254.47 muHz (P=7.4
min). Rotationally split frequency triplets give two choices for the
rotation period, P_rot=2.085+/-0.005 or 4.163+/-0.010d, which agree with
possible rotation periods derived independently from mean light
observations. Both frequency triplet possibilities show the sum of the
rotational inclination, i, and the magnetic obliquity, beta, to be
greater than 90 deg. This makes HD80316 only the third roAp star, after
HR3831 and HD6532, to have a polarity-reversing dipole pulsation. The
two possible rotation periods are so close to multiples of 1 d^-1 that
alias problems in our single-site observations preclude a definite
solution for the oscillations of this star. Further progress requires
multisite observations, or long-term single-site observations to sample
all rotation phases.
Monthly Notices of the Royal Astronomical Society 07/1997; 289:645-650. · 4.90 Impact Factor
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ABSTRACT: We have analysed 77 h of new, multisite observations of HD 6532 obtained
in 1994, along with 20 h of observations from 1984 and 90 h from 1985. A
frequency quintuplet with frequency separations equal to exactly the
rotation frequency, and two harmonic frequencies, give a complete
solution. The amplitude spectrum of the residuals shows white noise with
its highest peaks of amplitude 0.1 mmag. We interpret the frequency
quintuplet in terms of a distorted oblique dipole mode, which we model
with a spherical harmonic series that is primarily dipolar with small
radial and quadrupole contributions. This model does not fully describe
the rotational modulation of pulsation amplitude and phase, which leads
us to suggest that a completely decentred dipole model needs
development. HD 6532 is only the second roAp star, after HR 3831, with a
polarity-reversing pulsation mode which has been studied in sufficient
detail to characterize the geometry of the mode.
Monthly Notices of the Royal Astronomical Society 07/1996; 281:883-892. · 4.90 Impact Factor
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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.
Astrophysics and Space Science 01/1995; 230(1):29-39. · 1.69 Impact Factor
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ABSTRACT: Results of observations of the new rapidly oscillating Ap star HD 119027
are reported. It is found that HD 119027 pulsates with at least five
nearly equally spaced frequencies in the vicinity of 1.9 mHz, with a
spacing of 26 microhertz between the frequencies. The data, however,
cannot be completely described by these five frequencies. There is a
residual mound of amplitude in the vicinity of 1.9 mHz for all data
subsets, suggesting a nonperiodic amplitude or phase modulation of these
five frequency components. There is no indication of the first or higher
harmonics of the frequencies at 1.9 mHz.
Monthly Notices of the Royal Astronomical Society 12/1992; 260:9-20. · 4.90 Impact Factor
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ABSTRACT: New high-speed photometric observations of HR 3831 were obtained in 1991
from the South African Astronomical Observatory and Cerro Tololo
Inter-American Observatory over a time span of 17 d with a 41 percent
duty cycle. A frequency analysis shows that this object pulsates in a
single mode which is rotationally modulated, giving rise to a
fundamental frequency septuplet, a first harmonic quintuplet, a second
harmonic triplet, and a probable detection of a single frequency at the
third harmonic. The fundamental frequency septuplet is described by a
sum of axisymmetric spherical harmonics with l = 0, 2, and 3. The
dominant component of the sum is in a mode called a distorted dipole,
whose form changes slightly from year to year.
Monthly Notices of the Royal Astronomical Society 12/1992; 260:343-364. · 4.90 Impact Factor
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ABSTRACT: HR 3831 is a rapidly oscillating Ap star with a pulsational period of
11.67 min. The amplitude of this pulsation ranges from 0 to about 5 mmag
as a function of the rotational aspect of the star. We use 268 hr of new
high-speed photometric observations obtained in 1990 and 1991 plus 238
hi of published high-speed photometry from 1980, 1981, 1985 and 1986 to
show that the period of pulsation amplitude modulation is equal to the
period of mean-light variation. We use this to show that an
interpretation of the pulsation-amplitude modulation as rotationally
perturbed m-modes requires that Cnl ≤ 2 ×
10-5 at the 3σ confidence level, making that
interpretation highly unlikely. We also show that the time of pulsation
maximum precedes the time of mean-light extremum by 0.177±0.007 d
(0.062±0.002 rotation periods), a 25σ difference. The time
of magnetic extremum coincides with the time of pulsation-amplitude
maximum but differs from mean-light extremum by 0.158±0.032 d, a
5σ difference. This argues that HR 3831 is an oblique pulsator
with the magnetic axis and pulsation axis aligned, rather than a spotted
pulsator with amplitude modulation caused by surface inhomogeneities. It
also indicates a deviation from cylindrical symmetry which makes the
interpretation of the pulsation-amplitude modulation in terms of the
oblique-pulsator model more complex than has yet been considered. By
shifting the pulsation-amplitude variations into phase with the
mean-light variations and normalizing both data sets, we refine the
rotation frequency for HR3831 to be
νrot=0.3506333±0.0000006
d-1=4058.256±0.007 nHz, or Prot =
2.851982±0.000005 d.
Monthly Notices of the Royal Astronomical Society 02/1992; 255:289-294. · 4.90 Impact Factor
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ABSTRACT: The first results of a systematic survey of the roAp phenomenon in the
southern hemisphere are presented. Consideration is given to survey
candidates from the 464 Ap SrCrEu stars listed in the four currently
available volumes of the Michigan Spectral Catalogue (1978), which cover
all the HD stars in the southern sky up to delta = -12 deg. Spectra and
light curves are presented for five new rapidly oscillating Ap stars
discovered on 10 photometric nights spanning one month. Of particular
interest is star HD 190290, because it appears to be a multimode
pulsator somewhat akin to HR 1217 (which has a rich p-mode spectrum).
Monthly Notices of the Royal Astronomical Society 05/1991; 250:666-678. · 4.90 Impact Factor
