369 research outputs found
Radial velocities of giant stars: an investigation of line profile variations
Since 1999, a radial velocity survey of 179 red giant stars is ongoing at
Lick Observatory with a one month cadence. At present ~20-100 measurements have
been collected per star with an accuracy of 5 to 8 m/s. Of the stars monitored,
145 (80%) show radial velocity (RV) variations at a level >20 m/s, of which 43
exhibit significant periodicities. Here, we investigate the mechanism causing
the observed radial velocity variations. Firstly, we search for a correlation
between the radial velocity amplitude and an intrinsic parameter of the star,
in this case surface gravity (log g). Secondly, we investigate line profile
variations and compare these with theoretical predictions.Comment: To appear in the proceedings of the Helas II workshop:
Helioseismology, Asteroseismology and MHD Connections; published in the
Journal of Physics: Conference Series, ed. L. Gizon; 7 pages, 5 figure
Atmospheric parameters of 82 red giants in the Kepler field
Context: Accurate fundamental parameters of stars are essential for the
asteroseismic analysis of data from the NASA Kepler mission. Aims: We aim at
determining accurate atmospheric parameters and the abundance pattern for a
sample of 82 red giants that are targets for the Kepler mission. Methods: We
have used high-resolution, high signal-to-noise spectra from three different
spectrographs. We used the iterative spectral synthesis method VWA to derive
the fundamental parameters from carefully selected high-quality iron lines.
After determination of the fundamental parameters, abundances of 13 elements
were measured using equivalent widths of the spectral lines. Results: We
identify discrepancies in log g and [Fe/H], compared to the parameters based on
photometric indices in the Kepler Input Catalogue (larger than 2.0 dex for log
g and [Fe/H] for individual stars). The Teff found from spectroscopy and
photometry shows good agreement within the uncertainties. We find good
agreement between the spectroscopic log g and the log g derived from
asteroseismology. Also, we see indications of a potential metallicity effect on
the stellar oscillations. Conclusions: We have determined the fundamental
parameters and element abundances of 82 red giants. The large discrepancies
between the spectroscopic log g and [Fe/H] and values in the Kepler Input
Catalogue emphasize the need for further detailed spectroscopic follow-up of
the Kepler targets in order to produce reliable results from the asteroseismic
analysis.Comment: 16 Pages, 12 Figures, accepted for publication in A&
Precise radial velocities of giant stars. IV. A correlation between surface gravity and radial velocity variation and a statistical investigation of companion properties
Since 1999, we have been conducting a radial velocity survey of 179 K giants
using the CAT at UCO/Lick observatory. At present ~20-100 measurements have
been collected per star with a precision of 5 to 8 m/s. Of the stars monitored,
145 (80%) show radial velocity (RV) variations at a level >20 m/s, of which 43
exhibit significant periodicities. Our aim is to investigate possible
mechanism(s) that cause these observed RV variations. We intend to test whether
these variations are intrinsic in nature, or possibly induced by companions, or
both. In addition, we aim to characterise the parameters of these companions. A
relation between log g and the amplitude of the RV variations is investigated
for all stars in the sample. Furthermore, the hypothesis that all periodic RV
variations are caused by companions is investigated by comparing their inferred
orbital statistics with the statistics of companions around main sequence
stars. A strong relation is found between the amplitude of the RV variations
and log g in K giant stars, as suggested earlier by Hatzes & Cochran (1998).
However, most of the stars exhibiting periodic variations are located above
this relation. These RV variations can be split in a periodic component which
is not correlated with log g and a random residual part which does correlate
with log g. Compared to main-sequence stars, K giants frequently exhibit
periodic RV variations. Interpreting these RV variations as being caused by
companions, the orbital param eters are different from the companions orbiting
dwarfs. Intrinsic mechanisms play an important role in producing RV variations
in K giants stars, as suggested by their dependence on log g. However, it
appears that periodic RV variations are additional to these intrinsic
variations, consistent with them being caused by companions.Comment: 10 pages, accepted by A&
The connection between stellar granulation and oscillation as seen by the Kepler mission
The long and almost continuous observations by Kepler show clear evidence of
a granulation background signal in a large sample of stars, which is
interpreted as the surface manifestation of convection. It has been shown that
its characteristic timescale and rms intensity fluctuation scale with the peak
frequency (\nu_{max}) of the solar-like oscillations. Various attempts have
been made to quantify the observed signal, to determine scaling relations, and
to compare them to theoretical predictions. We use a probabilistic method to
compare different approaches to extracting the granulation signal. We fit the
power density spectra of a large set of Kepler targets, determine the
granulation and global oscillation parameter, and quantify scaling relations
between them. We establish that a depression in power at about \nu_{max}/2,
known from the Sun and a few other main-sequence stars, is also statistically
significant in red giants and that a super-Lorentzian function with two
components is best suited to reproducing the granulation signal in the broader
vicinity of the pulsation power excess. We also establish that the specific
choice of the background model can affect the determination of \nu_{max},
introducing systematic uncertainties that can significantly exceed the random
uncertainties. We find the characteristic background frequency and amplitude to
tightly scale with \nu_{max} for a wide variety of stars, and quantify a mass
dependency of the latter. To enable comparison with theoretical predictions, we
computed effective timescales and intensity fluctuations and found them to
approximately scale as \tau_{eff} \propto g^{-0.85}\,T^{-0.4} and A_{gran}
\propto (g^2M)^{-1/4}, respectively. Similarly, the bolometric pulsation
amplitude scales approximately as A_{puls} \propto (g^2M)^{-1/3}, which
implicitly verifies a separate mass and luminosity dependence of A_{puls}.Comment: 18 pages, 12 figures, accepted for A&
Solar-like oscillations in red giants observed with Kepler: comparison of global oscillation parameters from different methods
The large number of stars for which uninterrupted high-precision photometric
timeseries data are being collected with \textit{Kepler} and CoRoT initiated
the development of automated methods to analyse the stochastically excited
oscillations in main-sequence, subgiant and red-giant stars. Aims: We
investigate the differences in results for global oscillation parameters of G
and K red-giant stars due to different methods and definitions. We also
investigate uncertainties originating from the stochastic nature of the
oscillations. Methods: For this investigation we use Kepler data obtained
during the first four months of operation. These data have been analysed by
different groups using already published methods and the results are compared.
