57 research outputs found
Solar-like oscillations in the G2 subgiant beta Hydri from dual-site observations
We have observed oscillations in the nearby G2 subgiant star beta Hyi using
high-precision velocity observations obtained over more than a week with the
HARPS and UCLES spectrographs. The oscillation frequencies show a regular comb
structure, as expected for solar-like oscillations, but with several l=1 modes
being strongly affected by avoided crossings. The data, combined with those we
obtained five years earlier, allow us to identify 28 oscillation modes. By
scaling the large frequency separation from the Sun, we measure the mean
density of beta Hyi to an accuracy of 0.6%. The amplitudes of the oscillations
are about 2.5 times solar and the mode lifetime is 2.3 d. A detailed comparison
of the mixed l=1 modes with theoretical models should allow a precise estimate
of the age of the star.Comment: 13 pages, 14 figures, accepted by ApJ. Fixed minor typo (ref to Fig
14
A multi-site campaign to measure solar-like oscillations in Procyon. II. Mode frequencies
We have analyzed data from a multi-site campaign to observe oscillations in
the F5 star Procyon. The data consist of high-precision velocities that we
obtained over more than three weeks with eleven telescopes. A new method for
adjusting the data weights allows us to suppress the sidelobes in the power
spectrum. Stacking the power spectrum in a so-called echelle diagram reveals
two clear ridges that we identify with even and odd values of the angular
degree (l=0 and 2, and l=1 and 3, respectively). We interpret a strong, narrow
peak at 446 muHz that lies close to the l=1 ridge as a mode with mixed
character. We show that the frequencies of the ridge centroids and their
separations are useful diagnostics for asteroseismology. In particular,
variations in the large separation appear to indicate a glitch in the
sound-speed profile at an acoustic depth of about 1000 s. We list frequencies
for 55 modes extracted from the data spanning 20 radial orders, a range
comparable to the best solar data, which will provide valuable constraints for
theoretical models. A preliminary comparison with published models shows that
the offset between observed and calculated frequencies for the radial modes is
very different for Procyon than for the Sun and other cool stars. We find the
mean lifetime of the modes in Procyon to be 1.29 +0.55/-0.49 days, which is
significantly shorter than the 2-4 days seen in the Sun.Comment: accepted for publication in Ap
Asteroseismology of the Hyades red giant and planet host epsilon Tauri
Asteroseismic analysis of solar-like stars allows us to determine physical
parameters such as stellar mass, with a higher precision compared to most other
methods. Even in a well-studied cluster such as the Hyades, the masses of the
red giant stars are not well known, and previous mass estimates are based on
model calculations (isochrones). The four known red giants in the Hyades are
assumed to be clump (core-helium-burning) stars based on their positions in
colour-magnitude diagrams, however asteroseismology offers an opportunity to
test this assumption. Using asteroseismic techniques combined with other
methods, we aim to derive physical parameters and the evolutionary stage for
the planet hosting star epsilon Tau, which is one of the four red giants
located in the Hyades. We analysed time-series data from both ground and space
to perform the asteroseismic analysis. By combining high signal-to-noise (S/N)
radial-velocity data from the ground-based SONG network with continuous
space-based data from the revised Kepler mission K2, we derive and characterize
27 individual oscillation modes for epsilon Tau, along with global oscillation
parameters such as the large frequency separation and the ratio between the
amplitude of the oscillations measured in radial velocity and intensity as a
function of frequency. The latter has been measured previously for only two
stars, the Sun and Procyon. Combining the seismic analysis with interferometric
and spectroscopic measurements, we derive physical parameters for epsilon Tau,
and discuss its evolutionary status.Comment: 13 pages, 13 figures, 4 tables, accepted for publication in Astronomy
& Astrophysic
Detection of solar-like oscillations from Kepler photometry of the open cluster NGC 6819
Asteroseismology of stars in clusters has been a long-sought goal because the
assumption of a common age, distance and initial chemical composition allows
strong tests of the theory of stellar evolution. We report results from the
first 34 days of science data from the Kepler Mission for the open cluster NGC
6819 -- one of four clusters in the field of view. We obtain the first clear
detections of solar-like oscillations in the cluster red giants and are able to
measure the large frequency separation and the frequency of maximum oscillation
power. We find that the asteroseismic parameters allow us to test
cluster-membership of the stars, and even with the limited seismic data in
hand, we can already identify four possible non-members despite their having a
better than 80% membership probability from radial velocity measurements. We
are also able to determine the oscillation amplitudes for stars that span about
two orders of magnitude in luminosity and find good agreement with the
prediction that oscillation amplitudes scale as the luminosity to the power of
0.7. These early results demonstrate the unique potential of asteroseismology
of the stellar clusters observed by Kepler.Comment: 5 pages, 4 figures, accepted by ApJ (Lett.
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