42 research outputs found
Asteroseismology of the Beta Cephei star Nu Eridani -- IV. The 2003-4 multisite photometric campaign and the combined 2002-4 data
The second multisite photometric campaign devoted to Nu Eri is reported. For
Nu Eri, analysis of the new data adds four independent frequencies to the nine
derived previously from the 2002-3 data, three in the range from 7.20 to 7.93
c/d, and a low one, equal to 0.614 c/d. Combining the new and the old data
results in two further independent frequencies, equal to 6.7322 and 6.2236 c/d.
Altogether, the oscillation spectrum is shown to consist of 12 high frequencies
and two low ones. The latter have u amplitudes about twice as large as the v
and y amplitudes, a signature of high radial-order g modes. Thus, the
suggestion that Nu Eri is both a Beta Cephei and an SPB star, put forward on
the basis of the first campaign's data, is confirmed.
Nine of the 12 high frequencies form three triplets, of which two are new.
The triplets represent rotationally split l=1 modes, although in case of the
smallest-amplitude one this may be questioned. Mean separations and asymmetries
of the triplets are derived with accuracy sufficient for meaningful comparison
with models.
The first comparison star, Mu Eri, is shown to be an SPB variable with an
oscillation spectrum consisting of six frequencies, three of which are
equidistant in period. The star is also found to be an eclipsing variable. The
eclipse is a transit, probably total, the secondary is fainter than the primary
by several magnitudes, and the system is widely detached.
The second comparison star, Xi Eri, is confirmed to be a Delta Scuti
variable. To the frequency of 10.8742 c/d seen already in the first campaign's
data, another one, equal to 17.2524 c/d, is added.Comment: 13 pages, 8 figures, MNRAS, in pres
The rapidly oscillating Ap star HD 99563 and its distorted dipole pulsation mode
We undertook a time-series photometric multi-site campaign for the rapidly
oscillating Ap star HD 99563 and also acquired mean light observations over two
seasons. The pulsations of the star, that show flatter light maxima than
minima, can be described with a frequency quintuplet centred on 1557.653
microHertz and some first harmonics of these. The amplitude of the pulsation is
modulated with the rotation period of the star that we determine with 2.91179
+/- 0.00007 d from the analysis of the stellar pulsation spectrum and of the
mean light data. We break the distorted oscillation mode up into its pure
spherical harmonic components and find it is dominated by the l=1 pulsation,
and also has a notable l=3 contribution, with weak l=0 and 2 components. The
geometrical configuration of the star allows one to see both pulsation poles
for about the same amount of time; HD 99563 is only the fourth roAp star for
which both pulsation poles are seen and only the third where the distortion of
the pulsation modes was modelled. We point out that HD 99563 is very similar to
the well-studied roAp star HR 3831. Finally, we note that the visual companion
of HD 99563 is located in the Delta Scuti instability strip and may thus show
pulsation. We show that if the companion was physical, the roAp star would be a
2.03 solar mass object, seen at a rotational inclination of 44 degrees, which
then predicts a magnetic obliquity of 86.4 degrees.Comment: 10 pages, 6 figures, accepted for publication by MNRA
The rapidly oscillating Ap star HD 99563 and its distorted dipole pulsation mode
We undertook a time-series photometric multisite campaign for the rapidly oscillating Ap (roAp) star HD 99563 and also acquired mean light observations over four seasons. The pulsations of the star, which show flatter light maxima than minima, can be described with a frequency quintuplet centred on 1557.653 μHz and some first harmonics of it. The amplitude of the pulsation is modulated with the rotation period of the star that we determine with 2.91179 ± 0.00007 d from the analysis of the stellar pulsation spectrum and of the mean light data. We break up the distorted oscillation mode into its pure spherical harmonic components and find it is dominated by the ℓ= 1 pulsation, and also has a notable ℓ= 3 contribution, with weak ℓ= 0 and 2 components. The geometrical configuration of the star allows us to see both pulsation poles for about the same amount of time; HD 99563 is only the fourth roAp star for which both pulsation poles are seen and only the third where the distortion of the pulsation modes has been modelled. We point out that HD 99563 is very similar to the well-studied roAp star HR 3831. Finally, we note that the visual companion of HD 99563 is located in the δ Scuti instability strip and may thus show pulsation. We show that if the companion was physical, the roAp star would be a 2.03-M⊙, object, seen at a rotational inclination of 44°, which then predicts a magnetic obliquit
A Tight Upper Limit on Oscillations in the Ap star Epsilon Ursae Majoris from WIRE Photometry
Observations of Epsilon UMa obtained with the star tracker on the Wide Field
Infrared Explorer (WIRE) satellite during a month in mid-2000 are analyzed.
This is one of the most precise photometry of an Ap star. The amplitude
spectrum is used to set an upper limit of 75 parts per million for the
amplitude of stellar pulsations in this star unless it accidentally oscillates
with a single mode at the satellite orbit, its harmonics or their one day
aliases. This is the tightest limit put on the amplitude of oscillations in an
Ap star. As the rotation period of Epsilon UMa is relatively short (5.1 d), it
cannot be argued that the observations were made at a wrong rotational phase.
