4,216 research outputs found
Quasi-Biennial variations in helioseismic frequencies: Can the source of the variation be localized?
We investigate the spherical harmonic degree (l) dependence of the "seismic"
quasi-biennial oscillation (QBO) observed in low-degree solar p-mode
frequencies, using Sun-as-a-star Birmingham Solar Oscillations Network (BiSON)
data. The amplitude of the seismic QBO is modulated by the 11-yr solar cycle,
with the amplitude of the signal being largest at solar maximum. The amplitude
of the signal is noticeably larger for the l=2 and 3 modes than for the l=0 and
1 modes. The seismic QBO shows some frequency dependence but this dependence is
not as strong as observed in the 11-yr solar cycle. These results are
consistent with the seismic QBO having its origins in shallow layers of the
interior (one possibility being the bottom of the shear layer extending 5per
cent below the solar surface). Under this scenario the magnetic flux
responsible for the seismic QBO is brought to the surface (where its influence
on the p modes is stronger) by buoyant flux from the 11-yr cycle, the strong
component of which is observed at predominantly low-latitudes. As the l=2 and 3
modes are much more sensitive to equatorial latitudes than the l=0 and 1 modes
the influence of the 11-yr cycle on the seismic QBO is more visible in l=2 and
3 mode frequencies. Our results imply that close to solar maximum the main
influence of the seismic QBO occurs at low latitudes (<45 degrees), which is
where the strong component of the 11-yr solar cycle resides. To isolate the
latitudinal dependence of the seismic QBO from the 11-yr solar cycle we must
consider epochs when the 11-yr solar cycle is weak. However, away from solar
maximum, the amplitude of the seismic QBO is weak making the latitudinal
dependence hard to constrain.Comment: 10 pages, 6 figures, accepted for publication in MNRA
Asteroseismology of Solar-Type and Red-Giant Stars
We are entering a golden era for stellar physics driven by satellite and
telescope observations of unprecedented quality and scope. New insights on
stellar evolution and stellar interiors physics are being made possible by
asteroseismology, the study of stars by the observation of natural, resonant
oscillations. Asteroseismology is proving to be particularly significant for
the study of solar-type and red-giant stars. These stars show rich spectra of
solar-like oscillations, which are excited and intrinsically damped by
turbulence in the outermost layers of the convective envelopes. In this review
we discuss the current state of the field, with a particular emphasis on recent
advances provided by the Kepler and CoRoT space missions and the wider
significance to astronomy of the results from asteroseismology, such as stellar
populations studies and exoplanet studies.Comment: The following paper will appear in the 2013 volume of Annual Reviews
of Astronomy and Astrophysics (88 pages, 7 figures; references updated;
further corrections to typos during galley-proof review
The more the merrier: grid based modelling of Kepler dwarfs with 5-dimensional stellar grids
We present preliminary results of our grid based modelling (GBM) of the
dwarf/subgiant sample of stars observed with Kepler including global
asteroseismic parameters. GBM analysis in this work is based on a large grid of
stellar models that is characterized by five independent parameters: model mass
and age, initial metallicity (\zini), initial helium (\yini), and mixing
length parameter (). Using this grid relaxes assumptions used in
all previous GBM work where the initial composition is determined by a single
parameter and that is fixed to a solar-calibrated value. The new
grid allows us to study, for example, the impact of different galactic chemical
enrichment models on the determination of stellar parameters such as mass
radius and age. Also, it allows to include new results from stellar atmosphere
models on in the GBM analysis in a simple manner. Alternatively,
it can be tested if global asteroseismology is a useful tool to constraint our
ignorance on quantities such as \yini and . Initial findings
show that mass determination is robust with respect to freedom in the latter
quantities, with a 4.4\% maximum deviation for extreme assumptions regarding
prior information on \yini-\zini relations and . On the other
hand, tests carried out so far seem to indicate that global seismology does not
have much power to constrain \yini-\zini relations of values
without resourcing to additional information.Comment: To appear in the Proceedings of the joint TASC2/KASC9 workshop -
SPACEINN & HELAS8 conference. 4 page
A new method to detect solar-like oscillations at very low S/N using statistical significance testing
We introduce a new method to detect solar-like oscillations in frequency
power spectra of stellar observations, under conditions of very low signal to
noise. The Moving-Windowed-Power-Search, or MWPS, searches the power spectrum
for signatures of excess power, over and above slowly varying (in frequency)
background contributions from stellar granulation and shot or instrumental
noise. We adopt a false-alarm approach (Chaplin et al. 2011) to ascertain
whether flagged excess power, which is consistent with the excess expected from
solar-like oscillations, is hard to explain by chance alone (and hence a
candidate detection).
