311 research outputs found
Asymptotic and measured large frequency separations
With the space-borne missions CoRoT and Kepler, a large amount of
asteroseismic data is now available. So-called global oscillation parameters
are inferred to characterize the large sets of stars, to perform ensemble
asteroseismology, and to derive scaling relations. The mean large separation is
such a key parameter. It is therefore crucial to measure it with the highest
accuracy. As the conditions of measurement of the large separation do not
coincide with its theoretical definition, we revisit the asymptotic expressions
used for analysing the observed oscillation spectra. Then, we examine the
consequence of the difference between the observed and asymptotic values of the
mean large separation. The analysis is focused on radial modes. We use series
of radial-mode frequencies to compare the asymptotic and observational values
of the large separation. We propose a simple formulation to correct the
observed value of the large separation and then derive its asymptotic
counterpart. We prove that, apart from glitches due to stellar structure
discontinuities, the asymptotic expansion is valid from main-sequence stars to
red giants. Our model shows that the asymptotic offset is close to 1/4, as in
the theoretical development. High-quality solar-like oscillation spectra
derived from precise photometric measurements are definitely better described
with the second-order asymptotic expansion. The second-order term is
responsible for the curvature observed in the \'echelle diagrams used for
analysing the oscillation spectra and this curvature is responsible for the
difference between the observed and asymptotic values of the large separation.
Taking it into account yields a revision of the scaling relations providing
more accurate asteroseismic estimates of the stellar mass and radius.Comment: accepted in A&
Library Free Technology Mapping
This paper presents an efficient method for mapping a set of Boolean equations onto a set of Static CMOS Complex Gates (SCCGs) under a constraint in the number of serial transistors. This Library Free Technology Mapping (LFTM) approach uses a virtuallibrary of SCCGs available through a layout generator, instead of using a limited set of pre characterized cells. Our goal is to use a virtuallibrary of SCCGs to perform the mapping at the transistor leveI, in order to fit the topological constraints imposed by the CMOS technology. Limitations of previously proposed techniques to perform Library Free Technology Mapping are discussed. The proposed method, based on an one-to-one association of CMOS transistors with Binary Decision Diagram ares, is not dependent on the initial ordering of Boolean equations. Experimental results comparing this technique to previously published ones indicate that it generates good-quality solutions
The CoRoT target HD175726: an active star with weak solar-like oscillations
Context. The CoRoT short runs give us the opportunity to observe a large
variety of late-type stars through their solar-like oscillations. We report
observations of the star HD175726 that lasted for 27 days during the first
short run of the mission. The time series reveals a high-activity signal and
the power spectrum presents an excess due to solar-like oscillations with a low
signal-to-noise ratio. Aims. Our aim is to identify the most efficient tools to
extract as much information as possible from the power density spectrum.
Methods. The most productive method appears to be the autocorrelation of the
time series, calculated as the spectrum of the filtered spectrum. This method
is efficient, very rapid computationally, and will be useful for the analysis
of other targets, observed with CoRoT or with forthcoming missions such as
Kepler and Plato. Results. The mean large separation has been measured to be
97.2+-0.5 microHz, slightly below the expected value determined from solar
scaling laws.We also show strong evidence for variation of the large separation
with frequency. The bolometric mode amplitude is only 1.7+-0.25 ppm for radial
modes, which is 1.7 times less than expected. Due to the low signal-to-noise
ratio, mode identification is not possible for the available data set of
HD175726. Conclusions. This study shows the possibility of extracting a seismic
signal despite a signal-to-noise ratio of only 0.37. The observation of such a
target shows the efficiency of the CoRoT data, and the potential benefit of
longer observing runs.Comment: 8 pages. Accepted in A&
Hydrodynamical simulations of convection-related stellar micro-variability. II. The enigmatic granulation background of the COROT target HD49933
Local-box hydrodynamical model atmospheres provide statistical information
about a star's emergent radiation field which allows one to predict the level
of its granulation-related micro-variability. Space-based photometry is now
sufficiently accurate to test model predictions. We aim to model the
photometric granulation background of HD49933 as well as the Sun, and compare
the predictions to the measurements obtained by the COROT and SOHO satellite
missions. We construct hydrodynamical model atmospheres representing HD49933
and the Sun, and use a previously developed scaling technique to obtain the
observable disk-integrated brightness fluctuations. We further performed
exploratory magneto-hydrodynamical simulations to gauge the impact of small
scale magnetic fields on the synthetic light-curves. We find that the
granulation-related brightness fluctuations depend on metallicity. We obtain a
satisfactory correspondence between prediction and observation for the Sun,
validating our approach. For HD49933, we arrive at a significant
over-estimation by a factor of two to three in total power. Locally generated
magnetic fields are unlikely to be responsible, otherwise existing fields would
need to be rather strong to sufficiently suppress the granulation signal.
