95 research outputs found
Building Blue Stragglers with Stellar Collisions
The evolution of stellar collision products in cluster simulations has
usually been modelled using simplified prescriptions. Such prescriptions either
replace the collision product with an (evolved) main sequence star, or assume
that the collision product was completely mixed during the collision.
It is known from hydrodynamical simulations of stellar collisions that
collision products are not completely mixed, however. We have calculated the
evolution of stellar collision products and find that they are brighter than
normal main sequence stars of the same mass, but not as blue as models that
assume that the collision product was fully mixed during the collision.Comment: 2 pages, 1 figure. To appear in the proceedings of Dynamical
Evolution of Dense Stellar Systems, IAU Symposium 24
When Stars Collide
When two stars collide and merge they form a new star that can stand out
against the background population in a starcluster as a blue straggler. In so
called collision runaways many stars can merge and may form a very massive star
that eventually forms an intermediate mass blackhole. We have performed
detailed evolution calculations of merger remnants from collisions between main
sequence stars, both for lower mass stars and higher mass stars. These stars
can be significantly brighter than ordinary stars of the same mass due to their
increased helium abundance. Simplified treatments ignoring this effect give
incorrect predictions for the collision product lifetime and evolution in the
Hertzsprung-Russell diagram.Comment: 8 pages, 5 figures to appear in the proceedings for "Unsolved
Problems in Stellar Physics", Cambridge, 2-6 July 200
The Puzzling Frequencies of CEMP and NEMP Stars
We present the results of binary population simulations of carbon- and
nitrogen-enhanced metal-poor (CEMP and NEMP) stars. We show that the observed
paucity of very nitrogen-rich stars puts strong constraints on possible
modifications of the initial mass function at low metallicity.Comment: 3 pages, contribution to "The Origin of the Elements Heavier than
Iron" in honor of the 70th birthday of Roberto Gallino, Torino, Italy,
September 200
Critically rotating stars in binaries - an unsolved problem -
In close binaries mass and angular momentum can be transferred from one star
to the other during Roche-lobe overflow. The efficiency of this process is not
well understood and constitutes one of the largest uncertainties in binary
evolution.
One of the problems lies in the transfer of angular momentum, which will spin
up the accreting star. In very tight systems tidal friction can prevent
reaching critical rotation, by locking the spin period to the orbital period.
Accreting stars in systems with orbital periods larger than a few days reach
critical rotation after accreting only a fraction of their mass, unless there
is an effective mechanism to get rid of angular momentum. In low mass stars
magnetic field might help. In more massive stars angular momentum loss will be
accompanied by strong mass loss. This would imply that most interacting
binaries with initial orbital periods larger than a few days evolve very
non-conservatively.
In this contribution we wish to draw attention to the unsolved problems
related to mass and angular momentum transfer in binary systems. We do this by
presenting the first results of an implementation of spin up by accretion into
the TWIN version of the Eggleton stellar evolution code.Comment: 5 pages, 1 figure, to appear in the proceedings of the conference
"Unsolved Problems in Stellar Physics", Cambridge, 2-6 July 200
Models of Individual Blue Stragglers
This chapter describes the current state of models of individual blue
stragglers. Stellar collisions, binary mergers (or coalescence), and partial or
ongoing mass transfer have all been studied in some detail. The products of
stellar collisions retain memory of their parent stars and are not fully mixed.
Very high initial rotation rates must be reduced by an unknown process to allow
the stars to collapse to the main sequence. The more massive collision products
have shorter lifetimes than normal stars of the same mass, while products
between low mass stars are long-lived and look very much like normal stars of
their mass. Mass transfer can result in a merger, or can produce another binary
system with a blue straggler and the remnant of the original primary. The
products of binary mass transfer cover a larger portion of the colour-magnitude
diagram than collision products for two reasons: there are more possible
configurations which produce blue stragglers, and there are differing
contributions to the blended light of the system. The effects of rotation may
be substantial in both collision and merger products, and could result in
significant mixing unless angular momentum is lost shortly after the formation
event. Surface abundances may provide ways to distinguish between the formation
mechanisms, but care must be taking to model the various mixing mechanisms
properly before drawing strong conclusions. Avenues for future work are
outlined.Comment: Chapter 12, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
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
Orbital eccentricities of binary systems with a former AGB star
Many binary stellar systems in which the primary star is beyond the
asymptotic giant branch (AGB) evolutionary phase show significant orbital
eccentricities whereas current binary interaction models predict their orbits
to be circularised. We analyse how the orbital parameters in a system are
modified under mass loss and mass exchange among its binary components and
propose a model for enhanced mass-loss from the AGB star due to tidal
interaction with its companion, which allows a smooth transition between the
wind and Roche-lobe overflow mass-loss regimes. We explicitly follow its effect
along the orbit on the change of eccentricity and orbital semi-major axis, as
well as the effect of accretion by the companion. We calculate timescales for
the variation of these orbital parameters and compare them to the tidal
circularisation timescale. We find that in many cases, due to the enhanced mass
loss of the AGB component at orbital phases closer to the periastron, the net
eccentricity growth rate in one orbit is comparable to the rate of tidal
circularisation. We show that with this eccentricity enhancing mechanism it is
possible to reproduce the orbital period and eccentricity of the Sirius system,
which under the standard assumptions of binary interaction is expected to be
circularised. We also show that this mechanism may provide an explanation for
the eccentricities of most barium star systems, which are expected to be
circularised due to tidal dissipation. By proposing a tidally enhanced model of
mass loss from AGB stars we find a mechanism which efficiently works against
the tidal circularisation of the orbit, which explains the significant
eccentricities observed in binary systems containing a white dwarf and a less
evolved companion, such as Sirius and systems with barium stars.Comment: 9 pages, 5 figures, accepted for publication in Astronomy and
Astrophysics on 24th of October of 200
Evolution of stellar collision products in open clusters. I. Blue stragglers in N-body models of M67
Stellar collisions are an important formation channel for blue straggler
stars in globular and old open clusters. Hydrodynamical simulations have shown
that the remnants of such collisions are out of thermal equilibrium, are not
strongly mixed and can rotate very rapidly.
