525 research outputs found
Population of the lower part of the instability strip: Delta Scuti stars and dwarf Cepheids (or AI Velorum)
Some of the properties of the atmospheric variations in delta Scuti stars were investigated with emphasis on the amplitude and the shape of both light curves and radial velocity curves. It is shown that these curves are small and rapidly variable in the case of dwarf Scuti stars; for the evolved stars the situation is more complex. The relation between variables and nonvariables, and also the results on abundances in the atmospheres of these stars were surveyed with respect to the hydrodynamics of their envelopes. The abundance anomalies of Am stars were qualitatively examined. The coexistence of abundance anomalies and variability among giants were also studied. Attempts were made to relate the variability to the hydrogen ionization zone in an envelope deprived of helium. Specific results are reported
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&
Detecting circumbinary planets using eclipse timing of binary stars - numerical simulations
The presence of a body in an orbit around a close eclipsing binary star
manifests itself through the light time effect influencing the observed times
of eclipses as the close binary and the circumbinary companion both move around
the common centre of mass. This fact combined with the periodicity with which
the eclipses occur can be used to detect the companion. Given a sufficient
precision of the times of eclipses, the eclipse timing can be employed to
detect substellar or even planetary mass companions.
The main goal of the paper is to investigate the potential of the photometry
based eclipse timing of binary stars as a method of detecting circumbinary
planets. In the models we assume that the companion orbits a binary star in a
circular Keplerian orbit. We analyze both the space and ground based photometry
cases. In particular, we study the usefulness of the on-going COROT and Kepler
missions in detecting circumbinary planets. We also explore the relations
binding the planet discovery space with the physical parameters of the binaries
and the geometrical parameters of their light curves. We carry out detailed
numerical simulations of the eclipse timing by employing a relatively realistic
model of the light curves of eclipsing binary stars. We study the influence of
the white and red photometric noises on the timing precision. We determine the
sensitivity of the eclipse timing technique to circumbinary planets for the
ground and space based photometric observations. We provide suggestions for the
best targets, observing strategies and instruments for the eclipse timing
method. Finally, we compare the eclipse timing as a planet detection method
with the radial velocities and astrometry.Comment: 9 pages, 15 figures, accepted for publication in MNRA
Planetary eclipse mapping of CoRoT-2a. Evolution, differential rotation, and spot migration
The lightcurve of CoRoT-2 shows substantial rotational modulation and
deformations of the planet's transit profiles caused by starspots. We
consistently model the entire lightcurve, including both rotational modulation
and transits, stretching over approximately 30 stellar rotations and 79
transits. The spot distribution and its evolution on the noneclipsed and
eclipsed surface sections are presented and analyzed, making use of the high
resolution achievable under the transit path.
We measure the average surface brightness on the eclipsed section to be
(5\pm1) % lower than on the noneclipsed section. Adopting a solar spot
contrast, the spot coverage on the entire surface reaches up to 19 % and a
maximum of almost 40 % on the eclipsed section. Features under the transit
path, i.e. close to the equator, rotate with a period close to 4.55 days.
Significantly higher rotation periods are found for features on the noneclipsed
section indicating a differential rotation of . Spotted
and unspotted regions in both surface sections concentrate on preferred
longitudes separated by roughly 180 deg.Comment: Paper accepted by A&A 17/02/2010. For a better resolution paper
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TRUFAS, a wavelet based algorithm for the rapid detection of planetary transits
Aims: We describe a fast, robust and automatic detection algorithm, TRUFAS,
and apply it to data that are being expected from the CoRoT mission. Methods:
The procedure proposed for the detection of planetary transits in light curves
works in two steps: 1) a continuous wavelet transformation of the detrended
light curve with posterior selection of the optimum scale for transit
detection, and 2) a period search in that selected wavelet transformation. The
detrending of the light curves are based on Fourier filtering or a discrete
wavelet transformation. TRUFAS requires the presence of at least 3 transit
events in the data. Results: The proposed algorithm is shown to identify
reliably and quickly the transits that had been included in a standard set of
999 light curves that simulate CoRoT data. Variations in the pre-processing of
the light curves and in the selection of the scale of the wavelet transform
have only little effect on TRUFAS' results. Conclusions: TRUFAS is a robust and
quick transit detection algorithm, especially well suited for the analysis of
very large volumes of data from space or ground-based experiments, with long
enough durations for the target-planets to produce multiple transit events.Comment: 9 pages, 10 figures, accepted by A&
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
Short-lived spots in solar-like stars as observed by CoRoT
Context. CoRoT light curves have an unprecedented photometric quality, having
simultaneously a high signal-to-noise ratio, a long time span and a nearly
continuous duty-cycle. Aims. We analyse the light-curves of four bright targets
observed in the seismology field and study short-lived small spots in
solar-like stars. Methods. We present a simple spot modeling by iterative
analysis. Its ability to extract relevant parameters is ensured by implementing
relaxation steps to avoid convergence to local minima of the sum of the
residuals between observations and modeling. The use of Monte-Carlo simulations
allows us to estimate the performance of the fits. Results. Our starspot
modeling gives a representation of the spots on these stars in agreement with
other well tested methods. Within this framework, parameters such as rigid-body
rotation and spot lifetimes seem to be precisely determined. Then, the
lifetime/rotation period ratios are in the range 0.5 - 2, and there is clear
evidence for differential rotation.Comment: 11 pages Accepted in A&
Periodic mass loss episodes due to an oscillation mode with variable amplitude in the hot supergiant HD50064
We aim to interpret the photometric and spectroscopic variability of the
luminous blue variable supergiant HD\,50064 ().CoRoT space photometry
and follow-up high-resolution spectroscopy, with a time base of 137\,d and
169\,d, respectively, was gathered, analysed and interpreted using standard
time series analysis and light curve modelling methods as well as spectral line
diagnostics.The space photometry reveals one period of 37\,d, which undergoes a
sudden amplitude change with a factor 1.6. The pulsation period is confirmed in
the spectroscopy, which additionally reveals metal line radial velocity values
differing by km\,s depending on the spectral line and on the
epoch. We estimate \teff13\,500\,K, \logg1.5 from the equivalent
width of Si lines. The Balmer lines reveal that the star undergoes episodes of
changing mass loss on a time scale similar to the changes in the photometric
and spectroscopic variability, with an average value of (in M\,yr). We tentatively interpret the 37\,d
period as due to a strange mode oscillation.Comment: 4 pages, accepted for publication in Astronomy & Astrophysics Letter
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