We also performed simulations to investigate the uncertainty on the resulting
parameters due to different realizations of the stochastic signal. Results: We
obtain results for the frequency of maximum oscillation power (nu_max) and the
mean large separation () from different methods for over one thousand
red-giant stars. The results for these parameters agree within a few percent
and seem therefore robust to the different analysis methods and definitions
used here. The uncertainties for nu_max and due to differences in
realization noise are not negligible and should be taken into account when
using these results for stellar modelling.Comment: 11 pages, 9 Figures and 7 tables, accepted for publication in
Astronomy and Astrophysic
Dusty tails of evaporating exoplanets. II. Physical modelling of the KIC 12557548b light curve
Evaporating rocky exoplanets, such as KIC 12557548b, eject large amounts of
dust grains, which can trail the planet in a comet-like tail. When such objects
occult their host star, the resulting transit signal contains information about
the dust in the tail. We aim to use the detailed shape of the Kepler light
curve of KIC 12557548b to constrain the size and composition of the dust grains
that make up the tail, as well as the mass loss rate of the planet. Using a
self-consistent numerical model of the dust dynamics and sublimation, we
calculate the shape of the tail by following dust grains from their ejection
from the planet to their destruction due to sublimation. From this dust cloud
shape, we generate synthetic light curves (incorporating the effects of
extinction and angle-dependent scattering), which are then compared with the
phase-folded Kepler light curve. We explore the free-parameter space thoroughly
using a Markov chain Monte Carlo method. Our physics-based model is capable of
reproducing the observed light curve in detail. Good fits are found for initial
grain sizes between 0.2 and 5.6 micron and dust mass loss rates of 0.6 to 15.6
M_earth/Gyr (2-sigma ranges). We find that only certain combinations of
material parameters yield the correct tail length. These constraints are
consistent with dust made of corundum (Al2O3), but do not agree with a range of
carbonaceous, silicate, or iron compositions. Using a detailed, physically
motivated model, it is possible to constrain the composition of the dust in the
tails of evaporating rocky exoplanets. This provides a unique opportunity to
probe to interior composition of the smallest known exoplanets.Comment: 18 pages, 11 figures, A&A accepte
Non-radial oscillations in the red giant HR7349 measured by CoRoT
Convection in red giant stars excites resonant acoustic waves whose
frequencies depend on the sound speed inside the star, which in turn depends on
the properties of the stellar interior. Therefore, asteroseismology is the most
robust available method for probing the internal structure of red giant stars.
Solar-like oscillations in the red giant HR7349 are investigated. Our study is
based on a time series of 380760 photometric measurements spread over 5 months
obtained with the CoRoT satellite. Mode parameters were estimated using maximum
likelihood estimation of the power spectrum. The power spectrum of the
high-precision time series clearly exhibits several identifiable peaks between
19 and 40 uHz showing regularity with a mean large and small spacing of Dnu =
3.47+-0.12 uHz and dnu_02 = 0.65+-0.10 uHz. Nineteen individual modes are
identified with amplitudes in the range from 35 to 115 ppm. The mode damping
time is estimated to be 14.7+4.7-2.9 days.Comment: 8 pages, A&A accepte
Modelling a high-mass red giant observed by CoRoT
The G6 giant HR\,2582 (HD\,50890) was observed by CoRoT for approximately 55
days. Mode frequencies are extracted from the observed Fourier spectrum of the
light curve. Numerical stellar models are then computed to determine the
characteristics of the star (mass, age, etc...) from the comparison with
observational constraints. We provide evidence for the presence of solar-like
oscillations at low frequency, between 10 and 20\,Hz, with a regular
spacing of Hz between consecutive radial orders. Only radial
modes are clearly visible. From the models compatible with the observational
constraints used here, We find that HR\,2582 (HD\,50890) is a massive star with
a mass in the range (3--\,5\,), clearly above the red clump. It
oscillates with rather low radial order ( = 5\,--\,12) modes. Its
evolutionary stage cannot be determined with precision: the star could be on
the ascending red giant branch (hydrogen shell burning) with an age of
approximately 155 Myr or in a later phase (helium burning). In order to obtain
a reasonable helium amount, the metallicity of the star must be quite subsolar.
Our best models are obtained with a mixing length significantly smaller than
that obtained for the Sun with the same physical description (except
overshoot). The amount of core overshoot during the main-sequence phase is
found to be mild, of the order of 0.1\,.Comment: Accepted in A&
Characterisation of red-giant stars in the public Kepler data
The first public release of long-cadence stellar photometric data collected
by the NASA Kepler mission has now been made available. In this paper we
characterise the red-giant (G-K) stars in this large sample in terms of their
solar-like oscillations. We use published methods and well-known scaling
relations in the analysis. Just over 70% of the red giants in the sample show
detectable solar-like oscillations, and from these oscillations we are able to
estimate the fundamental properties of the stars. This asteroseismic analysis
reveals different populations: low-luminosity H-shell burning red-giant branch
stars, cool high-luminosity red giants on the red-giant branch and He-core
burning clump and secondary-clump giants.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Main Journa
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