118 research outputs found
Asteroseismology and interferometry of the red giant star epsilon Oph
The GIII red giant star epsilon Oph has been found to exhibit several modes
of oscillation by the MOST mission. We interpret the observed frequencies of
oscillation in terms of theoretical radial p-mode frequencies of stellar
models. Evolutionary models of this star, in both shell H-burning and core
He-burning phases of evolution, are constructed using as constraints a
combination of measurements from classical ground-based observations (for
luminosity, temperature, and chemical composition) and seismic observations
from MOST. Radial frequencies of models in either evolutionary phase can
reproduce the observed frequency spectrum of epsilon Oph almost equally well.
The best-fit models indicate a mass in the range of 1.85 +/- 0.05 Msun with
radius of 10.55 +/- 0.15 Rsun. We also obtain an independent estimate of the
radius of epsilon Oph using high accuracy interferometric observations in the
infrared K' band, using the CHARA/FLUOR instrument. The measured limb darkened
disk angular diameter of epsilon Oph is 2.961 +/- 0.007 mas. Together with the
Hipparcos parallax, this translates into a photospheric radius of 10.39 +/-
0.07 Rsun. The radius obtained from the asteroseismic analysis matches the
interferometric value quite closely even though the radius was not constrained
during the modelling.Comment: 11 pages, accepted for publication in Astronomy & Astrophysic
Period-luminosity relations of pulsating M giants in the solar neighbourhood and the Magellanic Clouds
We analyse the results of a 5.5-yr photometric campaign that monitored 247
southern, semi-regular variables with relatively precise Hipparcos parallaxes
to demonstrate an unambiguous detection of Red Giant Branch (RGB) pulsations in
the solar neighbourhood. We show that Sequence A' contains a mixture of AGB and
RGB stars, as indicated by a temperature related shift at the TRGB. Large
Magellanic Cloud (LMC) and Galactic sequences are compared in several ways to
show that the P-L sequence zero-points have a negligible metallicity
dependence. We describe a new method to determine absolute magnitudes from
pulsation periods and calibrate the LMC distance modulus using Hipparcos
parallaxes to find \mu (LMC) = 18.54 +- 0.03 mag. Several sources of systematic
error are discussed to explain discrepancies between the MACHO and OGLE
sequences in the LMC. We derive a relative distance modulus of the Small
Magellanic Cloud (SMC) relative to the LMC of \Delta \mu = 0.41 +- 0.02 mag. A
comparison of other pulsation properties, including period-amplitude and
luminosity-amplitude relations, confirms that RGB pulsation properties are
consistent and universal, indicating that the RGB sequences are suitable as
high-precision distance indicators. The M giants with the shortest periods
bridge the gap between G and K giant solar-like oscillations and M-giant
pulsation, revealing a smooth continuity as we ascend the giant branch.Comment: 12 pages, 17 figures, 1 table. Accepted for publication in MNRA
Causal Effects of the Timing of Life-course Events Age at Retirement and Subsequent Health
In this article, we combine the extensive literature on the analysis of life-course trajectories as sequences with the literature on causal inference and propose a new matching approach to investigate the causal effect of the timing of life-course events on subsequent outcomes. Our matching approach takes into account pre-event confounders that are both time-independent and time-dependent as well as life-course trajectories. After matching, treated and control individuals can be compared using standard statistical tests or regression models. We apply our approach to the study of the consequences of the age at retirement on subsequent health outcomes, using a unique data set from Swedish administrative registers. Once selectivity in the timing of retirement is taken into account, effects on hospitalization are small, while early retirement has negative effects on survival. Our approach also allows for heterogeneous treatment effects. We show that the effects of early retirement differ according to preretirement income, with higher income individuals tending to benefit from early retirement, while the opposite is true for individuals with lower income
Isolation and identification of a South African lentivirus from jaagsiekte lungs
In the course of attempts to grow the jaagsiekte retrovirus in cell culture, a typical lentivirus was isolated for the first time in South Africa from adenomatous lungs. Morphologically the virus could not be distinguished from other lentiviruses, but serologically it was shown to be more closely related to visna virus than to caprine arthritis-encephalitis virus. However, a preliminary restriction enzyme analysis of the linear proviral DNA of this new lentivirus (SA-DMVV) revealed that it is significantly district from visna virus and CAEV and therefore may represent a third type of lentivirus. Antibodies to the virus were demonstrated in a number of sheep in various parts of the country, but a direct link to a disease condition was not found. Attempts to produce lung lesions by
intratracheal injection of the virus have been unsuccessful to date but a transient arthritis was produced by intra-articular inoculation. Viral replication seems to be enhanced in jaagsiekte lungs.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi.
Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.mn201
Oscillation mode frequencies of 61 main sequence and subgiant stars observed by Kepler
Solar-like oscillations have been observed by Kepler and CoRoT in several
solar-type stars, thereby providing a way to probe the stars using
asteroseismology.
We provide the mode frequencies of the oscillations of various stars required
to perform a comparison with those obtained from stellar modelling.