Presently suggested updates on the fundamental stellar parameters do not
improve the correspondence; however, an ad-hoc increase of the HD49933 surface
gravity by about 0.2dex would eliminate most of the discrepancy. We diagnose a
puzzling discrepancy between the predicted and observed granulation background
in HD49933, with only rather ad-hoc ideas for remedies at hand.Comment: 7 pages, 5 figures, accepted for publication in A&
Visibilities and bolometric corrections for stellar oscillation modes observed by Kepler
Kepler produces a large amount of data used for asteroseismological analyses,
particularly of solar-like stars and red giants. The mode amplitudes observed
in the Kepler spectral band have to be converted into bolometric amplitudes to
be compared to models. We give a simple bolometric correction for the
amplitudes of radial modes observed with Kepler, as well as the relative
visibilities of non-radial modes. We numerically compute the bolometric
correction c_{K-bol} and mode visibilities for different effective temperatures
Teff within the range 4000-7500 K, using a similar approach to a recent one
from the literature (Michel et al. 2009, A&A 495, 979). We derive a law for the
correction to bolometric values: c_{K-bol} = 1 + a_1 (Teff-To) + a_2
(Teff-To)^2, with To = 5934 K, a_1 = 1.349e-4 K^{-1}, and a_2 = -3.120e-9
K^{-2} or, alternatively, as the power law c_{K-bol} = (Teff/To)^alpha with
alpha = 0.80. We give tabulated values for the mode visibilities based on
limb-darkening (LD), computed from ATLAS9 model atmospheres for Teff in
[4000,7500] K, log g in [2.5,4.5], and [M/H] in [-1.0,+1.0]. We show that using
LD profiles already integrated over the spectral band provides quick and good
approximations for visibilities. We point out the limits of these classical
visibility estimations.Comment: 5 pages, 4 figures, 1 table, minor language edition. Published in A&
On detecting the large separation in the autocorrelation of stellar oscillation times series
The observations carried out by the space missions CoRoT and Kepler provide a
large set of asteroseismic data. Their analysis requires an efficient procedure
first to determine if the star is reliably showing solar-like oscillations,
second to measure the so-called large separation, third to estimate the
asteroseismic information that can be retrieved from the Fourier spectrum. We
develop in this paper a procedure, based on the autocorrelation of the seismic
Fourier spectrum. We have searched for criteria able to predict the output that
one can expect from the analysis by autocorrelation of a seismic time series.
First, the autocorrelation is properly scaled for taking into account the
contribution of white noise. Then, we use the null hypothesis H0 test to assess
the reliability of the autocorrelation analysis. Calculations based on solar
and CoRoT times series are performed in order to quantify the performance as a
function of the amplitude of the autocorrelation signal. We propose an
automated determination of the large separation, whose reliability is
quantified by the H0 test. We apply this method to analyze a large set of red
giants observed by CoRoT. We estimate the expected performance for photometric
time series of the Kepler mission. Finally, we demonstrate that the method
makes it possible to distinguish l=0 from l=1 modes. The envelope
autocorrelation function has proven to be very powerful for the determination
of the large separation in noisy asteroseismic data, since it enables us to
quantify the precision of the performance of different measurements: mean large
separation, variation of the large separation with frequency, small separation
and degree identification.Comment: A&A, in pres
HD 51106 and HD 50747: an ellipsoidal binary and a triple system observed with CoRoT
We present an analysis of the observations of HD 51106 and HD 50747 by the
satellite CoRoT, obtained during its initial run, and of the spectroscopic
preparatory observations.
AIMS: We complete an analysis of the light curve, extract the main
frequencies observed, and discuss some preliminary interpretations about the
stars.
Methods: We used standard Fourier transform and pre-whitening methods to
extract information about the periodicities of the stars.
Results: HD 51106 is an ellipsoidal binary, the light curve of which can be
completely explained by the tidal deformation of the star and smaller secondary
effects. HD 50747 is a triple system containing a variable star, which exhibits
many modes of oscillation with periods in the range of a few hours. On the
basis of this period range and the analysis of the physical parameters of the
star, we conclude that HD 50747 is a Gamma-Doradus star.Comment: 7 pages, 8 figures, use (Astronomy-Astrophysics format/macro LAtex
An asteroseismic study of the O9V star HD 46202 from CoRoT space-based photometry
The O9V star HD 46202, which is a member of the young open cluster NGC 2244,
was observed by the CoRoT satellite in October/November 2008 during a short run
of 34 days. From the very high-precision light curve, we clearly detect beta
Cep-like pulsation frequencies with amplitudes of ~0.1 mmag and below. A
comparison with stellar models was performed using a chi^2 as a measure for the
goodness-of-fit between the observed and theoretically computed frequencies.
The physical parameters of our best-fitting models are compatible with the ones
deduced spectroscopically. A core overshooting parameter alpha_ov = 0.10 +-
0.05 pressure scale height is required. None of the observed frequencies are
theoretically excited with the input physics used in our study. More
theoretical work is thus needed to overcome this shortcoming in how we
understand the excitation mechanism of pulsation modes in such a massive star.
A similar excitation problem has also been encountered for certain pulsation
modes in beta Cep stars recently modelled asteroseismically.Comment: Accepted for publication in Astronomy and Astrophysics on 17/12/2010,
9 pages, 7 figures, 4 table
- …