360 research outputs found
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 , 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 star formation rates for the highest-redshift and
highest-SFR galaxies. In addition this may resolve several outstanding problems
in the chemical evolution of galactic halos.Comment: 9 pages, 5 figures. Published in MNRAS. Added further reference
A two-dimensional mixing length theory of convective transport
The helioseismic observations of the internal rotation profile of the Sun
raise questions about the two-dimensional (2D) nature of the transport of
angular momentum in stars. Here we derive a convective prescription for
axisymmetric (2D) stellar evolution models. We describe the small scale motions
by a spectrum of unstable linear modes in a Boussinesq fluid. Our saturation
prescription makes use of the angular dependence of the linear dispersion
relation to estimate the anisotropy of convective velocities. We are then able
to provide closed form expressions for the thermal and angular momentum fluxes
with only one free parameter, the mixing length.
We illustrate our prescription for slow rotation, to first order in the
rotation rate. In this limit, the thermodynamical variables are spherically
symetric, while the angular momentum depends both on radius and latitude. We
obtain a closed set of equations for stellar evolution, with a self-consistent
description for the transport of angular momentum in convective regions. We
derive the linear coefficients which link the angular momentum flux to the
rotation rate (- effect) and its gradient (-effect). We
compare our results to former relevant numerical work.Comment: MNRAS accepted, 10 pages, 1 figure, version prior to language editio
Confinement of the Sun's interior magnetic field: some exact boundary-layer solutions
High-latitude laminar confinement of the Sun's interior magnetic field is
shown to be possible, as originally proposed by Gough and McIntyre (1998) but
contrary to a recent claim by Brun and Zahn (A&A 2006). Mean downwelling as
weak as 2x10^-6cm/s -- gyroscopically pumped by turbulent stresses in the
overlying convection zone and/or tachocline -- can hold the field in
advective-diffusive balance within a confinement layer of thickness scale ~
1.5Mm ~ 0.002 x (solar radius) while transmitting a retrograde torque to the
Ferraro-constrained interior. The confinement layer sits at the base of the
high-latitude tachocline, near the top of the radiative envelope and just above
the `tachopause' marking the top of the helium settling layer. A family of
exact, laminar, frictionless, axisymmetric confinement-layer solutions is
obtained for uniform downwelling in the limit of strong rotation and
stratification. A scale analysis shows that the flow is dynamically stable and
the assumption of laminar flow realistic. The solution remains valid for
downwelling values of the order of 10^-5cm/s but not much larger. This suggests
that the confinement layer may be unable to accept a much larger mass
throughput. Such a restriction would imply an upper limit on possible internal
field strengths, perhaps of the order of hundreds of gauss, and would have
implications also for ventilation and lithium burning.
The solutions have interesting chirality properties not mentioned in the
paper owing to space restrictions, but described at
http://www.atmos-dynamics.damtp.cam.ac.uk/people/mem/papers/SQBO/solarfigure.htmlComment: 6 pages, 3 figures, to appear in conference proceedings: Unsolved
Problems in Stellar Physic
Statistical Description of a Magnetized Corona above a Turbulent Accretion Disk
We present a physics-based statistical theory of a force-free magnetic field
in the corona above a turbulent accretion disk. The field is represented by a
statistical ensemble of loops tied to the disk. Each loop evolves under several
physical processes: Keplerian shear, turbulent random walk of the disk
footpoints, and reconnection with other loops. To build a statistical
description, we introduce the distribution function of loops over their sizes
and construct a kinetic equation that governs its evolution. This loop kinetic
equation is formally analogous to Boltzmann's kinetic equation, with loop-loop
reconnection described by a binary collision integral. A dimensionless
parameter is introduced to scale the (unknown) overall rate of reconnection
relative to Keplerian shear. After solving for the loop distribution function
numerically, we calculate self-consistently the distribution of the mean
magnetic pressure and dissipation rate with height, and the equilibrium shapes
of loops of different sizes. We also compute the energy and torque associated
with a given loop, as well as the total magnetic energy and torque in the
corona. We explore the dependence of these quantities on the reconnection
parameter and find that they can be greatly enhanced if reconnection between
loops is suppressed.Comment: 22 pages, 15 figures. Submitted to the Astrophysical Journa
Long GRBs from binary stars: runaway, Wolf-Rayet progenitors
The collapsar model for long gamma-ray bursts requires a rapidly rotating
Wolf-Rayet star as progenitor. We test the idea of producing rapidly rotating
Wolf-Rayet stars in massive close binaries through mass accretion and
consecutive quasi-chemically homogeneous evolution; the latter had previously
been shown to provide collapsars below a certain metallicity threshold for
single stars. The binary channel presented here may provide a means for massive
stars to obtain the high rotation rates required to evolve quasi-chemically
homogeneous and fulfill the collapsar scenario. Moreover, it suggests that a
possibly large fraction of long gamma-ray bursts occurs in runaway stars.Comment: To appear in the proceedings of the conference "Unsolved problems in
stellar physics" - Cambridge, July 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
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Resolving dichotomy in compact objects through continuous gravitational waves observation
More than two dozen soft gamma-ray repeaters (SGRs) and anomalous X-ray
pulsars (AXPs) have been detected so far. These are isolated compact objects.
Many of them are either found to be associated with supernova remnants or their
surface magnetic fields are directly measured, confirming that they are neutron
stars (NSs). However, it has been argued that some SGRs and AXPs are highly
magnetized white dwarfs (WDs). Meanwhile, the existence of super-Chandrasekhar
WDs has remained to be a puzzle. However, not even a single such massive WD has
been observed directly. Moreover, some WD pulsars are detected in
electromagnetic surveys and some of their masses are still not confirmed. Here
we calculate the signal-to-noise ratio for all these objects, considering
different magnetic field configurations and thereby estimate the required time
for their detection by various gravitational wave (GW) detectors. For SGRs and
AXPs, we show that, if these are NSs, they can hardly be detected by any of the
GW detectors, while if they are WDs, Big Bang Observer (BBO), DECi-hertz
Interferometer Gravitational wave Observatory (DECIGO) and Advanced Laser
Interferometer Antenna (ALIA) would be able to detect them within a few days to
a year of integration, depending on the magnetic field strength and its
configuration. Similarly, if a super-Chandrasekhar WD has a dominant toroidal
field, we show that even Laser Interferometer Space Antenna (LISA) and TianQin
would be able to detect it within one year of integration. We also discuss how
GWs can confirm the masses of the WD pulsars
The Probability Distribution of Binary Pulsar Coalescence Rate Estimates. II. Neutron Star-White Dwarf Binaries
We consider the statistics of pulsar binaries with white dwarf companions
(NS-WD). Using the statistical analysis method developed by Kim et al. (2003)
we calculate the Galactic coalescence rate of NS-WD binaries due to
gravitational-wave emission. We find that the most likely values for the total
Galactic coalescence rate (R_tot) of NS-WD binaries lie in the range 0.2--10
per Myr depending on different assumed pulsar population models. For our
reference model, we obtain R_tot=4.11_(-2.56)^(+5.25) per Myr at a 68%
statistical confidence level. These rate estimates are not corrected for pulsar
beaming and as such they are found to be about a factor of 20 smaller than the
Galactic coalescence rate estimates for double neutron star systems. Based on
our rate estimates, we calculate the gravitational-wave background due to
coalescing NS-WD binaries out to extragalactic distances within the frequency
band of the Laser Interferometer Space Antenna. We find the contribution from
NS-WD binaries to the gravitational-wave background to be negligible.Comment: 20 pages, 2 figures, 2 tables, Accepted for publication in Ap
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