75 research outputs found
Dynamo driven accretion discs and dwarf nova eruptions
We explore the consequences of a magnetic dynamo origin for the viscosity in
accretion discs, for the structure and evolution of discs in dwarf nova
systems. We propose that the rapid cooling that sets in at the end of a dwarf
nova eruption acts to inhibit the Balbus-Hawley instability, and thereby to
quench dynamo action and so reduce disc viscosity. We demonstrate that a
modified disc instability model can reproduce the basic properties of dwarf
nova eruptions, as well as some properties of quiescent discs. We also discuss
some observational consequences of our model.Comment: uu-encoded gz-compressed Postscript file, 18 pages including 6
figures. ApJ in pres
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A dynamical gravitational wave source in a dense cluster
Making use of a new N-body model to describe the evolution of a moderate-size globular cluster we in- vestigate the characteristics of the population of black holes within such a cluster. This model reaches core-collapse and achieves a peak central density typical of the dense globular clusters of the Milky Way. Within this high-density environment we see direct confirmation of the merging of two stellar remnant black-holes in a dynamically-formed binary, a gravitational wave source. We describe how the formation, evolution and ultimate ejection/destruction of binary systems containing black holes impacts the evolution of the cluster core. Also, through comparison with previous models of lower density, we show that the period distribution of black hole binaries formed through dynamical interactions in this high-density model favours the production of gravitational wave sources. We confirm that the number of black holes remaining in a star cluster at late times and the characteristics of the binary black hole population depend on the nature of the star cluster, critically on the number density of stars and by extension the relaxation timescale
Enhanced rotational mixing in the radiative zones of massive stars
Convection in the cores of massive stars becomes anisotropic when they
rotate. This anisotropy leads to a misalignment of the thermal gradient and the
thermal flux, which in turn results in baroclinicity and circulation currents
in the upper radiative zone. We show that this induces a much stronger
meridional flow in the radiative zone than previously thought. This drives
significantly enhanced mixing, though this mixing does not necessarily reach
the surface. The extra mixing takes on a similar form to convective
overshooting, and is relatively insensitive to the rotation rate above a
threshold, and may help explain the large overshoot distances inferred from
observations. This has significant consequences for the evolution of these
stars by enhancing core-envelope mixing
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Modified virial theorem for highly magnetized white dwarfs
ABSTRACT
Generally the virial theorem provides a relation between various components of energy integrated over a system. This helps us to understand the underlying equilibrium. Based on the virial theorem we can estimate, for example, the maximum allowed magnetic field in a star. Recent studies have proposed the existence of highly magnetized white dwarfs (B-WDs), with masses significantly higher than the Chandrasekhar limit. Surface magnetic fields of such white dwarfs could be more than G with the central magnitude several orders higher. These white dwarfs could be significantly smaller in size than their ordinary counterparts (with surface fields restricted to about G). In this paper, we reformulate the virial theorem for non-rotating B-WDs in which, unlike in previous formulations, the contribution of the magnetic pressure to the magnetohydrostatic balance cannot be neglected. Along with the new equation of magnetohydrostatic equilibrium, we approach the problem by invoking magnetic flux conservation and by varying the internal magnetic field with the matter density as a power law. Either of these choices is supported by previous independent work and neither violates any important physics. They are useful while there is no prior knowledge of field profile within a white dwarf. We then compute the modified gravitational, thermal, and magnetic energies and examine how the magnetic pressure influences the properties of such white dwarfs. Based on our results we predict important properties of these B-WDs, which turn out to be independent of our chosen field profiles.</jats:p
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The cosmic microwave background and the stellar initial mass function
We argue that an increased temperature in star-forming clouds alters the stellar initial mass function to be more bottom-light than in the Milky Way. At redshifts z ≳ 6, heating from the cosmic microwave background radiation produces this effect in all galaxies, and it is also present at lower redshifts in galaxies with very high star formation rates (SFRs). A failure to account for it means that at present photometric template fitting likely overestimates stellar masses and SFRs for the highest redshift and highest SFR galaxies. In addition, this may resolve several outstanding problems in the chemical evolution of galactic haloes
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Tidal interactions of close hot subdwarf binaries
