2,907 research outputs found
A new method to detect solar-like oscillations at very low S/N using statistical significance testing
We introduce a new method to detect solar-like oscillations in frequency
power spectra of stellar observations, under conditions of very low signal to
noise. The Moving-Windowed-Power-Search, or MWPS, searches the power spectrum
for signatures of excess power, over and above slowly varying (in frequency)
background contributions from stellar granulation and shot or instrumental
noise. We adopt a false-alarm approach (Chaplin et al. 2011) to ascertain
whether flagged excess power, which is consistent with the excess expected from
solar-like oscillations, is hard to explain by chance alone (and hence a
candidate detection).
We apply the method to solar photometry data, whose quality was
systematically degraded to test the performance of the MWPS at low
signal-to-noise ratios. We also compare the performance of the MWPS against the
frequently applied power-spectrum-of-power-spectrum (PSxPS) detection method.
The MWPS is found to outperform the PSxPS method.Comment: 10 pages, 7 figures, accepted for publication in MNRAS, Added
reference
Solar-like oscillations of semiregular variables
Oscillations of the Sun and solar-like stars are believed to be excited
stochastically by convection near the stellar surface. Theoretical modeling
predicts that the resulting amplitude increases rapidly with the luminosity of
the star. Thus one might expect oscillations of substantial amplitudes in red
giants with high luminosities and vigorous convection. Here we present evidence
that such oscillations may in fact have been detected in the so-called
semiregular variables, extensive observations of which have been made by
amateur astronomers in the American Association for Variable Star Observers
(AAVSO). This may offer a new opportunity for studying the physical processes
that give rise to the oscillations, possibly leading to further information
about the properties of convection in these stars.Comment: Astrophys. J. Lett., in the press. Processed with aastex and
emulateap
Measuring stellar oscillations using equivalent widths of absorption lines
Kjeldsen et al. (1995, AJ 109, 1313; astro-ph/9411016) have developed a new
technique for measuring stellar oscillations and claimed a detection in the G
subgiant eta Boo. The technique involves monitoring temperature fluctuations in
a star via their effect on the equivalent width of Balmer lines. In this paper
we use synthetic stellar spectra to investigate the temperature dependence of
the Balmer lines, Ca II, Fe I, the Mg b feature and the G~band. We present a
list of target stars likely to show solar-like oscillations and estimate their
expected amplitudes. We also show that centre-to-limb variations in Balmer-line
profiles allow one to detect oscillation modes with l<=4, which accounts for
the detection by Kjeldsen et al. of modes with degree l=3 in integrated
sunlight.Comment: MNRAS (accepted); 7 pages, LaTeX with necessary style file and
PostScript figures in a single uuencoded Z-compressed .tar fil
Characteristics of solar-like oscillations in red giants observed in the CoRoT exoplanet field
Observations during the first long run (~150 days) in the exo-planet field of
CoRoT increase the number of G-K giant stars for which solar-like oscillations
are observed by a factor of 100. This opens the possibility to study the
characteristics of their oscillations in a statistical sense. We aim to
understand the statistical distribution of the frequencies of maximum
oscillation power (nu_max) in red giants and to search for a possible
correlation between nu_max and the large separation (delta_nu). The nu_max
distribution shows a pronounced peak between 20 - 40 microHz. For about half of
the stars we obtain delta_nu with at least two methods. The correlation between
nu_max and delta_nu follows the same scaling relation as inferred for
solar-like stars. The shape of the nu_max distribution can partly be explained
by granulation at low frequencies and by white noise at high frequencies, but
the population density of the observed stars turns out to be also an important
factor. From the fact that the correlation between delta_nu and nu_max for red
giants follows the same scaling relation as obtained for sun-like stars, we
conclude that the sound travel time over the pressure scale height of the
atmosphere scales with the sound travel time through the whole star
irrespective of evolution.Comment: Accepted for publication in Astronomy and Astrophysics (CoRoT special
issue), 5 pages, 7 figures and 1 tabl
Estimating stellar mean density through seismic inversions
Determining the mass of stars is crucial both to improving stellar evolution
theory and to characterising exoplanetary systems. Asteroseismology offers a
promising way to estimate stellar mean density. When combined with accurate
radii determinations, such as is expected from GAIA, this yields accurate
stellar masses. The main difficulty is finding the best way to extract the mean
density from a set of observed frequencies.
We seek to establish a new method for estimating stellar mean density, which
combines the simplicity of a scaling law while providing the accuracy of an
inversion technique.
We provide a framework in which to construct and evaluate kernel-based linear
inversions which yield directly the mean density of a star. We then describe
three different inversion techniques (SOLA and two scaling laws) and apply them
to the sun, several test cases and three stars.
