52 research outputs found
Simulations of the tidal interaction and mass transfer of a star in an eccentric orbit around an intermediate-mass black hole: the case of HLX-1
The X-ray source HLX-1 near the spiral galaxy ESO 243-49 is currently the
best intermediate-mass black hole candidate. It has a peak bolometric
luminosity of erg s, which implies a mass inflow rate of
MSun yr, but the origin of this mass is unknown. It has
been proposed that there is a star on an eccentric orbit around the black hole
which transfers mass at pericentre. To investigate the orbital evolution of
this system, we perform stellar evolution simulations using mesa and SPH
simulations of a stellar orbit around an intermediate-mass black hole using fi.
We run and couple these simulations using the amuse framework. We find that
mass is lost through both the first and second Lagrange points and that there
is a delay of up to 10 days between the pericentre passage and the peak mass
loss event. The orbital evolution timescales we find in our simulations are
larger than what is predicted by analytical models, but these models fall
within the errors of our results. Despite the fast orbital evolution, we are
unable to reproduce the observed change in outburst period. We conclude that
the change in the stellar orbit with the system parameters investigated here is
unable to account for all observed features of HLX-1.Comment: accepted for publication in mnra
Evolution of binary stars and the effect of tides on binary populations
We present a rapid binary evolution algorithm that enables modelling of even
the most complex binary systems. In addition to all aspects of single star
evolution, features such as mass transfer, mass accretion, common-envelope
evolution, collisions, supernova kicks and angular momentum loss mechanisms are
included. In particular, circularization and synchronization of the orbit by
tidal interactions are calculated for convective, radiative and degenerate
damping mechanisms. We use this algorithm to study the formation and evolution
of various binary systems. We also investigate the effect that tidal friction
has on the outcome of binary evolution. Using the rapid binary code, we
generate a series of large binary populations and evaluate the formation rate
of interesting individual species and events. By comparing the results for
populations with and without tidal friction we quantify the hitherto ignored
systematic effect of tides and show that modelling of tidal evolution in binary
systems is necessary in order to draw accurate conclusions from population
synthesis work. Tidal synchronism is important but because orbits generally
circularize before Roche-lobe overflow the outcome of the interactions of
systems with the same semi-latus rectum is almost independent of eccentricity.
It is not necessary to include a distribution of eccentricities in population
synthesis of interacting binaries, however, the initial separations should be
distributed according to the observed distribution of semi-latera recta rather
than periods or semi-major axes.Comment: 36 pages, 12 figures, to be published in the Monthly Notices of the
Royal Astronomical Societ
Direct N-body Modelling of Stellar Populations: Blue Stragglers in M67
We present a state-of-the-art N-body code which includes a detailed treatment
of stellar and binary evolution as well as the cluster dynamics. This code is
ideal for investigating all aspects relating to the evolution of star clusters
and their stellar populations. It is applicable to open and globular clusters
of any age. We use the N-body code to model the blue straggler population of
the old open cluster M67. Preliminary calculations with our binary population
synthesis code show that binary evolution alone cannot explain the observed
numbers or properties of the blue stragglers. On the other hand, our N-body
model of M67 generates the required number of blue stragglers and provides
formation paths for all the various types found in M67. This demonstrates the
effectiveness of the cluster environment in modifying the nature of the stars
it contains and highlights the importance of combining dynamics with stellar
evolution. We also perform a series of N = 10000 simulations in order to
quantify the rate of escape of stars from a cluster subject to the Galactic
tidal field.Comment: 26 pages, 18 figures, accepted for publication in MNRA
A Complete N-body Model of the Old Open Cluster M67
The old open cluster M67 is an ideal testbed for current cluster evolution
models because of its dynamically evolved structure and rich stellar
populations that show clear signs of interaction between stellar, binary and
cluster evolution. Here we present the first truly direct N-body model for M67,
evolved from zero age to 4 Gyr taking full account of cluster dynamics as well
as stellar and binary evolution. Our preferred model starts with 12000 single
stars and 12000 binaries placed in a Galactic tidal field at 8.0 kpc from the
Galactic Centre. Our choices for the initial conditions and for the primordial
binary population are explained in detail. At 4 Gyr, the age of M67, the total
mass has reduced by 90% as a result of mass loss and stellar escapes. The mass
and half-mass radius of luminous stars in the cluster are a good match to
observations although the model is more centrally concentrated than
observations indicate. The stellar mass and luminosity functions are
significantly flattened by preferential escape of low-mass stars. We find that
M67 is dynamically old enough that information about the initial mass function
is lost, both from the current luminosity function and from the current mass
fraction in white dwarfs. The model contains 20 blue stragglers at 4 Gyr which
is slightly less than the 28 observed in M67. Nine are in binaries. The blue
stragglers were formed by a variety of means and we find formation paths for
the whole variety observed in M67. Both the primordial binary population and
the dynamical cluster environment play an essential role in shaping the
population. A substantial population of short-period primordial binaries (with
periods less than a few days) is needed to explain the observed number of blue
stragglers in M67.Comment: 32 pages, 17 figures, submitted to MNRA
Simulating stellar winds in AMUSE
We present stellar_wind.py, a module that provides multiple methods of
simulating stellar winds using smoothed particle hydrodynamics codes (SPH)
within the astrophysical multipurpose software environment (AMUSE) framework.
