31 research outputs found
Exploiting the open clusters in the Kepler and CoRoT fields
The open clusters in the Kepler and CoRoT fields potentially provide tight
constraints for tests of stellar models and observational methods because they
allow a combination of complementary methods. We are in the process of identi-
fying and measuring parameters for detached eclipsing binaries (dEBs) in the
open clusters in the Kepler and CoRoT fields. We make use of measurements of
dEBs in the clusters to test the accuracy of asteroseismic scaling relations
for mass. We are able to provide strong indications that the asteroseismic
scaling relations over- estimate the stellar mass, but we are not yet able to
distinguish between different proposed corrections from the literature. We
argue how our ongoing measurements of more dEBs in more clusters, complemented
by dEBs in the field, should be able to break the degeneracy. We also briefly
describe how we can identify cluster stars that have evolved through
non-standard evolution by making use of ensemble asteroseismology.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
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
Systematic Differences in Spectroscopic Analysis of Red Giants
A spectroscopic analysis of stellar spectra can be carried out using
different approaches; different methods, line lists, atomic parameters, solar
abundances etc. The resulting atmospheric parameters from these choices can
vary beyond quoted uncertainties in the literature. Here we characterize these
differences by systematically comparing some of the commonly adopted
ingredients; line lists, equivalent width measurements and atomic parameters.
High resolution and high signal-to-noise spectroscopic data of one
helium-core-burning red giant star in each of the three open clusters, NGC6819,
M67 and NGC188 have been obtained with the FIES spectrograph at the Nordic
Optical Telescope. The M67 target has been used to benchmark the analysis, as
it is a well studied cluster with asteroseismic data from the K2 mission. For
the other two clusters we have obtained higher quality data than previously
analyzed, which allows us to establish their chemical composition more
securely. Using a line by line analysis, we tested several different
combinations of line lists and programs to measure equivalent widths of
absorption lines to characterize systematic differences within the same
spectroscopic method. The obtained parameters for the benchmark star in M67
vary up to ~170 K in T_eff, ~0.4 dex in log g and ~0.25 dex in [Fe/H] between
the tested setups. Using the combination of equivalent width measurement
program and line list that best reproduce the inferred log g from
asteroseismology, we determined the atmospheric parameters for the three stars
and securely established the chemical composition of NGC6819 to be ~solar,
[Fe/H]=-0.02+-0.01 dex. We have highlighted the significantly different results
obtained with different combinations of line lists, programs and atomic
parameters, which emphasize the importance of benchmark stars studied with
several different methods to anchor spectroscopic analyses.Comment: Accepted for publication in Astronomy and Astrophysic
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
The M 4 Core Project with HST - III. Search for variable stars in the primary field
We present the results of a photometric search for variable stars in the core of the Galactic globular cluster Messier 4 (M 4). The input data are a large and unprecedented set of deep Hubble Space Telescope WFC3 images (large programme GO-12911; 120 orbits allocated), primarily aimed at probing binaries with massive companions by detecting their astrometric wobbles. Though these data were not optimized to carry out a time-resolved photometric survey, their exquisite precision, spatial resolution and dynamic range enabled us to firmly detect 38 variable stars, of which 20 were previously unpublished. They include 19 clustermember eclipsing binaries (confirming the large binary fraction ofM4), RR Lyrae and objects with known X-ray counterparts. We improved and revised the parameters of some among published variables
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
An asteroseismic membership study of the red giants in three open clusters observed by Kepler: NGC6791, NGC6819, and NGC6811
Studying star clusters offers significant advances in stellar astrophysics
due to the combined power of having many stars with essentially the same
distance, age, and initial composition. This makes clusters excellent test
benches for verification of stellar evolution theory. To fully exploit this
potential, it is vital that the star sample is uncontaminated by stars that are
not members of the cluster. Techniques for determining cluster membership
therefore play a key role in the investigation of clusters. We present results
on three clusters in the Kepler field of view based on a newly established
technique that uses asteroseismology to identify fore- or background stars in
the field, which demonstrates advantages over classical methods such as
kinematic and photometry measurements. Four previously identified seismic
non-members in NGC6819 are confirmed in this study, and three additional
non-members are found -- two in NGC6819 and one in NGC6791. We further
highlight which stars are, or might be, affected by blending, which needs to be
taken into account when analysing these Kepler data.Comment: 12 pages, 9 figures, 5 tables, accepted by Ap