1,267 research outputs found
KIC 8410637: a 408-day period eclipsing binary containing a pulsating red giant
Detached eclipsing binaries (dEBs) are ideal targets for accurate measurement
of masses and radii of ther component stars. If at least one of the stars has
evolved off the main sequence (MS), the masses and radii give a strict
constraint on the age of the stars. Several dEBs containing a bright K giant
and a fainter MS star have been discovered by the Kepler satellite. The mass
and radius of a red giant (RG) star can also be derived from its asteroseismic
signal. The parameters determined in this way depend on stellar models and may
contain systematic errors. It is important to validate the asteroseismically
determined mass and radius with independent methods. This can be done when
stars are members of stellar clusters or members of dEBs. KIC 8410637 consists
of an RG and an MS star. The aim is to derive accurate masses and radii for
both components and provide the foundation for a strong test of the
asteroseismic method and the accuracy of the deduced mass, radius and age. We
analyse high-resolution spectra from three different spectrographs. We also
calculate a fit to the Kepler light curve and use ground-based photometry to
determine the flux ratios between the component stars in the BVRI passbands. We
measured the masses and radii of the stars in the dEB, and the classical
parameters Teff, log g and [Fe/H] from the spectra and ground-based photometry.
The RG component of KIC 8410637 is most likely in the core helium-burning red
clump phase of evolution and has an age and composition very similar to the
stars in the open cluster NGC 6819. The mass of the RG in KIC 8410637 should
therefore be similar to the mass of RGs in NGC 6819, thus lending support to
the most up-to-date version of the asteroseismic scaling relations. This is the
first direct measurement of both mass and radius for an RG to be compared with
values for RGs from asteroseismic scaling relations.Comment: Accepted 20.6.2013 for publication in Astronomy and Astrophysic
Sounding stellar cycles with Kepler - II. Ground-based observations
We have monitored 20 Sun-like stars in the Kepler field-of-view for excess
flux with the FIES spectrograph on the Nordic Optical Telescope since the
launch of Kepler spacecraft in 2009. These 20 stars were selected based on
their asteroseismic properties to sample the parameter space (effective
temperature, surface gravity, activity level etc.) around the Sun. Though the
ultimate goal is to improve stellar dynamo models, we focus the present paper
on the combination of space-based and ground-based observations can be used to
test the age-rotation-activity relations.
In this paper we describe the considerations behind the selection of these 20
Sun-like stars and present an initial asteroseismic analysis, which includes
stellar age estimates. We also describe the observations from the Nordic
Optical Telescope and present mean values of measured excess fluxes. These
measurements are combined with estimates of the rotation periods obtained from
a simple analysis of the modulation in photometric observations from Kepler
caused by starspots, and asteroseismic determinations of stellar ages, to test
relations between between age, rotation and activity.Comment: Accepted for publication in MNRA
SPB stars in the open SMC cluster NGC 371
Pulsation in beta Cep and SPB stars are driven by the kappa mechanism which
depends critically on the metallicity. It has therefore been suggested that
beta Cep and SPB stars should be rare in the Magellanic Clouds which have lower
metallicities than the solar neighborhood. To test this prediction we have
observed the open SMC cluster NGC 371 for 12 nights in order to search for beta
Cep and SPB stars. Surprisingly, we find 29 short-period B-type variables in
the upper part of the main sequence, many of which are probably SPB stars. This
result indicates that pulsation is still driven by the kappa mechanism even in
low metallicity environments. All the identified variables have periods longer
than the fundamental radial period which means that they cannot be beta Cep
stars. Within an amplitude detection limit of 5 mmag no stars in the top of the
HR-diagram show variability with periods shorter than the fundamental radial
period. So if beta Cep stars are present in the cluster they oscillate with
amplitudes below 5 mmag, which is significantly lower than the mean amplitude
of beta Cep stars in the Galaxy. We see evidence that multimode pulsation is
more common in the upper part of the main sequence than in the lower. We have
also identified 5 eclipsing binaries and 3 periodic pulsating Be stars in the
cluster field.Comment: 8 pages, 11 figures. Accepted for publication in MNRA
CCD BV and 2MASS photometric study of the open cluster NGC 1513
We present CCD BV and JHK 2MASS photometric data for the open cluster
NGC 1513. We observed 609 stars in the direction of the cluster up to a
limiting magnitude of mag. The star count method shows that the
centre of the cluster lies at ,
and its angular size is arcmin.
The optical and near-infrared two-colour diagrams reveal the colour excesses in
the direction of the cluster as , and
mag. These results are consistent with normal
interstellar extinction values. Optical and near-infrared Zero Age
Main-Sequences (ZAMS) provided an average distance modulus of
mag, which can be translated into a distance of
pc. Finally, using Padova isochrones we determined the metallicity
and age of the cluster as ( dex) and
, respectively.Comment: 15 pages, 12 figures and 4 tables, accepted for publication in
Astrophysics & Space Scienc
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
Superweakly interacting dark matter from the Minimal Walking Technicolor
We study a superweakly interacting dark matter particle motivated by minimal
walking technicolor theories. Our WIMP is a mixture of a sterile state and a
state with the charges of a standard model fourth family neutrino. We show that
the model can give the right amount of dark matter over a range of the WIMP
mass and mixing angle. We compute bounds on the model parameters from the
current accelerator data including the oblique corrections to the precision
electroweak parameters, as well as from cryogenic experiments, Super-Kamiokande
and from the IceCube experiment. We show that consistent dark matter solutions
exist which satisfy all current constraints. However, almost the entire
parameter range of the model lies within the the combined reach of the next
generation experiments.Comment: 29 pages, 6 figure
- …