1,701 research outputs found
Equipotential Surfaces and Lagrangian points in Non-synchronous, Eccentric Binary and Planetary Systems
We investigate the existence and properties of equipotential surfaces and
Lagrangian points in non-synchronous, eccentric binary star and planetary
systems under the assumption of quasi-static equilibrium. We adopt a binary
potential that accounts for non-synchronous rotation and eccentric orbits, and
calculate the positions of the Lagrangian points as functions of the mass
ratio, the degree of asynchronism, the orbital eccentricity, and the position
of the stars or planets in their relative orbit. We find that the geometry of
the equipotential surfaces may facilitate non-conservative mass transfer in
non-synchronous, eccentric binary star and planetary systems, especially if the
component stars or planets are rotating super-synchronously at the periastron
of their relative orbit. We also calculate the volume-equivalent radius of the
Roche lobe as a function of the four parameters mentioned above. Contrary to
common practice, we find that replacing the radius of a circular orbit in the
fitting formula of Eggleton (1983) with the instantaneous distance between the
components of eccentric binary or planetary systems does not always lead to a
good approximation to the volume-equivalent radius of the Roche-lobe. We
therefore provide generalized analytic fitting formulae for the
volume-equivalent Roche lobe radius appropriate for non-synchronous, eccentric
binary star and planetary systems. These formulae are accurate to better than
1% throughout the relevant 2-dimensional parameter space that covers a dynamic
range of 16 and 6 orders of magnitude in the two dimensions.Comment: 12 pages, 10 figures, 2 Tables, Accepted by the Astrophysical Journa
Two-Dimensional Hydrodynamics of Pre-Core Collapse: Oxygen Shell Burning
By direct hydrodynamic simulation, using the Piecewise Parabolic Method (PPM)
code PROMETHEUS, we study the properties of a convective oxygen burning shell
in a SN 1987A progenitor star prior to collapse. The convection is too
heterogeneous and dynamic to be well approximated by one-dimensional
diffusion-like algorithms which have previously been used for this epoch.
Qualitatively new phenomena are seen.
The simulations are two-dimensional, with good resolution in radius and
angle, and use a large (90-degree) slice centered at the equator. The
microphysics and the initial model were carefully treated. Many of the
qualitative features of previous multi-dimensional simulations of convection
are seen, including large kinetic and acoustic energy fluxes, which are not
accounted for by mixing length theory. Small but significant amounts of
carbon-12 are mixed non-uniformly into the oxygen burning convection zone,
resulting in hot spots of nuclear energy production which are more than an
order of magnitude more energetic than the oxygen flame itself. Density
perturbations (up to 8%) occur at the `edges' of the convective zone and are
the result of gravity waves generated by interaction of penetrating flows into
the stable region. Perturbations of temperature and electron fraction at the
base of the convective zone are of sufficient magnitude to create angular
inhomogeneities in explosive nucleosynthesis products, and need to be included
in quantitative estimates of yields. Combined with the plume-like velocity
structure arising from convection, the perturbations will contribute to the
mixing of nickel-56 throughout supernovae envelopes. Runs of different
resolution, and angular extent, were performed to test the robustness of theseComment: For mpeg movies of these simulations, see
http://www.astrophysics.arizona.edu/movies.html Submitted to the
Astrophysical Journa
Detection of Gravitational Lensing in the Cosmic Microwave Background
Gravitational lensing of the cosmic microwave background (CMB), a