We apply the method to solar photometry data, whose quality was
systematically degraded to test the performance of the MWPS at low
signal-to-noise ratios. We also compare the performance of the MWPS against the
frequently applied power-spectrum-of-power-spectrum (PSxPS) detection method.
The MWPS is found to outperform the PSxPS method.Comment: 10 pages, 7 figures, accepted for publication in MNRAS, Added
reference
Reliability of P mode event classification using contemporaneous BiSON and GOLF observations
We carried out a comparison of the signals seen in contemporaneous BiSON and
GOLF data sets. Both instruments perform Doppler shift velocity measurements in
integrated sunlight, although BiSON perform measurements from the two wings of
potassium absorption line and GOLF from one wing of the NaD1 line.
Discrepancies between the two datasets have been observed. We show,in fact,
that the relative power depends on the wing in which GOLF data observes. During
the blue wing period, the relative power is much higher than in BiSON datasets,
while a good agreement has been observed during the red period.Comment: 7 pages, HELAS II: Helioseismology, Asteroseismology, and MHD
Connections, conference proceedin
Solar cycle variations of large frequency separations of acoustic modes: Implications for asteroseismology
We have studied solar cycle changes in the large frequency separations that
can be observed in Birmingham Solar Oscillations Network (BiSON) data. The
large frequency separation is often one of the first outputs from asteroseismic
studies because it can help constrain stellar properties like mass and radius.
We have used three methods for estimating the large separations: use of
individual p-mode frequencies, computation of the autocorrelation of
frequency-power spectra, and computation of the power spectrum of the power
spectrum. The values of the large separations obtained by the different methods
are offset from each other and have differing sensitivities to the realization
noise. A simple model was used to predict solar cycle variations in the large
separations, indicating that the variations are due to the well-known solar
cycle changes to mode frequency. However, this model is only valid over a
restricted frequency range. We discuss the implications of these results for
asteroseismology.Comment: 9 pages, 11 figures, accepted for publication in MNRAS, references
updated, corrections following proof
BiSON data preparation: A correction for differential extinction and the weighted averaging of contemporaneous data
The Birmingham Solar Oscillations Network (BiSON) has provided high-quality
high-cadence observations from as far back in time as 1978. These data must be
calibrated from the raw observations into radial velocity and the quality of
the calibration has a large impact on the signal-to-noise ratio of the final
time series. The aim of this work is to maximise the potential science that can
be performed with the BiSON data set by optimising the calibration procedure.
To achieve better levels of signal-to-noise ratio we perform two key steps in
the calibration process: we attempt a correction for terrestrial atmospheric
differential extinction; and the resulting improvement in the calibration
allows us to perform weighted averaging of contemporaneous data from different
BiSON stations. The improvements listed produce significant improvement in the
signal-to-noise ratio of the BiSON frequency-power spectrum across all
frequency ranges. The reduction of noise in the power spectrum will allow
future work to provide greater constraint on changes in the oscillation
spectrum with solar activity. In addition, the analysis of the low-frequency
region suggests we have achieved a noise level that may allow us to improve
estimates of the upper limit of g-mode amplitudes.Comment: Accepted for publication in MNRAS; 10 pages, 7 figure
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