3,113,724 research outputs found
Rapidly rotating red giants
Stellar oscillations give seismic information on the internal properties of
stars. Red giants are targets of interest since they present mixed modes, which
behave as pressure modes in the convective envelope and as gravity modes in the
radiative core. Mixed modes thus directly probe red giant cores, and allow in
particular the study of their mean core rotation. The high-quality data
obtained by CoRoT and Kepler satellites represent an unprecedented perspective
to obtain thousands of measurements of red giant core rotation, in order to
improve our understanding of stellar physics in deep stellar interiors. We
developed an automated method to obtain such core rotation measurements and
validated it for stars on the red giant branch. In this work, we particularly
focus on the specific application of this method to red giants having a rapid
core rotation. They show complex spectra where it is tricky to disentangle
rotational splittings from mixed-mode period spacings. We demonstrate that the
method based on the identification of mode crossings is precise and efficient.
The determination of the mean core rotation directly derives from the precise
measurement of the asymptotic period spacing {\Delta}{\Pi}1 and of the
frequency at which the crossing of the rotational components is observed.Comment: 4 pages, 2 figures, 2 tables, to be published in the Astro Fluid 2016
Conference Proceedings, editor EAS Publications Serie
Rapidly Rotating Fermi Gases
We show that the density profile of a Fermi gas in rapidly rotating potential
will develop prominent features reflecting the underlying Landau level like
energy spectrum. Depending on the aspect ratio of the trap, these features can
be a sequence of ellipsoidal volumes or a sequence of quantized steps.Comment: 4 pages, 1 postscript fil
Be Stars: Rapidly Rotating Pulsators
I will show that Be stars are, without exception, a class of rapidly rotating
stars, which are in the majority of cases pulsating stars as well, while none
of them does possess a large scale (i.e. with significant dipolar contribution)
magnetic field.Comment: Review talk given at "XX Stellar Pulsation Conference Series: Impact
of new instrumentation and new insights in stellar pulsations", Granada, 5-9
September 2011, in press in AIP Conf. Se
Genealogies of rapidly adapting populations
The genetic diversity of a species is shaped by its recent evolutionary
history and can be used to infer demographic events or selective sweeps. Most
inference methods are based on the null hypothesis that natural selection is a
weak or infrequent evolutionary force. However, many species, particularly
pathogens, are under continuous pressure to adapt in response to changing
environments. A statistical framework for inference from diversity data of such
populations is currently lacking. Toward this goal, we explore the properties
of genealogies in a model of continual adaptation in asexual populations. We
show that lineages trace back to a small pool of highly fit ancestors, in which
almost simultaneous coalescence of more than two lineages frequently occurs.
While such multiple mergers are unlikely under the neutral coalescent, they
create a unique genetic footprint in adapting populations. The site frequency
spectrum of derived neutral alleles, for example, is non-monotonic and has a
peak at high frequencies, whereas Tajima's D becomes more and more negative
with increasing sample size. Since multiple merger coalescents emerge in many
models of rapid adaptation, we argue that they should be considered as a
null-model for adapting populations.Comment: to appear in PNA
Rapidly rotating neutron star progenitors
Rotating proto-neutron stars can be important sources of gravitational waves
to be searched for by present-day and future interferometric detectors. It was
demonstrated by Imshennik that in extreme cases the rapid rotation of a
collapsing stellar core may lead to fission and formation of a binary
proto-neutron star which subsequently merges due to gravitational wave
emission. In the present paper, we show that such dynamically unstable
collapsing stellar cores may be the product of a former merger process of two
stellar cores in a common envelope. We applied population synthesis
calculations to assess the expected fraction of such rapidly rotating stellar
cores which may lead to fission and formation of a pair of proto-neutron stars.
We have used the BSE population synthesis code supplemented with a new
treatment of stellar core rotation during the evolution via effective
core-envelope coupling, characterized by the coupling time, . The
validity of this approach is checked by direct MESA calculations of the
evolution of a rotating 15 star. From comparison of the calculated
spin distribution of young neutron stars with the observed one, reported by
Popov and Turolla, we infer the value years. We
show that merging of stellar cores in common envelopes can lead to collapses
with dynamically unstable proto-neutron stars, with their formation rate being
of the total core collapses, depending on the common envelope
efficiency.Comment: 10 pages, 4 figures, accepted for publication in MNRA
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