Over half of all observed hot subdwarf B (sdB) stars are found in binaries, and over half of these are found in close configurations with orbital periods of 10 d or less. In order to estimate the companion masses in these predominantly single-lined systems, tidal locking has frequently been assumed for sdB binaries with periods less than half a day. Observed non-synchronicity of a number of close sdB binaries challenges that assumption and hence provides an ideal testbed for tidal theory. We solve the second-order differential equations for detailed 1D stellar models of sdB stars to obtain the tidal dissipation strength and hence to estimate the tidal synchronization time-scale owing to Zahn's dynamical tide. The results indicate synchronization time-scales longer than the sdB lifetime in all observed cases. Further, we examine the roles of convective overshooting and convective dissipation in the core of sdB stars and find no theoretical framework in which tidally induced synchronization should occur
Post-common envelope binary systems experiencing helium-shell-driven stable mass transfer
We evolve stellar models to study the common envelope (CE) interaction of an
early asymptotic giant branch star of initial mass with a
companion star of mass ranging from to . We model the
CE as a fast stripping phase in which the primary experiences rapid mass loss
and loses about 80 per cent of its mass. The post-CE remnant is then allowed to
thermally readjust during a Roche-lobe overflow (RLOF) phase and the final
binary system and its orbital period are investigated. We find that the post-CE
RLOF phase is long enough to allow nuclear burning to proceed in the helium
shell. By the end of this phase, the donor is stripped of both its hydrogen and
helium and ends up as carbon-oxygen white dwarf of mass about . We study the sensitivity of our results to initial conditions of
different companion masses and orbital separations at which the stripping phase
begins. We find that the companion mass affects the final binary separation and
that helium-shell burning causes the star to refill its Roche lobe leading to
post-CE RLOF. Our results show that double mass transfer in such a binary
interaction is able to strip the helium and hydrogen layers from the donor star
without the need for any special conditions or fine tuning of the binary
parameters
Origin of magnetic fields in cataclysmic variables
In a series of recent papers, it has been proposed that high field magnetic
white dwarfs are the result of close binary interaction and merging. Population
synthesis calculations have shown that the origin of isolated highly magnetic
white dwarfs is consistent with the stellar merging hypothesis. In this
picture, the observed fields are caused by an alpha-Omega dynamo driven by
differential rotation. The strongest fields arise when the differential
rotation equals the critical break-up velocity and result from the merging of
two stars (one of which has a degenerate core) during common envelope evolution
or from the merging of two white dwarfs. We now synthesise a population of
binary systems to investigate the hypothesis that the magnetic fields in the
magnetic cataclysmic variables also originate during stellar interaction in the
common envelope phase. Those systems that emerge from common envelope more
tightly bound form the cataclysmic variables with the strongest magnetic
fields. We vary the common envelope efficiency parameter and compare the
results of our population syntheses with observations of magnetic cataclysmic
variables. We find that common envelope interaction can explain the observed
characteristics of these magnetic systems if the envelope ejection efficiency
is low
Genesis of magnetic fields in isolated white dwarfs
A dynamo mechanism driven by differential rotation when stars merge has been
proposed to explain the presence of strong fields in certain classes of
magnetic stars. In the case of the high field magnetic white dwarfs (HFMWDs),
the site of the differential rotation has been variously thought to be the
common envelope, the hot outer regions of a merged degenerate core or an
accretion disc formed by a tidally disrupted companion that is subsequently
accreted by a degenerate core. We have shown previously that the observed
incidence of magnetism and the mass distribution in HFMWDs are consistent with
the hypothesis that they are the result of merging binaries during common
envelope evolution. Here we calculate the magnetic field strengths generated by
common envelope interactions for synthetic populations using a simple
prescription for the generation of fields and find that the observed magnetic
field distribution is also consistent with the stellar merging hypothesis. We
use the Kolmogorov-Smirnov test to study the correlation between the calculated
and the observed field strengths and find that it is consistent for low
envelope ejection efficiency. We also suggest that field generation by the
plunging of a giant gaseous planet on to a white dwarf may explain why
magnetism among cool white dwarfs (including DZ white dwarfs) is higher than
among hot white dwarfs. In this picture a super Jupiter residing in the outer
regions of the planetary system of the white dwarf is perturbed into a highly
eccentric orbit by a close stellar encounter and is later accreted by the white
dwarf
Overshoot inwards from the bottom of the intershell convective zone in (S)AGB stars
We estimate the extent of overshooting inwards from the bottom of the
intershell convective zone in thermal pulses in (S)AGB stars. We find that the
buoyancy is so strong that any overshooting should be negligible. The
temperature inversion at the bottom of the convective zone adds to the
stability of the region. Any mixing that occurs in this region is highly
unlikely to be due to convective overshooting, and so must be due to another
process
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