1,439 research outputs found
Can very massive stars avoid Pair-instability Supernovae?
Very massive primordial stars () are
supposed to end their lives as pair-instability supernovae. Such an event can
be traced by a typical chemical signature in low metallicity stars, but at the
present time, this signature is lacking in the extremely metal-poor stars we
are able to observe. Does it mean that those very massive objects did not form,
contrarily to the primordial star formation scenarios? Could they avoid this
tragical fate?
We explore the effects of rotation, anisotropic mass loss and magnetic fields
on the core size of a very massive Population III model, in order to check if
its mass is sufficiently modified to prevent the pair instability.
We obtain that a Population III model of with
computed with the inclusion of wind
anisotropy and Tayler-Spruit dynamo avoids the pair instability explosion.Comment: to be published in the conference proceedings of First Stars III,
Santa Fe, 200
Pion-less effective field theory for atomic nuclei and lattice nuclei
We compute the medium-mass nuclei O and Ca using pionless
effective field theory (EFT) at next-to-leading order (NLO). The low-energy
coefficients of the EFT Hamiltonian are adjusted to experimantal data for
nuclei with mass numbers and , or alternatively to results from
lattice quantum chromodynamics (QCD) at an unphysical pion mass of 806 MeV. The
EFT is implemented through a discrete variable representation in the harmonic
oscillator basis. This approach ensures rapid convergence with respect to the
size of the model space and facilitates the computation of medium-mass nuclei.
At NLO the nuclei O and Ca are bound with respect to decay into
alpha particles. Binding energies per nucleon are 9-10 MeV and 30-40 MeV at
pion masses of 140 MeV and 806 MeV, respectively.Comment: 26 page
Fast rotating stars resulting from binary evolution will often appear to be single
Rapidly rotating stars are readily produced in binary systems. An accreting
star in a binary system can be spun up by mass accretion and quickly approach
the break-up limit. Mergers between two stars in a binary are expected to
result in massive, fast rotating stars. These rapid rotators may appear as Be
or Oe stars or at low metallicity they may be progenitors of long gamma-ray
bursts.
Given the high frequency of massive stars in close binaries it seems likely
that a large fraction of rapidly rotating stars result from binary interaction.
It is not straightforward to distinguish a a fast rotator that was born as a
rapidly rotating single star from a fast rotator that resulted from some kind
of binary interaction. Rapidly rotating stars resulting from binary interaction
will often appear to be single because the companion tends to be a low mass,
low luminosity star in a wide orbit. Alternatively, they became single stars
after a merger or disruption of the binary system during the supernova
explosion of the primary.
The absence of evidence for a companion does not guarantee that the system
did not experience binary interaction in the past. If binary interaction is one
of the main causes of high stellar rotation rates, the binary fraction is
expected to be smaller among fast rotators. How this prediction depend on
uncertainties in the physics of the binary interactions requires further
investigation.Comment: 2 pages, 1 figure, to be published in the proceedings of IAU 272
"Active OB stars: structure, evolution, mass loss and critical limit", Paris
19-23 July 201
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
Close binary evolution I. The tidally induced shear mixing in rotating binaries
We study how tides in a binary system induce some specific internal shear
mixing, able to substantially modify the evolution of close binaries prior to
mass transfer. We construct numerical models accounting for tidal interactions,
meridional circulation, transport of angular momentum, shears and horizontal
turbulence and consider a variety of orbital periods and initial rotation
velocities. Depending on orbital periods and rotation velocities, tidal effects
may spin down (spin down Case) or spin up (spin up Case) the axial rotation. In
both cases, tides may induce a large internal differential rotation. The
resulting tidally induced shear mixing (TISM) is so efficient that the internal
distributions of angular velocity and chemical elements are greatly influenced.
The evolutionary tracks are modified, and in both cases of spin down and spin
up, large amounts of nitrogen can be transported to the stellar surfaces before
any binary mass transfer. Meridional circulation, when properly treated as an
advection, always tends to counteract the tidal interaction, tending to spin up
the surface when it is braked down and vice versa. As a consequence, the times
needed for the axial angular velocity to become equal to the orbital angular
velocity may be larger than given by typical synchronization timescales. Also,
due to meridional circulation some differential rotation remains in tidally
locked binary systems.Comment: 10 pages, 18 figures, Accepted for publication in Astronomy and
Astrophysic
Effects of three-nucleon forces and two-body currents on Gamow-Teller strengths
We optimize chiral interactions at next-to-next-to leading order to
observables in two- and three-nucleon systems, and compute Gamow-Teller
transitions in carbon-14, oxygen-22 and oxygen-24 using consistent two-body
currents. We compute spectra of the daughter nuclei nitrogen-14, fluorine-22
and fluorine-24 via an isospin-breaking coupled-cluster technique, with several
predictions. The two-body currents reduce the Ikeda sum rule, corresponding to
a quenching factor q^2 ~ 0.84-0.92 of the axial-vector coupling. The half life
of carbon-14 depends on the energy of the first excited 1+ state, the
three-nucleon force, and the two-body current
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