891 research outputs found
Merger rates of double neutron stars and stellar origin black holes: The Impact of Initial Conditions on Binary Evolution Predictions
The initial mass function (IMF), binary fraction and distributions of binary
parameters (mass ratios, separations and eccentricities) are indispensable
input for simulations of stellar populations. It is often claimed that these
are poorly constrained significantly affecting evolutionary predictions.
Recently, dedicated observing campaigns provided new constraints on the initial
conditions for massive stars. Findings include a larger close binary fraction
and a stronger preference for very tight systems. We investigate the impact on
the predicted merger rates of neutron stars and black holes.
Despite the changes with previous assumptions, we only find an increase of
less than a factor 2 (insignificant compared with evolutionary uncertainties of
typically a factor 10-100). We further show that the uncertainties in the new
initial binary properties do not significantly affect (within a factor of 2)
our predictions of double compact object merger rates. An exception is the
uncertainty in IMF (variations by a factor of 6 up and down). No significant
changes in the distributions of final component masses, mass ratios, chirp
masses and delay times are found.
We conclude that the predictions are, for practical purposes, robust against
uncertainties in the initial conditions concerning binary parameters with
exception of the IMF. This eliminates an important layer of the many uncertain
assumptions affecting the predictions of merger detection rates with the
gravitational wave detectors aLIGO/aVirgo.Comment: Accepted for publication in Ap
Binaries at Low Metallicity: ranges for case A, B and C mass transfer
The evolution of single stars at low metallicity has attracted a large
interest, while the effect of metallicity on binary evolution remains still
relatively unexplored. We study the effect of metallicity on the number of
binary systems that undergo different cases of mass transfer. We find that
binaries at low metallicity are more likely to start transferring mass after
the onset of central helium burning, often referred to as case C mass transfer.
In other words, the donor star in a metal poor binary is more likely to have
formed a massive CO core before the onset of mass transfer.
At solar metallicity the range of initial binary separations that result in
case C evolution is very small for massive stars, because they do not expand
much after the ignition of helium and because mass loss from the system by
stellar winds causes the orbit to widen, preventing the primary star to fill
its Roche lobe. This effect is likely to have important consequences for the
metallicity dependence of the formation rate of various objects through binary
evolution channels, such as long GRBs, double neutron stars and double white
dwarfs.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
Binaries are the best single stars
Stellar models of massive single stars are still plagued by major
uncertainties. Testing and calibrating against observations is essential for
their reliability. For this purpose one preferably uses observed stars that
have never experienced strong binary interaction, i.e. "true single stars".
However, the binary fraction among massive stars is high and identifying "true
single stars" is not straight forward. Binary interaction affects systems in
such a way that the initially less massive star becomes, or appears to be,
single. For example, mass transfer results in a widening of the orbit and a
decrease of the luminosity of the donor star, which makes it very hard to
detect. After a merger or disruption of the system by the supernova explosion,
no companion will be present.
The only unambiguous identification of "true single stars" is possible in
detached binaries, which contain two main-sequence stars. For these systems we
can exclude the occurrence of mass transfer since their birth. A further
advantage is that binaries can often provide us with direct measurements of the
fundamental stellar parameters. Therefore, we argue these binaries are worth
the effort needed to observe and analyze them. They may provide the most
stringent test cases for single stellar models.Comment: 5 pages, 1 figure, contribution to the proceedings of "The
multi-wavelength view of hot, massive stars", 39th Li`ege Int. Astroph.
Coll., 12-16 July 201
Can low metallicity binaries avoid merging?
Rapid mass transfer in a binary system can drive the accreting star out of
thermal equilibrium, causing it to expand. This can lead to a contact system,
strong mass loss from the system and possibly merging of the two stars. In low
metallicity stars the timescale for heat transport is shorter due to the lower
opacity. The accreting star can therefore restore thermal equilibrium more
quickly and possibly avoid contact.