Our results thus support the idea that some Ap stars do not pulsate at all.Comment: 4 pages, 4 figures, 2 style files, accepted for publication in ApJ
Asteroseismology of the Beta Cephei star Nu Eridani: photometric observations and pulsational frequency analysis
We undertook a multisite photometric campaign for the Beta Cephei star Nu
Eridani. More than 600 hours of differential photoelectric uvyV photometry were
obtained with 11 telescopes during 148 clear nights. The frequency analysis of
our measurements shows that the variability of Nu Eri can be decomposed into 23
sinusoidal components, eight of which correspond to independent pulsation
frequencies between 5 - 8 c/d. Some of these are arranged in multiplets, which
suggests rotational m-mode splitting of nonradial pulsation modes as the cause.
If so, the rotation period of the star must be between 30 - 60 d. One of the
signals in the light curves of Nu Eri has a very low frequency of 0.432 c/d. It
can be a high-order combination frequency or, more likely, an independent
pulsation mode. In the latter case Nu Eri would be both a Beta Cephei star and
a slowly pulsating B (SPB) star. The photometric amplitudes of the individual
pulsation modes of Nu Eri appear to have increased by about 20 per cent over
the last 40 years. So do the amplitudes of the dominant combination frequencies
of the star. Among the latter, we only could identify sum frequencies with
certainty, not difference frequencies, which suggests that neither light-curve
distortion in its simplest form nor resonant mode coupling are their single
cause. One of our comparison stars, Mu Eridani, turned out to be variable with
a dominant time scale of 1.62 d. We believe that it is either an SPB star just
leaving its instability strip or that its variations are of rotational origin.Comment: 10 pages, 5 figure
A multisite photometric study of two unusual Beta Cep stars: the magnetic V2052 Oph and the massive rapid rotator V986 Oph
We report a multisite photometric campaign for the Beta Cep stars V2052 Oph
and V986 Oph. 670 hours of high-quality differential photoelectric Stromgren,
Johnson and Geneva time-series photometry were obtained with eight telescopes
on five continents during 182 nights. Frequency analyses of the V2052 Oph data
enabled the detection of three pulsation frequencies, the first harmonic of the
strongest signal, and the rotation frequency with its first harmonic.
Pulsational mode identification from analysing the colour amplitude ratios
confirms the dominant mode as being radial, whereas the other two oscillations
are most likely l=4. Combining seismic constraints on the inclination of the
rotation axis with published magnetic field analyses we conclude that the
radial mode must be the fundamental. The rotational light modulation is in
phase with published spectroscopic variability, and consistent with an oblique
rotator for which both magnetic poles pass through the line of sight. The
inclination of the rotation axis is 54o <i< 58o and the magnetic obliquity 58o
<beta< 66o. The possibility that V2052 Oph has a magnetically confined wind is
discussed. The photometric amplitudes of the single oscillation of V986 Oph are
most consistent with an l=3 mode, but this identification is uncertain.
Additional intrinsic, apparently temporally incoherent, light variations of
V986 Oph are reported. Different interpretations thereof cannot be
distinguished at this point, but this kind of variability appears to be present
in many OB stars. The prospects of obtaining asteroseismic information for more
rapidly rotating Beta Cep stars, which appear to prefer modes of higher l, are
briefly discussed.Comment: 12 pages, 8 figures, MNRAS, in pres
Short time-scale frequency and amplitude variations in the pulsations of an roAp star: HD 217522
Photometric observations of HD 217522 in 1981 revealed only one pulsation frequency ν1 = 1.215 29 mHz. Subsequent observations in 1989 showed the presence of an additional frequency ν2 = 2.0174 mHz. New observations in 2008 confirm the presence of the mode with ν2 = 2.0174 mHz. Examination of the 1989 data shows amplitude modulation over a time-scale of the order of a day, much shorter than what has been observed in other rapidly oscillating Ap (roAp) stars. High spectral and time resolution data obtained using the Very Large Telescope in 2008 confirm the presence of ν2 and short-term modulations in the radial velocity amplitudes of rare earth elements. This suggests growth and decay times shorter than a day, more typical of solar-like oscillations. The driving mechanism of roAp stars and the Sun are different, and the growth and decay seen in the Sun are due to stochastic nature of the driving mechanism. The driving mechanism in roAp stars usually leads to mode stability on a longer time-scale than in the Sun. We interpret the reported change in ν1 between the 1982 and 1989 data as part of the general frequency variability observed in this star on many time-scales
A photometric study of the southern Blazhko star SS For Unambiguous detection of quintuplet components
We present our analysis of photometric data in the Johnson B and V filter of
the southern Blazhko star SS For. In parallel, we analyzed the V observations
obtained with the ASAS-3 photometry of the star gathered between 2000 and 2008.
In the frequency spectra resulting from a Fourier analysis of our data, the
triplet structure is detectable up to high order, both in the B and V data.
Moreover, we find evidence for quintuplet components. We confirm from our data
that the modulation components decrease less steeply than the harmonics of the
main frequency. We derived the variations of the Fourier parameters quantifying
the light curve shape over the Blazhko cycle. There is good agreement between
the spectroscopic abundance and the metallicity determined from the Fourier
parameters of the average light curve. SS For is peculiar as a Blazhko star
because of its strong variations around minimum light.Comment: 15 pages, 14 figures, accepted for publication in MNRA