1,175 research outputs found
Residual stress measurements of alumina-zirconia ceramics by time-of-flight neutron diffraction
Neutron strain scanning and Rietveld analysis were used to study the residual stresses in Al2O3/Y-TZP ceramic composites fabricated by different green processing techniques (a novel tape casting and conventional slip casting) and with different zirconia content. The results show that the residual stresses in zirconia particulates are tensile and the ones in alumina matrix are compressive, with almost flat through-thickness residual stress profiles in all bulk samples. The residual stresses for both phases were mainly dependent on the zirconia content, irrespective of the measurement direction and the fabrication process
Oscillating red giants in the CoRoT exo-field: Asteroseismic mass and radius determination
Context. Observations and analysis of solar-type oscillations in red-giant
stars is an emerging aspect of asteroseismic analysis with a number of open
questions yet to be explored. Although stochastic oscillations have previously
been detected in red giants from both radial velocity and photometric
measurements, those data were either too short or had sampling that was not
complete enough to perform a detailed data analysis of the variability. The
quality and quantity of photometric data as provided by the CoRoT satellite is
necessary to provide a breakthrough in observing p-mode oscillations in red
giants. We have analyzed continuous photometric time-series of about 11 400
relatively faint stars obtained in the exofield of CoRoT during the first 150
days long-run campaign from May to October 2007. We find several hundred stars
showing a clear power excess in a frequency and amplitude range expected for
red-giant pulsators. In this paper we present first results on a sub-sample of
these stars. Aims. Knowing reliable fundamental parameters like mass and radius
is essential for detailed asteroseismic studies of red-giant stars. As the
CoRoT exofield targets are relatively faint (11-16 mag) there are no (or only
weak) constraints on the star's location in the H-R diagram. We therefore aim
to extract information about such fundamental parameters solely from the
available time series. Methods. We model the convective background noise and
the power excess hump due to pulsation with a global model fit and deduce
reliable estimates for the stellar mass and radius from scaling relations for
the frequency of maximum oscillation power and the characteristic frequency
separation.Comment: 10 pages, 7 figures, accepted for publication in A&
The universal red-giant oscillation pattern; an automated determination with CoRoT data
The CoRoT and Kepler satellites have provided thousands of red-giant
oscillation spectra. The analysis of these spectra requires efficient methods
for identifying all eigenmode parameters. The assumption of new scaling laws
allows us to construct a theoretical oscillation pattern. We then obtain a
highly precise determination of the large separation by correlating the
observed patterns with this reference. We demonstrate that this pattern is
universal and are able to unambiguously assign the eigenmode radial orders and
angular degrees. This solves one of the current outstanding problems of
asteroseismology hence allowing precise theoretical investigation of red-giant
interiors.Comment: Accepted in A&A letter
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
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