Abstract
Since close WR+O binaries are the result of a strong interaction
of both stars in
massive close binary systems, they can be used to constrain
the highly uncertain mass and angular momentum budget
during the major mass transfer phase.
We explore the progenitor evolution of the three
best suited WR+O binaries sHD\,90657, HD\,186943 and
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HD\,211853,
which are characterized by a WR/O mass ratio of $\sim$0.5 and periods of 6..10 days.
We are doing so at three different levels of approximation:
predicting the massive binary evolution through
simple mass loss and angular momentum loss estimates, through
full binary evolution models with parametrized mass transfer
efficiency, and through binary evolution models including rotation
of both components and a physical model which allows to
compute mass and angular momentum loss from the binary system
as function of time during the mass transfer process.
All three methods give consistently the same answers.
Our results show that, if these systems formed through stable mass transfer,
their initial periods were smaller than their current ones,
which implies that mass transfer has started during the
core hydrogen burning phase of the initially more massive star.
Furthermore, the mass transfer in all three cases must have
been highly non-conservative, with on average only $\sim$10\%
of the transferred mass being retained by the mass receiving star.
This result gives support to our system mass and angular momentum
loss model, which predicts that, in the considered systems,
about 90\% of the overflowing matter is expelled by the
rapid rotation of the mass receiver close to the $\Omega$-limit,
which is reached through the accretion of the remaining 10\%.
The collapsar model for gamma-ray bursts requires three
essential ingredients: a massive core, a removed hydrogen envelope,
and a high enough core specific angular momentum. We explore
up-to-date massive star evolutionary models of solar metallicity in order to
find out which massive star physics is capable to produce these
ingredients. In particular, we investigate the role of hydrodynamic and magnetic
internal angular momentum transport and binary mass and angular momentum transfer. To pursue this,
we compute evolutionary models of rotating single stars, and of binary systems which include rotational processes
for both stars.
Neglecting magnetic fields, we show that the cores of massive single stars can
maintain a high specific angular momentum ($j$$\sim$10$^{17}\rm cm^2s^{-1}$)
when evolved with the assumption that mean molecular weight gradients suppress rotational mixing processes.
In binary systems which undergo mass transfer during core hydrogen burning
the mass receiving star gains large amounts of high angular momentum material,
which leads to a spin-up of the core. However, we find that this does not lead to cores which rotate faster than in single stars,
but merely compensates for the tidal angular momentum loss due to spin-orbit coupling, which leads
to synchronous rotation before the mass transfer event. We show that some accretion stars
become Wolf-Rayet stars towards core helium exhaustion and form CO-cores which are massive enough to form a
black hole. We also present models which include magnetic fields produced due to
differential rotation and internal angular momentum transport
by magnetic torques (Spruit 2002). While magnetic single star models are known to develop rather slowly rotating
cores (Heger et al. 2002) --- with specific angular momenta close to those in
observed young pulsars ($j$$\sim$10$^{14}\rm cm^2s^{-1}$), we investigate
the capability of magnetic torques to efficiently pump angular into the
cores of accreting stars. While this mechanism is found to work, the magnetic coupling of core and envelope after the accretion star ends
its core hydrogen burning leads to slower rotation $j$$\sim$10$^{{15}-{16}}\rm cm^2s^{-1}$)
than in the non-magnetic case, so GRBs at near solar metallicity need to be produced in rather exotic binary channels,
or the magnetic effects are currently overestimated in our models.
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