168 research outputs found
Asteroseismology and evolution of EHB stars
The properties of the Extreme Horizontal Branch stars are quite well
understood, but much uncertainty surrounds the many paths that bring a star to
this peculiar configuration. Asteroseismology of pulsating EHB stars has been
performed on a number of objects, bringing us to the stage where comparisons of
the inferred properties with evolutionary models becomes feasible. In this
review I outline our current understanding of the formation and evolution of
these stars, with emphasis on recent progress. The aim is to show how the
physical parameters derived by asteroseismology can enable the discrimination
between different evolutionary models.Comment: 13 pages, 6 figures, invited review to appear in Communications in
Asteroseismology vol.159, "Proceedings of the JENAM 2008 Symposium No 4:
Asteroseismology and Stellar Evolution
Pulsating stars in NGC 6231 Frequency analysis and photometric mode identification near the main sequence
We used Johnson UBV photometric CCD observations to identify pulsating and
other variable stars in the young open cluster NGC 6231. The multi-color
information was used to classify pulsating variables, perform frequency
analysis, and - where possible - to compare observed to theoretical amplitude
ratios for mode identification. The data reduction was performed with standard
IRAF tools. Differential light curves have been obtained by identifying a set
of suitable comparison stars and the frequency analysis was then conducted on
the basis of Fourier methods. Our classification of pulsating stars was based
on the time scales and amplitudes of the variability with respect to the
different filters and stellar parameters as calculated from published
Str\"omgren and Geneva photometry. We identified 32 variable stars in the field
of the cluster out of which 21 are confirmed members and twelve are newly
detected variable stars. Ten stars were classified as Slowly Pulsating B (SPB)
stars in NGC 6231 out of which seven are new discoveries. We also analyzed six
previously reported {\beta} Cephei variables in more detail. One of them may be
a hybrid {\beta} Cephei/SPB pulsator. In addition, we investigated five more
previously suspected pulsators of this group which we cannot convincingly
confirm. The remaining eleven variable stars are either not members of NGC 6231
or the membership status is questionable. Among them are three previously known
{\delta} Scuti stars, two newly detected pulsators of this class, one new and
two already known eclipsing binaries, one new SPB variable, one possible
Pre-Main-Sequence (PMS) pulsator and another new variable star for which we
cannot present a classification. With more than 20 main sequence pulsators of
spectral type B, NGC 6231 becomes the open cluster with the largest population
of such pulsating stars known.Comment: 27 pages, 35 figures, 3 Tables, accepted by A&A, abstract excessively
shorted due to character limit
Further progress on solar age calibration
We recalibrate a standard solar model seismologically to estimate the
main-sequence age of the Sun. Our procedure differs from what we have done in
the past by removing from the observed frequencies the effect of hydrogen
ionization and the superadiabatic convective boundary layer. Our preliminary
result is Gy.Comment: Accepted for publication in Comm. Asteroseis., 2 pages, 1 figur
Challenges for stellar pulsation and evolution theory
During the last few decades, great effort has been made towards understanding
hydrodynamical processes which determine the structure and evolution of stars.
Up to now, the most stringent constraints have been provided by helioseismology
and stellar cluster studies. However, the contribution of asteroseismology
becomes more and more important, giving us an opportunity to probe the
interiors and atmospheres of very different stellar objects. A variety of
pulsating variables allows us to test various parameters of micro- and
macrophysics by means of oscillation data. I will review the most outstanding
key problems, both observational and theoretical, of which our knowledge can be
improved by means of asteroseismology.Comment: Invited talk presented at the JENAM 2008 Symposium No.4
"Asteroseismology and stellar evolution", eds. S. Schuh and G. Handler.,
Communications in Asteroseismology, 159, in press; 11 pages, 1 figur
Search for exoplanets around pulsating stars of A--F type in Kepler Short Cadence data and the case of KIC 8197761
We searched for extrasolar planets around pulsating stars by examining
data for transit-like events hidden in the intrinsic
variability. All Short Cadence observations for targets with 6000 K 8500 K were visually inspected for transit-like events following the
removal of pulsational signals by sinusoidal fits. Clear transit-like events
were detected in KIC 5613330 and KIC 8197761. KIC 5613330 is a confirmed
exoplanet host (Kepler-635b), where the transit period determined here is
consistent with the literature value. KIC 8197761 is a Doradus -
Scuti star exhibiting eclipses/transits occurring every 9.8686667(27)
d, having durations of 8.37 h, and causing brightness drops . The star's pulsation spectrum contains several mode doublets
and triplets, identified as , with a mean spacing of 0.001659(15) c/d,
implying an internal rotation period of d. Trials to calculate the
size of the light travel time effect (LTTE) from the pulsations to constrain
the companion's mass ended inconclusive. Finding planets around
Doradus stars from the pulsational LTTE, therefore, is concluded to be
unrealistic. Spectroscopic monitoring of KIC 8197761 revealed sinusoidal radial
velocity variations with a semi-amplitude of km/s, while
individual spectra present rotational broadening consistent with km/s. This suggests that the stellar surface rotation is synchronized
with the orbit, whereas the stellar core rotates 30 times slower.
