87 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
Solar-like oscillations in the metal-poor subgiant nu Indi: constraining the mass and age using asteroseismology
Asteroseismology is a powerful method for determining fundamental properties
of stars. We report the first application to a metal-poor object, namely the
subgiant star nu Ind. We measured precise velocities from two sites, allowing
us to detect oscillations and infer a large frequency separation of Delta_nu =
24.25 +/- 0.25 microHz. Combining this value with the location of the star in
the H-R diagram and comparing with standard evolutionary models, we were able
to place constraints on the stellar parameters. In particular, our results
indicate that nu Ind has a low mass (0.85 +/- 0.04 M_sun) and is at least 9 Gyr
old.Comment: accpted for publication in Ap
Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars
Red giants are evolved stars that have exhausted the supply of hydrogen in
their cores and instead burn hydrogen in a surrounding shell. Once a red giant
is sufficiently evolved, the helium in the core also undergoes fusion.
Outstanding issues in our understanding of red giants include uncertainties in
the amount of mass lost at the surface before helium ignition and the amount of
internal mixing from rotation and other processes. Progress is hampered by our
inability to distinguish between red giants burning helium in the core and
those still only burning hydrogen in a shell. Asteroseismology offers a way
forward, being a powerful tool for probing the internal structures of stars
using their natural oscillation frequencies. Here we report observations of
gravity-mode period spacings in red giants that permit a distinction between
evolutionary stages to be made. We use high-precision photometry obtained with
the Kepler spacecraft over more than a year to measure oscillations in several
hundred red giants. We find many stars whose dipole modes show sequences with
approximately regular period spacings. These stars fall into two clear groups,
allowing us to distinguish unambiguously between hydrogen-shell-burning stars
(period spacing mostly about 50 seconds) and those that are also burning helium
(period spacing about 100 to 300 seconds).Comment: to appear as a Letter to Natur
Low-amplitude solar-like oscillations in the K5 V star Indi A
We have detected solar-like oscillations in the mid K-dwarf
Indi A, making it the coolest dwarf to have measured oscillations. The star is
noteworthy for harboring a pair of brown dwarf companions and a Jupiter-type
planet. We observed Indi A during two radial velocity campaigns,
using the high-resolution spectrographs HARPS (2011) and UVES (2021). Weighting
the time series, we computed the power spectra and established the detection of
solar-like oscillations with a power excess located at Hz
-- the highest frequency solar-like oscillations so far measured in any star.
The measurement of the center of the power excess allows us to compute a
stellar mass of based on scaling relations and a
known radius from interferometry. We also determine the amplitude of the peak
power and note that there is a slight difference between the two observing
campaigns, indicating a varying activity level. Overall, this work confirms
that low-amplitude solar-like oscillations can be detected in mid-K type stars
in radial velocity measurements obtained with high-precision spectrographs.Comment: 10 pages, 3 figures, accepted for publication in Ap
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
Fast core rotation in red-giant stars revealed by gravity-dominated mixed modes
When the core hydrogen is exhausted during stellar evolution, the central
region of a star contracts and the outer envelope expands and cools, giving
rise to a red giant, in which convection occupies a large fraction of the star.
Conservation of angular momentum requires that the cores of these stars rotate
faster than their envelopes, and indirect evidence supports this. Information
about the angular momentum distribution is inaccessible to direct observations,
but it can be extracted from the effect of rotation on oscillation modes that
probe the stellar interior. Here, we report the detection of non-rigid rotation
in the interiors of red-giant stars by exploiting the rotational frequency
splitting of recently detected mixed modes. We demonstrate an increasing
rotation rate from the surface of the star to the stellar core. Comparing with
theoretical stellar models, we conclude that the core must rotate at least ten
times faster than the surface. This observational result confirms the
theoretical prediction of a steep gradient in the rotation profile towards the
deep stellar interior.Comment: to appear as a Letter to Natur
Towards probing the internal angular momentum distribution in red giants from solar-like oscillations
status: publishe
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