We investigate the effect of accretion onto main sequence stars with
radiative envelopes with different metallicities. We find that a low
metallicity (Z<0.001), 4 solar mass star can endure a 10 to 30 times higher
accretion rate before it reaches a certain radius than a star at solar
metallicity. This could imply that up to two times fewer systems come into
contact during rapid mass transfer when we compare low metallicity. This factor
is uncertain due to the unknown distribution of binary parameters and the
dependence of the mass transfer timescale on metallicity. In a forthcoming
paper we will present analytic fits to models of accreting stars at various
metallicities intended for the use in population synthesis models.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 2 figure
Clues about the scarcity of stripped-envelope stars from the evolutionary state of the sdO+Be binary system phi Persei
Stripped-envelope stars (SESs) form in binary systems after losing mass
through Roche-lobe overflow. They bear astrophysical significance as sources of
UV and ionizing radiation in older stellar populations and, if sufficiently
massive, as stripped supernova progenitors. Binary evolutionary models predict
them to be common, but only a handful of subdwarfs (i.e., SESs) with B-type
companions are known. This could be the result of observational biases
hindering detection, or an incorrect understanding of binary evolution. We
reanalyze the well-studied post-interaction binary phi Persei. Recently, new
data improved the orbital solution of the system, which contains a ~1.2 Msun
SES and a rapidly rotating ~9.6 Msun Be star. We compare with an extensive grid
of evolutionary models using a Bayesian approach and find initial masses of the
progenitor of 7.2+/-0.4 Msun for the SES and 3.8+/-0.4 Msun for the Be star.
The system must have evolved through near-conservative mass transfer. These
findings are consistent with earlier studies. The age we obtain, 57+/-9 Myr, is
in excellent agreement with the age of the alpha Persei cluster. We note that
neither star was initially massive enough to produce a core-collapse supernova,
but mass exchange pushed the Be star above the mass threshold. We find that the
subdwarf is overluminous for its mass by almost an order of magnitude, compared
to the expectations for a helium core burning star. We can only reconcile this
if the subdwarf is in a late phase of helium shell burning, which lasts only
2-3% of the total lifetime as a subdwarf. This could imply that up to ~50 less
evolved, dimmer subdwarfs exist for each system similar to phi Persei. Our
findings can be interpreted as a strong indication that a substantial
population of SESs indeed exists, but has so far evaded detection because of
observational biases and lack of large-scale systematic searches.Comment: 11 pages, 5 figures, accepted for publication in A&
The rotation rates of massive stars: the role of binary interaction through tides, mass transfer and mergers
Rotation is thought to be a major factor in the evolution of massive stars,
especially at low metallicity, with consequences for their chemical yields,
ionizing flux and final fate. Determining the natal rotation-rate distribution
of stars is of high priority given its importance as a constraint on theories
of massive star formation and as input for models of stellar populations in the
local Universe and at high redshift. Recently, it has become clear that the
majority of massive stars interact with a binary companion before they die. We
investigate how this affects the distribution of rotation rates.
For this purpose, we simulate a massive binary-star population typical for
our Galaxy assuming continuous star formation. We find that, because of binary
interaction, 20^+5_-10% of all massive main-sequence stars have projected
rotational velocities in excess of 200km/s. We evaluate the effect of uncertain
input distributions and physical processes and conclude that the main
uncertainties are the mass transfer efficiency and the possible effect of
magnetic braking, especially if magnetic fields are generated or amplified
during mass accretion and stellar mergers.
The fraction of rapid rotators we derive is similar to that observed. If
indeed mass transfer and mergers are the main cause for rapid rotation in
massive stars, little room remains for rapidly rotating stars that are born
single. This implies that spin down during star formation is even more
efficient than previously thought. In addition, this raises questions about the
interpretation of the surface abundances of rapidly rotating stars as evidence
for rotational mixing. Furthermore, our results allow for the possibility that
all early-type Be stars result from binary interactions and suggest that
evidence for rotation in explosions, such as long gamma-ray bursts, points to a
binary origin.Comment: 14 pages, 5 figures, accepted for publication in ApJ., no changes
with v1 apart from fixed typos/ref
Massive binaries and the enrichment of the interstellar medium in globular clusters
Abundance anomalies observed in globular cluster stars indicate pollution
with material processed by hydrogen burning. Two main sources have been
suggested: asymptotic giant branch stars and massive stars rotating near the
break-up limit. We discuss the potential of massive binaries as an interesting
alternative source of processed material.
We discuss observational evidence for mass shedding from interacting
binaries. In contrast to the fast, radiatively driven winds of massive stars,
this material is typically ejected with low velocity. We expect that it remains
inside the potential well of a globular cluster and becomes available for the
formation or pollution of a second generation of stars. We estimate that the
amount of processed low-velocity material that can be ejected by massive
binaries is larger than the contribution of two previously suggested sources
combined.Comment: 6 pages, 2 figures, to appear in the proceedings of IAU Symposium
266, "Star Clusters - Basic Galactic Building Blocks throughout Time and
Space", 10-14 August 2009, at the general assembly in Rio de Janeiro, Brazi
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