8,882 research outputs found
Solar-like oscillations in a metal-poor globular cluster with the HST
We present analyses of variability in the red giant stars in the metal-poor
globular cluster NGC6397, based on data obtained with the Hubble Space
Telescope. We use an non-standard data reduction approach to turn a 23-day
observing run originally aimed at imaging the white dwarf population, into
time-series photometry of the cluster's highly saturated red giant stars. With
this technique we obtain noise levels in the final power spectra down to 50
parts per million, which allows us to search for low amplitude solar-like
oscillations. We compare the observed excess power seen in the power spectra
with estimates of the typical frequency range, frequency spacing and amplitude
from scaling the solar oscillations. We see evidence that the detected
variability is consistent with solar-like oscillations in at least one and
perhaps up to four stars. With metallicities two orders of magnitude lower than
of the Sun, these stars present so far the best evidence of solar-like
oscillations in such a low metallicity environment.Comment: 7 pages, 6 figures, accepted by Ap
COST Action ES0901: European procedures for flood frequency estimation (FloodFreq) [Keynote]
The aim of COST Action ES0901 European Procedures for Flood Frequency Estimation (FloodFreq) is to undertake
a Pan-European comparison and evaluation of different methods for flood frequency estimation under the
various climatologic and geographic conditions found in Europe, and different levels of data availability. A scientific framework for assessing the ability of these methods to predict the impact of environmental change (climate change, land-use and river engineering works) on future flood frequency characteristics (flood occurrence and magnitude) will be developed and tested. The availability of such procedures is crucial for the formulation of robust flood risk management strategies as required by the Directive of the European Parliament on the assessment and management of floods. The outputs from FloodFreq will be disseminated to: academics, professionals involved in operational flood risk management from private and public institutions, and relevant policy makers from national and international regulatory bodies. This Action enable cooperation between researchers involved in nationally funded research projects to, thereby enabling testing of methods free from the constraints of administrative boundaries, and allowing a more efficient use of European flood research funding
Correcting stellar oscillation frequencies for near-surface effects
In helioseismology, there is a well-known offset between observed and
computed oscillation frequencies. This offset is known to arise from improper
modeling of the near-surface layers of the Sun, and a similar effect must occur
for models of other stars. Such an effect impedes progress in asteroseismology,
which involves comparing observed oscillation frequencies with those calculated
from theoretical models. Here, we use data for the Sun to derive an empirical
correction for the near-surface offset, which we then apply three other stars
(alpha Cen A, alpha Cen B and beta Hyi). The method appears to give good
results, in particular providing an accurate estimate of the mean density of
each star.Comment: accepted for publication in ApJ Letter
P-mode observations on Alpha Cen A
We have made a clear detection of p-mode oscillations in the nearest
solar-like G2V star Cen A with the CORALIE spectrograph on the 1.2-m
Swiss telescope at the ESO La Silla Observatory. We report the 5 nights of
observation on this star in May 2001 during which 1260 high precision radial
velocity measurements were obtained. The power spectrum clearly shows several
identifiable peaks between 1.7 and 3 mHz. The average large splitting of 105.7
Hz and the amplitude of about 35 cm s of these modes are in
agreement with theoretical expectations.Comment: 4 pages, 4 figures, A&A Letter accepte
Joint Time Frequency Analysis Techniques: A study of Transitional Dyamics in Sheet/Cloud Cavitation
An introduction to, and the use of, Joint Time Frequency Analysis techniques is given. Special emphasis is made on Time Frequency Distribution series. This method is demonstrated on selected experimental data. Attention is given the transition region in the dynamic nature of sheet/cloud cavitation. Wavelets and a method that is a variation of phase-portraits are also presented and discussed with regard to cavitation dynamics. An example on
how experimental techniques have been improved by the above mentioned methods is given
Scaled oscillation frequencies and echelle diagrams as a tool for comparative asteroseismology
We describe a method for comparing the frequency spectra of oscillating
stars. We focus on solar-like oscillations, in which mode frequencies generally
follow a regular pattern. On the basis that oscillation frequencies of similar
stars scale homologously, we show how to display two stars on a single echelle
diagram. The result can be used to infer the ratio of their mean densities very
precisely, without reference to theoretical models. In addition, data from the
star with the better signal-to-noise ratio can be used to confirm weaker modes
and reject sidelobes in data from the second star. Finally, we show that scaled
echelle diagrams provide a solution to the problem of ridge identification in
F-type stars, such as those observed by the CoRoT space mission.Comment: accepted for publication in Communications in Asteroseismolog
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