42 research outputs found
Spectroscopic Study of the Open Cluster NGC 6811
The NASA space telescope Kepler has provided unprecedented time-series
observations which have revolutionised the field of asteroseismology, i.e. the
use of stellar oscillations to probe the interior of stars. The Kepler-data
include observations of stars in open clusters, which are particularly
interesting for asteroseismology. One of the clusters observed with Kepler is
NGC 6811, which is the target of the present paper. However, apart from
high-precision time-series observations, sounding the interiors of stars in
open clusters by means of asteroseismology also requires accurate and precise
atmospheric parameters as well as cluster membership indicators for the
individual stars. We use medium-resolution (R~25,000) spectroscopic
observations, and three independent analysis methods, to derive effective
temperatures, surface gravities, metallicities, projected rotational velocities
and radial velocities, for 15 stars in the field of the open cluster NGC 6811.
We discover two double-lined and three single-lined spectroscopic binaries.
Eight stars are classified as either certain or very probable cluster members,
and three stars are classified as non-members. For four stars, cluster
membership could not been assessed. Five of the observed stars are G-type
giants which are located in the colour-magnitude diagram in the region of the
red clump of the cluster. Two of these stars are surely identified as red clump
stars for the first time. For those five stars, we provide chemical abundances
of 31 elements. The mean radial-velocity of NGC 6811 is found to be
+6.680.08 km s and the mean metallicity and overall abundance
pattern are shown to be very close to solar with an exception of Ba which we
find to be overabundant.Comment: 18 pages, 11 tables, 7 figures, accepted for MNRA
Automated extraction of oscillation parameters for Kepler observations of solar-type stars
The recent launch of the Kepler space telescope brings the opportunity to
study oscillations systematically in large numbers of solar-like stars. In the
framework of the asteroFLAG project, we have developed an automated pipeline to
estimate global oscillation parameters, such as the frequency of maximum power
(nu_max) and the large frequency spacing (Delta_nu), for a large number of time
series. We present an effective method based on the autocorrelation function to
find excess power and use a scaling relation to estimate granulation timescales
as initial conditions for background modelling. We derive reliable
uncertainties for nu_max and Delta_nu through extensive simulations. We have
tested the pipeline on about 2000 simulated Kepler stars with magnitudes of
V~7-12 and were able to correctly determine nu_max and Delta_nu for about half
of the sample. For about 20%, the returned large frequency spacing is accurate
enough to determine stellar radii to a 1% precision. We conclude that the
methods presented here are a promising approach to process the large amount of
data expected from Kepler.Comment: 14 pages, 9 figures, accepted for publication in Communications in
Asteroseismolog
The amplitude of solar oscillations using stellar techniques
The amplitudes of solar-like oscillations depend on the excitation and
damping, both of which are controlled by convection. Comparing observations
with theory should therefore improve our understanding of the underlying
physics. However, theoretical models invariably compute oscillation amplitudes
relative to the Sun, and it is therefore vital to have a good calibration of
the solar amplitude using stellar techniques. We have used daytime spectra of
the Sun, obtained with HARPS and UCLES, to measure the solar oscillations and
made a detailed comparison with observations using the BiSON helioseismology
instrument. We find that the mean solar amplitude measured using stellar
techniques, averaged over one full solar cycle, is 18.7 +/- 0.7 cm/s for the
strongest radial modes (l=0) and 25.2 +/- 0.9 cm/s for l=1. In addition, we use
simulations to establish an equation that estimates the uncertainty of
amplitude measurements that are made of other stars, given that the mode
lifetime is known. Finally, we also give amplitudes of solar-like oscillations
for three stars that we measured from a series of short observations with HARPS
(gamma Ser, beta Aql and alpha For), together with revised amplitudes for five
other stars for which we have previously published results (alpha Cen A, alpha
Cen B, beta Hyi, nu Ind and delta Pav).Comment: 8 pages, accepted by ApJ. Minor wording changes and added a referenc
Establishing the accuracy of asteroseismic mass and radius estimates of giant stars. II. Revised stellar masses and radii for KIC 8430105
Asteroseismic scaling relations can provide high-precision measurements of
mass and radius for red giant (RG) stars displaying solar-like oscillations.
Their accuracy can be validated and potentially improved using independent and
accurate observations of mass, radius, effective temperature and metallicity.