long-standing prediction of the standard cosmolgical model, is ultimately
expected to be an important source of cosmological information, but first
detection has not been achieved to date. We report a 3.4 sigma detection, by
applying quadratic estimator techniques to all sky maps from the Wilkinson
Microwave Anisotropy Probe (WMAP) satellite, and correlating the result with
radio galaxy counts from the NRAO VLA Sky Survey (NVSS). We present our
methodology including a detailed discussion of potential contaminants. Our
error estimates include systematic uncertainties from density gradients in
NVSS, beam effects in WMAP, Galactic microwave foregrounds, resolved and
unresolved CMB point sources, and the thermal Sunyaev-Zeldovich effect.Comment: 27 pages, 20 figure
Physical Orbit for Lambda Virginis and a Test of Stellar Evolution Models
Lambda Virginis (LamVir) is a well-known double-lined spectroscopic Am binary
with the interesting property that both stars are very similar in abundance but
one is sharp-lined and the other is broad-lined. We present combined
interferometric and spectroscopic studies of LamVir. The small scale of the
LamVir orbit (~20 mas) is well resolved by the Infrared Optical Telescope Array
(IOTA), allowing us to determine its elements as well as the physical
properties of the components to high accuracy. The masses of the two stars are
determined to be 1.897 Msun and 1.721 Msun, with 0.7% and 1.5% errors
respectively, and the two stars are found to have the same temperature of 8280
+/- 200 K. The accurately determined properties of LamVir allow comparisons
between observations and current stellar evolution models, and reasonable
matches are found. The best-fit stellar model gives LamVir a subsolar
metallicity of Z=0.0097, and an age of 935 Myr. The orbital and physical
parameters of LamVir also allow us to study its tidal evolution time scales and
status. Although currently atomic diffusion is considered to be the most
plausible cause of the Am phenomenon, the issue is still being actively debated
in the literature. With the present study of the properties and evolutionary
status of LamVir, this system is an ideal candidate for further detailed
abundance analyses that might shed more light on the source of the chemical
anomalies in these A stars.Comment: 43 Pages, 13 figures. Accepted for publication in Ap
The Magnetic Fields at the Surface of Active Single G-K Giants
We investigate the magnetic field at the surface of 48 red giants selected as
promising for detection of Stokes V Zeeman signatures in their spectral lines.
We use the spectropolarimeters Narval and ESPaDOnS to detect circular
polarization within the photospheric absorption lines of our targets and use
the least-squares deconvolution (LSD) method. We also measure the classical
S-index activity indicator, and the stellar radial velocity. To infer the
evolutionary status of our giants and to interpret our results, we use
state-of-the-art stellar evolutionary models with predictions of convective
turnover times. We unambiguously detect magnetic fields via Zeeman signatures
in 29 of the 48 red giants in our sample. Zeeman signatures are found in all
but one of the 24 red giants exhibiting signs of activity, as well as 6 out of
17 bright giant stars.The majority of the magnetically detected giants are
either in the first dredge up phase or at the beginning of core He burning,
i.e. phases when the convective turnover time is at a maximum: this corresponds
to a 'magnetic strip' for red giants in the Hertzsprung-Russell diagram. A
close study of the 16 giants with known rotational periods shows that the
measured magnetic field strength is tightly correlated with the rotational
properties, namely to the rotational period and to the Rossby number Ro. Our
results show that the magnetic fields of these giants are produced by a dynamo.
