120 research outputs found
Asteroseismology with the WIRE satellite. I. Combining Ground- and Space-based Photometry of the Delta Scuti Star Epsilon Cephei
We have analysed ground-based multi-colour Stromgren photometry and
single-filter photometry from the star tracker on the WIRE satellite of the
delta scuti star Epsilon Cephei. The ground-based data set consists of 16
nights of data collected over 164 days, while the satellite data are nearly
continuous coverage of the star during 14 days. The spectral window and noise
level of the satellite data are superior to the ground-based data and this data
set is used to locate the frequencies. However, we can use the ground-based
data to improve the accuracy of the frequencies due to the much longer time
baseline. We detect 26 oscillation frequencies in the WIRE data set, but only
some of these can be seen clearly in the ground-based data. We have used the
multi-colour ground-based photometry to determine amplitude and phase
differences in the Stromgren b-y colour and the y filter in an attempt to
identify the radial degree of the oscillation frequencies. We conclude that the
accuracies of the amplitudes and phases are not sufficient to constrain
theoretical models of Epsilon Cephei. We find no evidence for rotational
splitting or the large separation among the frequencies detected in the WIRE
data set. To be able to identify oscillation frequencies in delta scuti stars
with the method we have applied, it is crucial to obtain more complete coverage
from multi-site campaigns with a long time baseline and in multiple filters.
This is important when planning photometric and spectroscopic ground-based
support for future satellite missions like COROT and KEPLER.Comment: 13 pages, 12 figures, 4 tables. Fig. 4 reduced in quality. Accepted
by A&
The sdB pulsating star V391 Peg and its putative giant planet revisited after 13 years of time-series photometric data
V391 Peg (alias HS 2201+2610) is a subdwarf B (sdB) pulsating star that shows both p- and g-modes. By studying the arrival times
of the p-mode maxima and minima through the O–C method, in a previous article the presence of a planet was inferred with an
orbital period of 3.2 years and a minimum mass of 3.2 MJup. Here we present an updated O–C analysis using a larger data set of
1066 h of photometric time series (∼2.5× larger in terms of the number of data points), which covers the period between 1999 and 2012
(compared with 1999–2006 of the previous analysis). Up to the end of 2008, the new O–C diagram of the main pulsation frequency (f1)
is compatible with (and improves) the previous two-component solution representing the long-term variation of the pulsation period
(parabolic component) and the giant planet (sine wave component). Since 2009, the O–C trend of f1 changes, and the time derivative
of the pulsation period (pË™) passes from positive to negative; the reason of this change of regime is not clear and could be related to
nonlinear interactions between different pulsation modes. With the new data, the O–C diagram of the secondary pulsation frequency
(f2) continues to show two components (parabola and sine wave), like in the previous analysis. Various solutions are proposed to fit
the O–C diagrams of f1 and f2, but in all of them, the sinusoidal components of f1 and f2 differ or at least agree less well than before.
The nice agreement found previously was a coincidence due to various small effects that are carefully analyzed. Now, with a larger
dataset, the presence of a planet is more uncertain and would require confirmation with an independent method. The new data allow
us to improve the measurement of p˙ for f1 and f2: using only the data up to the end of 2008, we obtain p˙ 1 = (1.34 ± 0.04) × 10−12 and
p˙ 2 = (1.62 ± 0.22) × 10−12. The long-term variation of the two main pulsation periods (and the change of sign of p˙ 1) is visible also in
direct measurements made over several years. The absence of peaks near f1 in the Fourier transform and the secondary peak close to
f2 confirm a previous identification as l = 0 and l = 1, respectively, and suggest a stellar rotation period of about 40 days. The new data
allow constraining the main g-mode pulsation periods of the star
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