Detailed evolution models of collision products are needed to interpret
observed blue straggler populations and to use them to probe the dynamical
history of a star cluster. We expand on previous studies by presenting an
efficient procedure to import the results of detailed collision simulations
into a fully implicit stellar evolution code. Our code is able to evolve
stellar collision products in a fairly robust manner and allows for a
systematic study of their evolution.
Using our code we have constructed detailed models of the collisional blue
stragglers produced in the -body simulation of M67 performed by Hurley
\emph{et al.} in 2005. We assume the collisions are head-on and thus ignore the
effects of rotation in this paper.
Our detailed models are more luminous than normal stars of the same mass and
in the same stage of evolution, but cooler than homogeneously mixed versions of
the collision products. The increased luminosity and inefficient mixing
decrease the remaining main-sequence lifetimes of the collision products, which
are much shorter than predicted by the simple prescription commonly used in
-body simulations.Comment: To be published in A&
Formation of the black-hole binary M33 X-7 via mass-exchange in a tight massive system
M33 X-7 is among the most massive X-Ray binary stellar systems known, hosting
a rapidly spinning 15.65 Msun black hole orbiting an underluminous 70 Msun Main
Sequence companion in a slightly eccentric 3.45 day orbit. Although
post-main-sequence mass transfer explains the masses and tight orbit, it leaves
unexplained the observed X-Ray luminosity, star's underluminosity, black hole's
spin, and eccentricity. A common envelope phase, or rotational mixing, could
explain the orbit, but the former would lead to a merger and the latter to an
overluminous companion. A merger would also ensue if mass transfer to the black
hole were invoked for its spin-up. Here we report that, if M33 X-7 started as a
primary of 85-99 Msun and a secondary of 28-32 Msun, in a 2.8-3.1 day orbit,
its observed properties can be consistently explained. In this model, the Main
Sequence primary transferred part of its envelope to the secondary and lost the
rest in a wind; it ended its life as a ~16 Msun He star with a Fe-Ni core which
collapsed to a black hole (with or without an accompanying supernova). The
release of binding energy and, possibly, collapse asymmetries "kicked" the
nascent black hole into an eccentric orbit. Wind accretion explains the X-Ray
luminosity, while the black hole spin can be natal.Comment: Manuscript: 18 pages, 2 tables, 2 figure. Supplementary Information:
34 pages, 6 figures. Advance Online Publication (AOP) on
http://www.nature.com/nature on October 20, 2010. To Appear in Nature on
November 4, 201
Slowing down atomic diffusion in subdwarf B stars: mass loss or turbulence?
Subdwarf B stars show chemical peculiarities that cannot be explained by
diffusion theory alone. Both mass loss and turbulence have been invoked to slow
down atomic diffusion in order to match observed abundances. The fact that some
sdB stars show pulsations gives upper limits on the amount of mass loss and
turbulent mixing allowed. Consequently, non-adiabatic asteroseismology has the
potential to decide which process is responsible for the abundance anomalies.
We compute for the first time seismic properties of sdB models with atomic
diffusion included consistently during the stellar evolution. The diffusion
equations with radiative forces are solved for H, He, C, N, O, Ne, Mg, Fe and
Ni. We examine the effects of various mass-loss rates and mixed surface masses
on the abundances and mode stability. It is shown that the mass-loss rates
needed to simulate the observed He abundances (10^{-14}<=Mdot
[Msun/yr]<=10^{-13}) are not consistent with observed pulsations. We find that
for pulsations to be driven the rates should be Mdot<=10^{-15} Msun/yr. On the
other hand, weak turbulent mixing of the outer 10^{-6} Msun can explain the He
abundance anomalies while still allowing pulsations to be driven. The origin of
the turbulence remains unknown but the presence of pulsations gives tight
constraints on the underlying turbulence model.Comment: 12 pages, 8 figures, 1 table, accepted for publication in MNRA
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