1,633 research outputs found
Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities
The availability of asteroseismic constraints for a large sample of stars
from the missions CoRoT and Kepler paves the way for various statistical
studies of the seismic properties of stellar populations. In this paper, we
evaluate the impact of rotation-induced mixing and thermohaline instability on
the global asteroseismic parameters at different stages of the stellar
evolution from the Zero Age Main Sequence to the Thermally Pulsating Asymptotic
Giant Branch to distinguish stellar populations. We present a grid of stellar
evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and
0.014) in the mass range between 0.85 to 6.0 Msun. The models are computed
either with standard prescriptions or including both thermohaline convection
and rotation-induced mixing. For the whole grid we provide the usual stellar
parameters (luminosity, effective temperature, lifetimes, ...), together with
the global seismic parameters, i.e. the large frequency separation and
asymptotic relations, the frequency corresponding to the maximum oscillation
power {\nu}_{max}, the maximal amplitude A_{max}, the asymptotic period spacing
of g-modes, and different acoustic radii. We discuss the signature of
rotation-induced mixing on the global asteroseismic quantities, that can be
detected observationally. Thermohaline mixing whose effects can be identified
by spectroscopic studies cannot be caracterized with the global seismic
parameters studied here. But it is not excluded that individual mode
frequencies or other well chosen asteroseismic quantities might help
constraining this mixing.Comment: 15 pages, 11 figures, accepted for publication in A&
The independent association of overweight and obesity with breathlessness in adults: a cross-sectional, population-based study
Obesity is an independent risk factor for chronic breathlessness and should be assessed in people with this symptom
Analysis of glacial earthquakes
In 2003, Ekström et al. reported on the detection of a new class of earthquakes that occur in glaciated regions, with the vast majority being in Greenland. The events have a characteristic radiation pattern and lack the high-frequency content typical of tectonic earthquakes. It was proposed that the events correspond to large and sudden sliding motion of glaciers. Here we present an analysis of all 184 such events detected in Greenland between 1993 and 2005. Fitting the teleseismic long-period surface waves to a landslide model of the source, we obtain improved locations, timing, force amplitudes, and force directions. After relocation, the events cluster into seven regions, all of which correspond to regions of very high ice flow and most of which are named outlet glaciers. These regions are Daugaard Jensen Glacier, Kangerdlugssuaq Glacier, Helheim Glacier, the southeast Greenland glaciers, the northwest Greenland glaciers, Rinks Isbrae, and Jakobshavn Isbrae. Event amplitudes range from 0.1 to 2.0 × 10^(14) kg m. Force directions are consistent with sliding in the direction of glacial flow over a period of about 50 s. Each region has a different temporal distribution of events. All glaciers are more productive in the summer, but have their peak activity in different months. Over the study period, Kangerdlugssuaq has had a constant number of events each year, whereas Jakobshavn had most events in 1998–1999, and the number of events in Helheim and the northwest Greenland glaciers has increased substantially between 1993 and 2005. The size distribution of events in Kangerdlugssuaq is peaked above the detection threshold, suggesting that glacial earthquakes have a characteristic size
Populations of rotating stars II. Rapid rotators and their link to Be-type stars
Even though it is broadly accepted that single Be stars are rapidly rotating
stars surrounded by a flat rotating circumstellar disk, there is still a debate
about how fast these stars rotate and also about the mechanisms involved in the
angular-momentum and mass input in the disk. We study the properties of stars
that rotate near their critical-rotation rate and investigate the properties of
the disks formed by equatorial mass ejections. We used the most recent Geneva
stellar evolutionary tracks for rapidly rotating stars that reach the critical
limit and used a simple model for the disk structure. We obtain that for a 9
Msun star at solar metallicity, the minimum average velocity during the Main
Sequence phase to reach the critical velocity is around 330 km/s, whereas it
would be 390 km/s at the metallicity of the Small Magellanic Cloud (SMC). Red
giants or supergiants originating from very rapid rotators rotate six times
faster and show N/C ratios three times higher than those originating from
slowly rotating stars. This difference becomes stronger at lower metallicity.
It might therefore be very interesting to study the red giants in clusters that
show a large number of Be stars on the MS band. On the basis of our single-star
models, we show that the observed Be-star fraction with cluster age is
compatible with the existence of a temperature-dependent lower limit in the
velocity rate required for a star to become a Be star. The mass, extension, and
diffusion time of the disks produced when the star is losing mass at the
critical velocity, obtained from simple parametrized expressions, are not too
far from those estimated for disks around Be-type stars. At a given
metallicity, the mass and the extension of the disk increase with the initial
mass and with age on the MS phase. Denser disks are expected in low-metallicity
regions.Comment: Accepted for publication in A&A, language edite
The Cloud Condensation Nuclei (CCN) properties of 2-methyltetrols and C3-C6 polyols from osmolality and surface tension measurements
A significant fraction of the organic material in aerosols is made of highly soluble compounds such as sugars (mono- and polysaccharides) and polyols such as the 2-methyltetrols, methylerythritol and methyltreitol. Because of their high solubility these compounds are considered as potentially efficient CCN material. For the 2-methyltetrols, this would have important implications for cloud formation at global scale because they are thought to be produced by the atmospheric oxidation of isoprene. To investigate this question, the complete Köhler curves for C3-C6 polyols and the 2-methyltetrols have been determined experimentally from osmolality and surface tension measurements. Contrary to what was expected, none of these compounds displayed a higher CCN efficiency than organic acids. Their Raoult terms show that this limited CCN efficiency is due to their absence of dissociation in water, this in spite of slight surface-tension effects for the 2-methyltetrols. Thus, compounds such as saccharides and polyols would not contribute more to cloud formation than other organic compounds studied so far. In particular, the presence of 2-methyltetrols in aerosols would not particularly enhance cloud formation in the atmosphere, in contrary to recently suggested
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