With the simple wind mode, we create SPH wind particles in a spherically
symmetric shell. We inject the wind particles with a velocity equal to their
terminal velocity. The accelerating wind mode is similar, but with this method
particles can be injected with a lower initial velocity than the terminal
velocity and they are accelerated away from the star according to an
acceleration function. With the heating wind mode, SPH particles are created
with zero initial velocity with respect to the star, but instead wind particles
are given an internal energy based on the integrated mechanical luminosity of
the star. This mode is designed to be used on longer timescales and larger
spatial scales compared to the other two modes and assumes that the star is
embedded in a gas cloud. For fast winds, we find that both the simple and
accelerating mode can reproduce the desired velocity, density and temperature
profiles. For slow winds, the simple wind mode is insufficient due to dominant
hydrodynamical effects that change the wind velocities. The accelerating mode,
with additional options to account for these hydrodynamical effects, can still
reproduce the desired wind profiles. We test the heating mode by simulating
both a normal wind and a supernova explosion of a single star in a uniform
density medium. The stellar wind simulation results matches the analytical
solution for an expanding wind bubble. The supernova simulation gives
qualitatively correct results, but the simulated bubble expands faster than the
analytical solution predicts. We conclude with an example of a triple star
system which includes the colliding winds of all three stars.Comment: Accepted for publication in A&
Massive donors in interacting binaries: effect of metallicity
Metallicity is known to significantly affect the radial expansion of a
massive star: the lower the metallicity, the more compact the star, especially
during its post-MS evolution. We study this effect in the context of binary
evolution. Using the stellar-evolution code MESA, we computed evolutionary
tracks of stars at different metallicities, exploring variations of factors
known to affect the radial expansion (e.g. semiconvection, overshooting,
rotation). We find observational support for an evolution in which already at
metallicity massive stars remain relatively compact during the
Hertzprung-Gap (HG) phase and most of their expansion occurs during core-helium
burning (CHeB). Consequently, we show that metallicity has a strong influence
on the type of mass transfer evolution in binary systems. At solar metallicity,
a case-B mass transfer is initiated shortly after the end of MS, and a giant
donor is almost always a rapidly expanding HG star. At lower metallicity, the
parameter space for mass transfer from a more evolved CHeB star increases
dramatically. This means that envelope stripping and formation of helium stars
in low-metallicity environments occurs later in the evolution of the donor,
implying a much shorter duration of the Wolf-Rayet phase (even by an order of
magnitude) and higher final core masses. This metallicity effect is independent
of the impact of metallicity-dependent stellar winds. At very low
metallicities, a significant fraction of massive stars in binaries engages in
the first episode of mass transfer very late into their evolution, when they
already have a well-developed CO core. The remaining lifetime ( yr) is
unlikely to be enough to strip the entire H-rich envelope. We also briefly
discuss the extremely small parameter space for mass transfer from massive
convective-envelope donors in the context of binary black hole merger
formation.Comment: 15 pages, 8 figures (+ 4 pages, 4 fig. appendix), to appear in A&
Semi-analytic modelling of the europium production by neutron star mergers in the halo of the Milky Way
Neutron star mergers (NSM) are likely to be the main production sites for the
rapid (r-) neutron capture process elements. We study the r-process enrichment
of the stellar halo of the Milky Way through NSM, by tracing the typical
r-process element Eu in the Munich-Groningen semi-analytic galaxy formation
model, applied to three high resolution Aquarius dark matter simulations. In
particular, we investigate the effect of the kick velocities that neutron star
binaries receive upon their formation, in the building block galaxies (BBs)
that partly formed the stellar halo by merging with our Galaxy. When this kick
is large enough to overcome the escape velocity of the BB, the NSM takes place
outside the BB with the consequence that there is no r-process enrichment. We
find that a standard distribution of NS kick velocities decreases [Eu/Mg]
abundances of halo stars by ~dex compared to models where NS do not
