107 research outputs found
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
Reaction Rate Uncertainties: NeNa and MgAl in AGB Stars
We study the effect of uncertainties in the proton-capture reaction rates of
the NeNa and MgAl chains on nucleosynthesis due to the operation of hot bottom
burning (HBB) in intermediate-mass asymptotic giant branch (AGB) stars. HBB
nucleosynthesis is associated with the production of sodium, radioactive Al26
and the heavy magnesium isotopes, and it is possibly responsible for the O, Na,
Mg and Al abundance anomalies observed in globular cluster stars.
We model HBB with an analytic code based on full stellar evolution models so
we can quickly cover a large parameter space. The reaction rates are varied
first individually, then all together. This creates a knock-on effect, where an
increase of one reaction rate affects production of an isotope further down the
reaction chain. We find the yields of Ne22, Na23 and Al26 to be the most
susceptible to current nuclear reaction rate uncertainties.Comment: Presented at NIC-IX, International Symposium on Nuclear Astrophysics
- Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland, 25-30 June, 200
The strange evolution of the Large Magellanic Cloud Cepheid OGLE-LMC-CEP1812
Classical Cepheids are key probes of both stellar astrophysics and cosmology
as standard candles and pulsating variable stars. It is important to understand
Cepheids in unprecedented detail in preparation for upcoming GAIA, JWST and
extremely-large telescope observations. Cepheid eclipsing binary stars are
ideal tools for achieving this goal, however there are currently only three
known systems. One of those systems, OGLE-LMC-CEP1812, raises new questions
about the evolution of classical Cepheids because of an apparent age
discrepancy between the Cepheid and its red giant companion. We show that the
Cepheid component is actually the product of a stellar merger of two main
sequence stars that has since evolved across the Hertzsprung gap of the HR
diagram. This post-merger product appears younger than the companion, hence the
apparent age discrepancy is resolved. We discuss this idea and consequences for
understanding Cepheid evolution.Comment: 5 pages, 3 figures, accepted to A&
Planetary nebulae after common-envelope phases initiated by low-mass red giants
It is likely that at least some planetary nebulae are composed of matter
which was ejected from a binary star system during common-envelope (CE)
evolution. For these planetary nebulae the ionizing component is the hot and
luminous remnant of a giant which had its envelope ejected by a companion in
the process of spiralling-in to its current short-period orbit. A large
fraction of CE phases which end with ejection of the envelope are thought to be
initiated by low-mass red giants, giants with inert, degenerate helium cores.
We discuss the possible end-of-CE structures of such stars and their subsequent
evolution to investigate for which structures planetary nebulae are formed. We
assume that a planetary nebula forms if the remnant reaches an effective
temperature greater than 30 kK within 10^4 yr of ejecting its envelope. We
assume that the composition profile is unchanged during the CE phase so that
possible remnant structures are parametrized by the end-of-CE core mass,
envelope mass and entropy profile. We find that planetary nebulae are expected
in post-CE systems with core masses greater than about 0.3 solar masses if
remnants end the CE phase in thermal equilibrium. We show that whether the
remnant undergoes a pre-white dwarf plateau phase depends on the prescribed
end-of-CE envelope mass. Thus, observing a young post-CE system would constrain
the end-of CE envelope mass and post-CE evolution.Comment: Published in MNRAS. 12 pages, 12 figures. Minor changes to match
published versio
Numerical Tests of Rotational Mixing in Massive Stars with the new Population Synthesis Code BONNFIRES
We use our new population synthesis code BONNFIRES to test how surface
abundances predicted by rotating stellar models depend on the numerical
treatment of rotational mixing, such as spatial resolution, temporal resolution
and computation of mean molecular weight gradients. We find that even with
identical numerical prescriptions for calculating the rotational mixing
coefficients in the diffusion equation, different timesteps lead to a deviation
of the coefficients and hence surface abundances. We find the surface
abundances vary by 10-100% between the model sequences with short timestep of
0.001Myr to model sequences with longer timesteps. Model sequences with
stronger surface nitrogen enrichment also have longer main-sequence lifetimes
because more hydrogen is mixed to the burning cores. The deviations in
main-sequence lifetimes can be as large as 20%. Mathematically speaking, no
numerical scheme can give a perfect solution unless infinitesimally small
timesteps are used. However, we find that the surface abundances eventually
converge within 10% between modelling sequences with sufficiently small
timesteps below 0.1Myr. The efficiency of rotational mixing depends on the
implemented numerical scheme and critically on the computation of the mean
molecular weight gradient. A smoothing function for the mean molecular weight
gradient results in stronger rotational mixing. If the discretization scheme or
the computational recipe for calculating the mean molecular weight gradient is
altered, re-calibration of mixing parameters may be required to fit
observations. If we are to properly understand the fundamental physics of
rotation in stars, it is crucial that we minimize the uncertainty introduced
into stellar evolution models when numerically approximating rotational mixing
processes.Comment: 8 pages, 6 figures, accepted by A&
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