Combining the observed radial velocity variability with the transit photometry,
constrains the companion's mass to be M, ruling out an
exoplanet hypothesis.Comment: 12 pages, 9 figures, accepted for publication in MNRA
BRITE view of Scorpii, Cephei-type star studied for over a century
Preliminary results of the analysis of the combined space-based BRITE and
SMEI, and ground-based Stroemgren photometry are presented. The BRITE data
allowed to find seven p and three g modes in the frequency spectrum of this
star; only four p modes were known in this star prior to this study. The first
results of seismic modelling are also presented.Comment: 3 pages, 1 figure, to appear in the Proceedings of the 3rd BRITE
Science Conference, Auberge du Lac Taureau, Canad
Simulation and evaluation of freeze-thaw cryoablation scenarios for the treatment of cardiac arrhythmias
BACKGROUND: Cardiac cryoablation is a minimally invasive procedure to treat cardiac arrhythmias by cooling cardiac tissues responsible for the cardiac arrhythmia to freezing temperatures. Although cardiac cryoablation offers a gentler treatment than radiofrequency ablation, longer interventions and higher recurrence rates reduce the clinical acceptance of this technique. Computer models of ablation scenarios allow for a closer examination of temperature distributions in the myocardium and evaluation of specific effects of applied freeze-thaw protocols in a controlled environment. METHODS: In this work multiple intervention scenarios with two freeze-thaw cycles were simulated with varying durations and starting times of the interim thawing phase using a finite element model verified by in-vivo measurements and data from literature. To evaluate the effects of different protocols, transmural temperature distributions and iceball dimensions were compared over time. Cryoadhesion durations of the applicator were estimated in the interim thawing phase with varying thawing phase starting times. In addition, the increase of cooling rates was compared between the freezing phases, and the thawing rates of interim thawing phases were analyzed over transmural depth. RESULTS: It could be shown that the increase of cooling rate, the regions undergoing additional phase changes and depths of selected temperatures depend on the chosen ablation protocol. Only small differences of the estimated cryoadhesion duration were found for ablation scenarios with interim thawing phase start after 90 s freezing. CONCLUSIONS: By the presented model a quantification of effects responsible for cell death is possible, allowing for the analysis and optimization of cryoablation scenarios which contribute to a higher clinical acceptance of cardiac cryoablation
Constraining the near-core rotation of the gamma Doradus star 43 Cygni using BRITE-Constellation data
Photometric time series of the Dor star 43 Cyg obtained with the
BRITE-Constellation nano-satellites allow us to study its pulsational
properties in detail and to constrain its interior structure. We aim to find a
g-mode period spacing pattern that allows us to determine the near-core
rotation rate of 43 Cyg and redetermine the star's fundamental atmospheric
parameters and chemical composition. We conducted a frequency analysis using
the 156-days long data set obtained with the BRITE-Toronto satellite and
employed a suite of MESA/GYRE models to derive the mode identification,
asymptotic period spacing and near-core rotation rate. We also used
high-resolution, high signal-to-noise ratio spectroscopic data obtained at the
1.2m Mercator telescope with the HERMES spectrograph to redetermine the
fundamental atmospheric parameters and chemical composition of 43 Cyg using the
software Spectroscopy Made Easy (SME). We detected 43 intrinsic pulsation
frequencies and identified 18 of them to be part of a period spacing pattern
consisting of prograde dipole modes with an asymptotic period spacing of . The near-core rotation rate was
determined to be . The
atmosphere of 43 Cyg shows solar chemical composition at an effective
temperature of 7150 150 K, a log g of 4.2 0.6 dex and a projected
rotational velocity, , of 44 4 kms. The morphology
of the observed period spacing patterns shows indications of the presence of a
significant chemical gradient in the stellar interior.Comment: 9 pages, 8 figures, accepted by A&
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