receive a kick. With low NS kick velocities, our simulations match observed
[Eu/Mg] abundances of halo stars reasonably well, for stars with metallicities
[Mg/H]. Only in Aquarius halo B-2 also the lower metallicity stars
have [Eu/Mg] values similar to observations. We conclude that our assumption of
instantaneous mixing is most likely inaccurate for modelling the r-process
enrichment of the Galactic halo, or an additional production site for r-process
elements is necessary to explain the presence of low-metallicity halo stars
with high Eu abundances.Comment: 15 pages, 9 figures, accepted for publication in MNRA
Disc-binary interactions in depleted post-AGB binaries
Binary post-asymptotic giant branch (post-AGB) stars have orbital periods in
the range of 100--2500 days in eccentric orbits. They are surrounded by
circumbinary dusty discs. They are the immediate result of unconstrained binary
interaction processes. Their observed orbital properties do not correspond to
model predictions: Neither the periods nor the high eccentricities are
expected. Our goal is to investigate if interactions between a binary and its
circumbinary disc during the post-AGB phase can result in their eccentric
orbits, while simultaneously explaining the chemical anomaly known as
depletion. For this paper, we selected three binaries (EP Lyr, RU Cen, HD
46703) with well-constrained orbits, luminosities, and chemical abundances. We
used the MESA code to evolve post-AGB models, while including the accretion of
metal-poor gas. This allows us to constrain the evolution of the stars and
study the impact of circumbinary discs on the orbital properties of the models.
We investigate the effect of torques produced by gas inside the binary cavity
and the effect of Lindblad resonances on the orbit, while also including the
tidal interaction following the equilibrium tide model. We find that none of
our models are able to explain the high orbital eccentricities of the binaries
in our sample. The accretion torque does not significantly impact the binary
orbit, while Lindblad resonances can pump the eccentricity up to only . At higher eccentricities, the tidal interaction becomes too
strong, so the high observed eccentricities cannot be reproduced. However, even
if we assume tides to be ineffective, the eccentricities in our models do not
exceed . We conclude that either our knowledge of disc-binary
interactions is still incomplete, or the binaries must have left their phase of
strong interaction in an eccentric orbit.Comment: 17 pages + 8 pages appendix, 12 figures, accepted for publication in
Astronomy and Astrophysic
Orbital properties of binary post-AGB stars
Binary post-asymptotic giant branch (post-AGB) stars are thought to be the
products of a strong but poorly-understood interaction during the AGB phase.
The aim of this contribution is to update the orbital elements of a sample of
galactic post-AGB binaries observed in a long-term radial-velocity monitoring
campaign. Radial velocities are computed from high signal-to-noise spectra by
use of a cross-correlation method. The radial-velocity curves are fitted by
using both a least-squares algorithm and a Nelder-Mead simplex algorithm. We
use a Monte Carlo method to compute uncertainties on the orbital elements. The
resulting mass functions are used to derive a companion mass distribution by
optimising the predicted to the observed cumulative mass-function
distributions, after correcting for observational bias. As a result, we derive
and update orbital elements for 33 galactic post-AGB binaries, among which 3
are new orbits. The orbital periods of the systems range from 100 to about 3000
days. Over 70 percent (23 out of 33) of our binaries have significant non-zero
eccentricities ranging over all periods. Their orbits are non-circular despite
the fact that the Roche-lobe radii are smaller than the maximum size of a
typical AGB star and tidal circularisation should have been strong when the
objects were on the AGB. We derive a distribution of companion masses that is
peaked around 1.09 with a standard deviation of 0.62 . The
large spread in companion masses highlights the diversity of post-AGB binary
systems. Furthermore, we find that only post-AGB stars with high effective
temperatures (> 5500 K) in wide orbits are depleted in refractory elements,
suggesting that re-accretion of material from a circumbinary disc is an ongoing
process. It appears, however, that chemical depletion is inefficient for the
closest orbits irrespective of the actual surface temperature.Comment: 21 pages total, 3 appendices, 8 figures excluding appendix figures, 3
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