1,063 research outputs found
Characteristics of solar-like oscillations of clusters simulated by stellar population synthesis
Using a stellar population-synthesis method, we studied the distributions of
{\nu}_max and {\Delta}{\nu} of simulated clusters with various ages and
metallicities. Except for the confirmed peak (RC peak) of {\Delta}{\nu} of
red-clump (RC) stars, i.e. core-helium burning stars, there are a gap and a
main sequence (MS) peak in the distributions of {\nu}_max and {\Delta}{\nu} of
young clusters. The gap corresponds mainly to the Hertzsprung gap phase of
evolution. The RC peak is caused by the fact that the radius of many RC stars
near the zero-age horizontal branch concentrates in a certain range. The MS
peak also results from the fact that many MS stars which are located in a
certain mass range have an approximate radius in the early phase of MS. The MS
peak barely exists in the simulated clusters with age < 1.0 Gyr. The location
of the MS peak moves to a lower frequency with increasing age or metallicity,
which may be applied to constrain the age and metallicity of young clusters.
For the simulated clusters with Z = 0.02, the frequency of the location of the
dominant RC peak increases with age when age < 1.2 Gyr, and then decreases with
age when age > 1.2 Gyr; but it scarcely varies when age > 2.4 Gyr. This is
relative to the degeneracy of the hydrogen-exhausted core at the time of helium
ignition. In addition, the RC peak is not sensitive to the metallicity,
especially for the clusters with age > 2.4 Gyr. Asteroseismical observation for
clusters with age < 2.4 Gyr may aid in testing the theory of the degeneracy of
the hydrogen-exhausted core. Moreover, for the clusters with 1.1 M{\odot} <
M_hook < 1.3 M{\odot}, there are a MS gap and a peak on the left of the MS gap
in the distributions of{\nu}_max and {\Delta}{\nu}, which may be applied to
constrain the central hydrogen abundance of stars in the MS gap and the peak.Comment: Accepted for publication in MNRAS. 8 pages, 8 figure
Masses of subgiant stars from asteroseismology using the coupling strengths of mixed modes
Since few decades, asteroseismology, the study of stellar oscillations,
enables us to probe the interiors of stars with great precision. It allows
stringent tests of stellar models and can provide accurate radii, masses and
ages for individual stars. Of particular interest are the mixed modes that
occur in subgiant solar-like stars since they can place very strong constraints
on stellar ages. Here we measure the characteristics of the mixed modes,
particularly the coupling strength, using a grid of stellar models for stars
with masses between 0.9 and 1.5 M_{\odot}. We show that the coupling strength
of the mixed modes is predominantly a function of stellar mass and
appears to be independent of metallicity. This should allow an accurate mass
evaluation, further increasing the usefulness of mixed modes in subgiants as
asteroseismic tools.Comment: 7 pages, 4 figures, 1 table, Accepted (ApJL
WFIRST Ultra-Precise Astrometry II: Asteroseismology
WFIRST microlensing observations will return high-precision parallaxes,
sigma(pi) < 0.3 microarcsec, for the roughly 1 million stars with H<14 in its
2.8 deg^2 field toward the Galactic bulge. Combined with its 40,000 epochs of
high precision photometry (~0.7 mmag at H_vega=14 and ~0.1 mmag at H=8), this
will yield a wealth of asteroseismic data of giant stars, primarily in the
Galactic bulge but including a substantial fraction of disk stars at all
Galactocentric radii interior to the Sun. For brighter stars, the astrometric
data will yield an external check on the radii derived from the two
asteroseismic parameters, and nu_max, while for the fainter ones, it
will enable a mass measurement from the single measurable asteroseismic
parameter nu_max. Simulations based on Kepler data indicate that WFIRST will be
capable of detecting oscillations in stars from slightly less luminous than the
red clump to the tip of the red giant branch, yielding roughly 1 million
detections.Comment: 13 pages, 6 figures, submitted to JKA
Oscillations in β Ursae Minoris
Aims. From observations of the K4III star β UMi we attempt to determine whether oscillations or any other form of variability is present.
Methods. A high-quality photometric time series of ≈1000 days in length obtained from the SMEI instrument on the Coriolis satellite is analysed. Various statistical tests were performed to determine the significance of features seen in the power density spectrum of the light curve.
Results. Two oscillations with frequencies 2.44 and 2.92 μHz have been identified. We interpret these oscillations as consecutive overtones of an acoustic spectrum, implying a large frequency spacing of 0.48 μHz. Using derived asteroseismic parameters in combination with known astrophysical parameters, we estimate the mass of β UMi to be 1.3 ± 0.3 M. Peaks of the oscillations in the
power density spectrum show width, implying that modes are stochastically excited and damped by convection. The mode lifetime is estimated at 18 ± 9 days
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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