286 research outputs found
A comparison of Bayesian and Fourier methods for frequency determination in asteroseismology
Bayesian methods are becoming more widely used in asteroseismic analysis. In
particular, they are being used to determine oscillation frequencies, which are
also commonly found by Fourier analysis. It is important to establish whether
the Bayesian methods provide an improvement on Fourier methods. We compare,
using simulated data, the standard iterative sine-wave fitting method against a
Markov Chain Monte Carlo (MCMC) code that has been introduced to infer purely
the frequencies of oscillation modes (Brewer et al. 2007). A uniform prior
probability distribution function is used for the MCMC method. We find the
methods do equally well at determining the correct oscillation frequencies,
although the Bayesian method is able to highlight the possibility of a
misidentification due to aliasing, which can be useful. In general, we suggest
that the least computationally intensive method is preferable.Comment: 11 pages, 8 figures, accepted for publication in Communications in
Asterosesimolog
The treatment of mixing in core helium burning models - I. Implications for asteroseismology
The detection of mixed oscillation modes offers a unique insight into the
internal structure of core helium burning (CHeB) stars. The stellar structure
during CHeB is very uncertain because the growth of the convective core, and/or
the development of a semiconvection zone, is critically dependent on the
treatment of convective boundaries. In this study we calculate a suite of
stellar structure models and their non-radial pulsations to investigate why the
predicted asymptotic g-mode period spacing is
systematically lower than is inferred from Kepler field stars. We find that
only models with large convective cores, such as those calculated with our
newly proposed "maximal-overshoot" scheme, can match the average
reported. However, we also find another possible solution that is related to
the method used to determine : mode trapping can raise the
observationally inferred well above its true value. Even after
accounting for these two proposed resolutions to the discrepancy in average
, models still predict more CHeB stars with low ( s) than are observed. We establish two possible remedies for this: i)
there may be a difficulty in determining for early CHeB stars
(when is lowest) because of the effect that the sharp composition
profile at the hydrogen burning shell has on the pulsations, or ii) the mass of
the helium core at the flash is higher than predicted. Our conclusions
highlight the need for the reporting of selection effects in asteroseismic
population studies in order to safely use this information to constrain stellar
evolution theory.Comment: 24 pages. 24 figures. Published in MNRA
Solar-like oscillations in a metal-poor globular cluster with the HST
We present analyses of variability in the red giant stars in the metal-poor
globular cluster NGC6397, based on data obtained with the Hubble Space
Telescope. We use an non-standard data reduction approach to turn a 23-day
observing run originally aimed at imaging the white dwarf population, into
time-series photometry of the cluster's highly saturated red giant stars. With
this technique we obtain noise levels in the final power spectra down to 50
parts per million, which allows us to search for low amplitude solar-like
oscillations. We compare the observed excess power seen in the power spectra
with estimates of the typical frequency range, frequency spacing and amplitude
from scaling the solar oscillations. We see evidence that the detected
variability is consistent with solar-like oscillations in at least one and
perhaps up to four stars. With metallicities two orders of magnitude lower than
of the Sun, these stars present so far the best evidence of solar-like
oscillations in such a low metallicity environment.Comment: 7 pages, 6 figures, accepted by Ap
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