396 research outputs found
Spin Angular Momentum Evolution of the Long Period Algols
We consider the spin angular momentum evolution of the accreting components
of Algol-type binary stars. In wider Algols the accretion is through a disc so
that the accreted material can transfer enough angular momentum to the gainer
that material at its equator should be spinning at break-up. We demonstrate
that even a small amount of mass transfer, much less than required to produce
today's mass ratios, transfers enough angular momentum to spin the gainer up to
this critical rotation velocity. However the accretors in these systems have
spins typically between 10 and per cent of the critical rate. So some
mechanism for angular momentum loss from the gainers is required. We consider
generation of magnetic fields in the radiative atmospheres in a differentially
rotating star and the possibility of angular momentum loss driven by strong
stellar winds in the intermediate mass stars, such as the primaries of the
Algols. Differential rotation, induced by the accretion itself, may produce
such winds which carry away enough angular momentum to reduce their rotational
velocities to the today's observed values. We apply this model to two systems
with initial periods of 5\,d, one with initial masses 5 and
and the other with 3.2 and . Our calculations show that, if
the mass outflow rate in the stellar wind is about per cent of the
accretion rate and the dipole magnetic field is stronger than about kG,
the spin rate of the gainer is reduced to below break-up velocity even in the
fast phase of mass transfer. Larger mass loss is needed for smaller magnetic
fields. The slow rotation of the gainers in the classical Algol systems is
explained by a balance between the spin-up by mass accretion and spin-down by a
stellar wind linked to a magnetic field.Comment: 12 figures, 26 pages, accepted in MNRA
Accretion Disc Evolution in Single and Binary T Tauri Stars
We present theoretical models for the evolution of T Tauri stars surrounded
by circumstellar discs. The models include the effects of pre-main-sequence
stellar and time dependent disc evolution, and incorporate the effects of
stellar magnetic fields acting on the inner disc. For single stars, consistency
with observations in Taurus-Auriga demands that disc dispersal occurs rapidly,
on much less than the viscous timescale of the disc, at roughly the epoch when
heating by stellar radiation first dominates over internal viscous dissipation.
Applying the models to close binaries, we find that because the initial
conditions for discs in binaries are uncertain, studies of extreme mass ratio
systems are required to provide a stringent test of theoretical disc evolution
models. We also note that no correlation of the infra-red colours of T Tauri
stars with their rotation rate is observed, in apparent contradiction to the
predictions of simple magnetospheric accretion models.Comment: 11 pages, MNRAS in pres
Carbon Rich Extremely Metal Poor Stars: Signatures of Population-III AGB stars in Binary Systems
We use the Cambridge stellar evolution code STARS to model the evolution and
nucleosynthesis of zero-metallicity intermediate-mass stars. We investigate the
effect of duplicity on the nucleosynthesis output of these systems and the
potential abundances of the secondaries. The surfaces of zero-metallicity stars
are enriched in CNO elements after second dredge up. During binary interaction,
such as Roche lobe overflow or wind accretion, metals can be released from
these stars and the secondaries enriched in CNO isotopes. We investigate the
formation of the two most metal poor stars known, HE 0107-5240 and HE
1327-2326. The observed carbon and nitrogen abundances of HE 0107-5240 can be
reproduced by accretion of material from the companion-enhanced wind of a seven
solar star after second dredge-up, though oxygen and sodium are underproduced.
We speculate that HE 1327-2326, which is richer in nitrogen and strontium, may
similarly be formed by wind accretion in a later AGB phase after third
dredge-up.Comment: 16 pages, 1 figure, 7 tables, accepted by MNRA
A two-dimensional mixing length theory of convective transport
The helioseismic observations of the internal rotation profile of the Sun
raise questions about the two-dimensional (2D) nature of the transport of
angular momentum in stars. Here we derive a convective prescription for
axisymmetric (2D) stellar evolution models. We describe the small scale motions
by a spectrum of unstable linear modes in a Boussinesq fluid. Our saturation
prescription makes use of the angular dependence of the linear dispersion
relation to estimate the anisotropy of convective velocities. We are then able
to provide closed form expressions for the thermal and angular momentum fluxes
with only one free parameter, the mixing length.
We illustrate our prescription for slow rotation, to first order in the
rotation rate. In this limit, the thermodynamical variables are spherically
symetric, while the angular momentum depends both on radius and latitude. We
obtain a closed set of equations for stellar evolution, with a self-consistent
description for the transport of angular momentum in convective regions. We
derive the linear coefficients which link the angular momentum flux to the
rotation rate (- effect) and its gradient (-effect). We
compare our results to former relevant numerical work.Comment: MNRAS accepted, 10 pages, 1 figure, version prior to language editio
Approximate input physics for stellar modelling
We present a simple and efficient, yet reasonably accurate, equation of
state, which at the moderately low temperatures and high densities found in the
interiors of stars less massive than the Sun is substantially more accurate
than its predecessor by Eggleton, Faulkner & Flannery. Along with the most
recently available values in tabular form of opacities, neutrino loss rates,
and nuclear reaction rates for a selection of the most important reactions,
this provides a convenient package of input physics for stellar modelling. We
briefly discuss a few results obtained with the updated stellar evolution code.Comment: uuencoded compressed postscript. The preprint are also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
The C-flash and the ignition conditions of type Ia supernovae
Thanks to a stellar evolution code able to compute through the
C-flash we link the binary population synthesis of single degenerate
progenitors of type Ia supernovae (SNe Ia) to their physical condition at the
time of ignition. We show that there is a large range of possible ignition
densities and we detail how their probability distribution depends on the
accretion properties. The low density peak of this distribution qualitatively
reminds of the clustering of the luminosities of Branch-normal SNe Ia. We
tighten the possible range of initial physical conditions for explosion models:
they form a one-parameter family, independent of the metallicity. We discuss
how these results may be modified if we were to relax our hypothesis of a
permanent Hachisu wind or if we were to include electron captures.Comment: 10 pages, 14 figures, MNRAS accepte
Reconstructing the evolution of white dwarf binaries: further evidence for an alternative algorithm for the outcome of the common-envelope phase in close binaries
We determine the possible masses and radii of the progenitors of white dwarfs
in binaries from fits to detailed stellar evolution models and use these to
reconstruct the mass-transfer phase in which the white dwarf was formed. We
confirm the earlier finding that in the first phase of mass transfer in the
binary evolution leading to a close pair of white dwarfs, the standard
common-envelope formalism equating the energy balance in the system, does not
work. An algorithm equating the angular momentum balance can explain the
observations. This conclusion is now based on ten observed systems rather than
three. With the latter algorithm the separation does not change much for
approximately equal mass binaries. Assuming constant efficiency in the standard
\alpha-formalism and a constant value of \gamma, we investigate the effect of
both methods on the change in separation in general and conclude that when
there is observational evidence for strong shrinkage of the orbit, the
\gamma-algorithm also leads to this. We then extend our analysis to all close
binaries with at least one white dwarf component and reconstruct the mass
transfer phases that lead to these binaries. We find that all observations can
be explained with a single value of \gamma, making the \gamma-algorithm a
useful tool to predict the outcome of common-envelope evolution. We discuss the
consequences of our findings for different binary populations in the Galaxy,
including massive binaries, for which the reconstruction method cannot be used
(abbriged).Comment: Accepted for publication in MNRA
- …