We seek to achieve this using long period SB2 eclipsing binaries hosting
oscillating RGs. We explore KIC 8430105, for which a previous study found
significant asteroseismic overestimation of mass and radius when compared with
eclipsing binary measurements. We measured dynamical masses and radii for both
components to be significantly lower than previously established, increasing
the discrepancy between asteroseismic and dynamical measurements. Our dynamical
measurements of the RG component were compared to corresponding measurements of
mass and radius using asteroseismic scaling relations. Uncorrected scaling
relations overestimated the mass of the RG by 26%, the radius by 11%, and the
average density by 7%, in agreement with studies for other systems. However,
using a theoretical correction to , we managed to obtain an
asteroseismic average density that is consistent with our dynamical
result. We obtained several measurements of that are not fully
consistent. With Hz, the
correction provided consistent mass and radius for the giant. The
age of the system was estimated to be Gyr
What asteroseismology can do for exoplanets
We describe three useful applications of asteroseismology in the context of
exoplanet science: (1) the detailed characterisation of exoplanet host stars;
(2) the measurement of stellar inclinations; and (3) the determination of
orbital eccentricity from transit duration making use of asteroseismic stellar
densities. We do so using the example system Kepler-410 (Van Eylen et al.
2014). This is one of the brightest (V = 9.4) Kepler exoplanet host stars,
containing a small (2.8 Rearth) transiting planet in a long orbit (17.8 days),
and one or more additional non-transiting planets as indicated by transit
timing variations. The validation of Kepler-410 (KOI-42) was complicated due to
the presence of a companion star, and the planetary nature of the system was
confirmed after analyzing a Spitzer transit observation as well as ground-based
follow-up observations.Comment: 4 pages, Proceedings of the CoRoT Symposium 3 / Kepler KASC-7 joint
meeting, Toulouse, 7-11 July 2014. To be published by EPJ Web of Conference
Precise radial velocities of giant stars XV. Mysterious nearly periodic radial velocity variations in the eccentric binary Cygni
Using the Hamilton Echelle Spectrograph at Lick Observatory, we have obtained
precise radial velocities (RVs) of a sample of 373 G- and K-giant stars over
more than 12 years, leading to the discovery of several single and multiple
planetary systems. The RVs of the long-period (~53 years) spectroscopic binary
Cyg (HIP 102488) are found to exhibit additional regular variations
with a much shorter period (~291 days). We intend to improve the orbital
solution of the Cyg system and attempt to identify the cause of the
nearly periodic shorter period variations, which might be due to an additional
substellar companion. We used precise RV measurements of the K-giant star
Cyg from Lick Observatory, in combination with a large set of RVs
collected more recently with the SONG telescope, as well as archival data sets.
Our Keplerian model to the RVs characterizes the orbit of the spectroscopic
binary to higher precision than achieved previously, resulting in a semi-major
axis of , an eccentricity of , and a minimum
mass of the secondary of . Additional short-period RV
variations closely resemble the signal of a Jupiter-mass planet orbiting the
evolved primary component with a period of , but the period and
amplitude of the putative orbit change strongly over time. Furthermore, in our
stability analysis of the system, no stable orbits could be found in a large
region around the best fit. Both of these findings deem a planetary cause of
the RV variations unlikely. Most of the investigated alternative scenarios,
such as an hierarchical triple or stellar spots, also fail to explain the
observed variability convincingly. Due to its very eccentric binary orbit, it
seems possible, however, that Cyg could be an extreme example of a
heartbeat system.Comment: 17 pages, 13 figures, accepted to A&
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
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
Solar-like oscillations and activity in Procyon: A comparison of the 2007 MOST and ground-based radial velocity campaigns
Publisher's version/PDFWe compare the simultaneous 2007 space-based MOST photometry and ground-based radial velocity (RV) observations of the F5 star Procyon. We identify slow variations in the MOST data that are similar to those reported in the RV time series and confirm by comparison with the Sun that these variations are likely the signature of stellar activity. The MOST power spectrum yields clear evidence for individual oscillation frequencies that match those found in the RV data by Bedding et al. We identify the same ridges due to modes of different spherical degree in both data sets, but are not able to confirm a definite ridge identification using the MOST data. We measure the luminosity amplitude per radial mode A[subscript l=0,phot] = 9.1 [plus or minus] 0.5 ppm. Combined with the estimate for the RV data by Arentoft et al., this gives a mean amplitude ratio of A[subscript l=0,phot/A[subscript l=0,RV] = 0.24 [plus or minus] 0.02 ppm cm[superscript −1] s, considerably higher than expected from scaling relations but in reasonable agreement with theoretical models by Houdek. We also compare the amplitude ratio as a function of frequency and find that the maximum of the oscillation envelope is shifted to higher frequencies in photometry than in velocity