The SOLA approach and the scaling law based on the surface correcting
technique described by Kjeldsen et al. (2008) yield comparable results which
can reach an accuracy of 0.5 % and are better than scaling the large frequency
separation. The reason for this is that the averaging kernels from the two
first methods are comparable in quality and are better than what is obtained
with the large frequency separation. It is also shown that scaling the large
frequency separation is more sensitive to near-surface effects, but is much
less affected by an incorrect mode identification. As a result, one can
identify pulsation modes by looking for an l and n assignment which provides
the best agreement between the results from the large frequency separation and
those from one of the two other methods. Non-linear effects are also discussed
as is the effects of mixed modes. In particular, it is shown that mixed modes
bring little improvement as a result of their poorly adapted kernels.Comment: Accepted for publication in A&A, 20 pages, 19 figure
A comparison of Bayesian and Fourier methods for frequency determination in asteroseismology
Bayesian methods are becoming more widely used in asteroseismic analysis. In
particular, they are being used to determine oscillation frequencies, which are
also commonly found by Fourier analysis. It is important to establish whether
the Bayesian methods provide an improvement on Fourier methods. We compare,
using simulated data, the standard iterative sine-wave fitting method against a
Markov Chain Monte Carlo (MCMC) code that has been introduced to infer purely
the frequencies of oscillation modes (Brewer et al. 2007). A uniform prior
probability distribution function is used for the MCMC method. We find the
methods do equally well at determining the correct oscillation frequencies,
although the Bayesian method is able to highlight the possibility of a
misidentification due to aliasing, which can be useful. In general, we suggest
that the least computationally intensive method is preferable.Comment: 11 pages, 8 figures, accepted for publication in Communications in
Asterosesimolog
Hot Electron Capture Dissociation Distinguishes Leucine from Isoleucine in a Novel Hemoglobin Variant, Hb Askew, β54(D5)Val→Ile
Population migration has led to the global dispersion of human hemoglobinopathies and has precipitated a need for their identification. An effective mass spectrometry-based procedure involves analysis of the intact α- and β-globin chains to determine their mass, followed by location of the variant amino acid residue by direct analysis of the enzymatically digested chains and low-energy collision induced dissociation of the variant peptide. Using this procedure, a variant was identified as either β54Val→Leu or β54Val→Ile, since the amino acids leucine and isoleucine cannot be distinguished using low-energy collisions. Here, we describe how hot electron capture dissociation on a Fourier transform-ion cyclotron resonance mass spectrometer was used to distinguish isoleucine from leucine and identify the mutation as β54(D5)Val→Ile. This is a novel variant, and we have named it Hb Askew
Characterizing temporary hydrological regimes at a European scale
Monthly duration curves have been constructed from climate data across Europe to help address the relative frequency of ecologically critical low flow stages in temporary rivers, when flow persists only in disconnected pools in the river bed. The hydrological model is 5 based on a partitioning of precipitation to estimate water available for evapotranspiration and plant growth and for residual runoff. The duration curve for monthly flows has then been analysed to give an estimate of bankfull flow based on recurrence interval. The corresponding frequency for pools is then based on the ratio of bank full discharge to pool flow, arguing from observed ratios of cross-sectional areas at flood 10 and low flows to estimate pool flow as 0.1% of bankfull flow, and so estimate the frequency of the pool conditions that constrain survival of river-dwelling arthropods and fish. The methodology has been applied across Europe at 15 km resolution, and can equally be applied under future climatic scenarios
Determination of stellar radii from asteroseismic Data
The NASA Kepler mission is designed to find planets through transits.
Accurate and precise radii of the detected planets depend on knowing the radius
of the host star accurately, which is difficult unless the temperature and
luminosity of the star are known precisely. Kepler, however, has an
asteroseismology programme that will provide seismic variables that can
characterise stellar radii easily, accurately, and extremely precisely. In this
paper we describe the Yale-Birmingham (YB) method to determine stellar radii
using a combination of seismic and conventional variables, and analyse the
effect of these variables on the result. We find that for main-sequence stars,
a knowledge of the parallax is not important to get accurate radii using the YB
method: we can get results to an accuracy and precision of better than a few
percent if we know the effective temperature and the seismic parameters for
these stars. Metallicity does not make much difference either. However, good
estimates of the effective temperature and metallicity, along with those of the
seismic parameters, are essential to determine radii of sub giants properly. On
the other hand, for red giants we find that determining radii properly is not
possible without a good estimate of the parallax. We find that the so called
"surface term" in the seismic data has minimal effect on the inferred radii.
Uncertainties in the convective mixing length can matter under some
circumstances and can cause a systematic shift in the inferred radii. Blind
tests with data simulated to match those expected from the asteroseismic Survey
Phase of Kepler show that it will be possible to infer stellar radii
successfully using our method.Comment: Submitted to Ap
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