Four stars for which the magnetic field is measured to be outstandingly strong
with respect to that expected from the rotational period/magnetic field
relation or their evolutionary status are interpreted as being probable
descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4
bright giants (Aldebaran, Alphard, Arcturus, eta Psc) are detected with
magnetic field strength at the sub-gauss level.Comment: 34 pages, 22 Figures, accepted for publication in Astronomy &
Astrophysic
Evolution in Binary and Triple Stars, with an application to SS Lac
We present equations governing the way in which both the orbit and the
intrinsic spins of stars in a close binary should evolve subject to a number of
perturbing forces, including the effect of a third body in a possibly inclined
wider orbit. We illustrate the solutions in some binary-star and triple-star
situations: tidal friction in a wide but eccentric orbit of a radio pulsar
about a B star, the Darwin and eccentricity instabilities in a more massive but
shorter-period massive X-ray binary, and the interaction of tidal friction with
Kozai cycles in a triple such as Algol (beta-Per), at an early stage in that
star's life when all 3 components were ZAMS stars. We also attempt to model in
some detail the interesting triple system SS Lac, which stopped eclipsing in
about 1950. We find that our model of SS Lac is quite constrained by the
relatively good observational data of this system, and leads to a specific
inclination (29 deg) of the outer orbit relative to the inner orbit at epoch
zero (1912). Although the intrinsic spins of the stars have little effect on
the orbit, the converse is not true: the spin axes can vary their orientation
relative to the close binary by up to 120 deg on a timescale of about a
century.Comment: 30 pages, 6 figure
CMBR Weak Lensing and HI 21-cm Cross-correlation Angular Power Spectrum
Weak gravitational lensing of the CMBR manifests as a secondary anisotropy in
the temperature maps. The effect, quantified through the shear and convergence
fields imprint the underlying large scale structure (LSS), geometry and
evolution history of the Universe. It is hence perceived to be an important
observational probe of cosmology. De-lensing the CMBR temperature maps is also
crucial for detecting the gravitational wave generated B-modes. Future
observations of redshifted 21-cm radiation from the cosmological neutral
hydrogen (HI) distribution hold the potential of probing the LSS over a large
redshift range. We have investigated the correlation between post-reionization
HI signal and weak lensing convergence field. Assuming that the HI follows the
dark matter distribution, the cross-correlation angular power spectrum at a
multipole \ell is found to be proportional to the cold dark matter power
spectrum evaluated at \ell/r, where r denotes the comoving distance to the
redshift where the HI is located. The amplitude of the ross-correlation depends
on quantities specific to the HI distribution, growth of perturbations and also
the underlying cosmological model. In an ideal ituation, we found that a
statistically significant detection of the cross-correlation signal is
possible. If detected, the cross-correlation signal hold the possibility of a
joint estimation of cosmological parameters and also test various CMBR
de-lensing estimators.Comment: 14 pages, 4 figures, publishe
Donor Stars in Black-Hole X-Ray Binaries
We study theoretically the formation of black-hole (BH) X-ray binaries.
Consistency of the models with the observed relative numbers of systems with
low-mass (<2 M_sun) and intermediate-mass (~2 M_sun - M_{BH}) donors leads to
severe constraints on the evolutionary parameters of the progenitors. In
particular, we find that (i) BH progenitor masses cannot exceed about 2 M_{BH};
(ii) high values of the common-envelope efficiency parameter (alpha_{CE} > 1)
are required, implying that energy sources other than orbital contraction must
be invoked to eject the envelope; (iii) the mass-loss fraction in helium-star
winds is limited to be <50%. Outside of this limited parameter space for
progenitors we find that either BH X-ray binary formation cannot occur at all
or donors do not have the full range of observed masses. We discuss the
implications of these results for the structure of massive hydrogen-rich stars,
the evolution of helium-stars, and BH formation. We also consider the possible
importance of asymmetric kicks.Comment: 29 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Pores in Bilayer Membranes of Amphiphilic Molecules: Coarse-Grained Molecular Dynamics Simulations Compared with Simple Mesoscopic Models
We investigate pores in fluid membranes by molecular dynamics simulations of
an amphiphile-solvent mixture, using a molecular coarse-grained model. The
amphiphilic membranes self-assemble into a lamellar stack of amphiphilic
bilayers separated by solvent layers. We focus on the particular case of
tension less membranes, in which pores spontaneously appear because of thermal
fluctuations. Their spatial distribution is similar to that of a random set of
repulsive hard discs. The size and shape distribution of individual pores can
be described satisfactorily by a simple mesoscopic model, which accounts only
for a pore independent core energy and a line tension penalty at the pore
edges. In particular, the pores are not circular: their shapes are fractal and
have the same characteristics as those of two dimensional ring polymers.
Finally, we study the size-fluctuation dynamics of the pores, and compare the
time evolution of their contour length to a random walk in a linear potential
Voros product and the Pauli principle at low energies
Using the Voros star product, we investigate the status of the two particle
correlation function to study the possible extent to which the previously
proposed violation of the Pauli principle may impact at low energies. The
results show interesting features which are not present in the computations
made using the Moyal star product.Comment: 5 pages LateX, minor correction
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