We used a time series of nine months of data for each star. The 61 stars
observed were categorised in three groups: simple, F-like and mixed-mode. The
simple group includes stars for which the identification of the mode degree is
obvious. The F-like group includes stars for which the identification of the
degree is ambiguous. The mixed-mode group includes evolved stars for which the
modes do not follow the asymptotic relation of low-degree frequencies.
Following this categorisation, the power spectra of the 61 main sequence and
subgiant stars were analysed using both maximum likelihood estimators and
Bayesian estimators, providing individual mode characteristics such as
frequencies, linewidths, and mode heights. We developed and describe a
methodology for extracting a single set of mode frequencies from multiple sets
derived by different methods and individual scientists. We report on how one
can assess the quality of the fitted parameters using the likelihood ratio test
and the posterior probabilities.
We provide the mode frequencies of 61 stars (with their 1-sigma error bars),
as well as their associated echelle diagrams.Comment: 83 pages, 17 figures, 61 tables, paper accepted by Astronomy and
Astrophysic
Solar-like oscillations in low-luminosity red giants: first results from Kepler
We have measured solar-like oscillations in red giants using time-series
photometry from the first 34 days of science operations of the Kepler Mission.
The light curves, obtained with 30-minute sampling, reveal clear oscillations
in a large sample of G and K giants, extending in luminosity from the red clump
down to the bottom of the giant branch. We confirm a strong correlation between
the large separation of the oscillations (Delta nu) and the frequency of
maximum power (nu_max). We focus on a sample of 50 low-luminosity stars (nu_max
> 100 muHz, L <~ 30 L_sun) having high signal-to-noise ratios and showing the
unambiguous signature of solar-like oscillations. These are H-shell-burning
stars, whose oscillations should be valuable for testing models of stellar
evolution and for constraining the star-formation rate in the local disk. We
use a new technique to compare stars on a single echelle diagram by scaling
their frequencies and find well-defined ridges corresponding to radial and
non-radial oscillations, including clear evidence for modes with angular degree
l=3. Measuring the small separation between l=0 and l=2 allows us to plot the
so-called C-D diagram of delta nu_02 versus Delta nu. The small separation
delta nu_01 of l=1 from the midpoint of adjacent l=0 modes is negative,
contrary to the Sun and solar-type stars. The ridge for l=1 is notably
broadened, which we attribute to mixed modes, confirming theoretical
predictions for low-luminosity giants. Overall, the results demonstrate the
tremendous potential of Kepler data for asteroseismology of red giants.Comment: accepted by ApJ Letters, to appear in special Kepler issue. Updated
reference
Stochastic excitation of acoustic modes in stars
For more than ten years, solar-like oscillations have been detected and
frequencies measured for a growing number of stars with various characteristics
(e.g. different evolutionary stages, effective temperatures, gravities, metal
abundances ...).
Excitation of such oscillations is attributed to turbulent convection and
takes place in the uppermost part of the convective envelope. Since the
pioneering work of Goldreich & Keely (1977), more sophisticated theoretical
models of stochastic excitation were developed, which differ from each other
both by the way turbulent convection is modeled and by the assumed sources of
excitation. We review here these different models and their underlying
approximations and assumptions.
We emphasize how the computed mode excitation rates crucially depend on the
way turbulent convection is described but also on the stratification and the
metal abundance of the upper layers of the star. In turn we will show how the
seismic measurements collected so far allow us to infer properties of turbulent
convection in stars.Comment: Notes associated with a lecture given during the fall school
organized by the CNRS and held in St-Flour (France) 20-24 October 2008 ; 39
pages ; 11 figure
Comparative blind test of five planetary transit detection algorithms on realistic synthetic light curves
Copyright © The European Southern Observatory (ESO)Because photometric surveys of exoplanet transits are very promising sources of future discoveries, many algorithms are being developed to detect transit signals in stellar light curves. This paper compares such algorithms for the next generation of space-based transit detection surveys like CoRoT, Kepler, and Eddington. Five independent analyses of a thousand synthetic light curves are presented. The light curves were produced with an end-to-end instrument simulator and include stellar micro-variability and a varied sample of stellar and planetary transits diluted within a much larger set of light curves. The results show that different algorithms perform quite differently, with varying degrees of success in detecting real transits and avoiding false positives. We also find that the detection algorithm alone does not make all the difference, as the way the light curves are filtered and detrended beforehand also has a strong impact on the detection limit and on the false alarm rate. The microvariability of sun-like stars is a limiting factor only in extreme cases, when the fluctuation amplitudes are large and the star is faint. In the majority of cases it does not prevent detection of planetary transits. The most sensitive analysis is performed with periodic box-shaped detection filters. False positives are method-dependent, which should allow reduction of their detection rate in real surveys. Background eclipsing binaries are wrongly identified as planetary transits in most cases, a result which confirms that contamination by background stars is the main limiting factor. With parameters simulating the CoRoT mission, our detection test indicates that the smallest detectable planet radius is on the order of 2 Earth radii for a 10-day orbital period planet around a K0 dwarf
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