27 research outputs found
The Evolution of Blue Stragglers Formed Via Stellar Collisions
We have used the results of recent smoothed particle hydrodynamic simulations
of colliding stars to create models appropriate for input into a stellar
evolution code. In evolving these models, we find that little or no surface
convection occurs, precluding angular momentum loss via a magnetically-driven
stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers
which have been formed by collisions. Angular momentum transfer to either a
circumstellar disk (possibly collisional ejecta) or a nearby companion are
plausible mechanisms for explaining the observed low rotation velocities of
blue stragglers. Under the assumption that the blue stragglers seen in NGC 6397
and 47 Tuc have been created solely by collisions, we find that the majority of
these blue stragglers cannot have been highly mixed by convection or meridional
circulation currents at anytime during their evolution. Also, on the basis of
the agreement between the predictions of our non-rotating models and the
observed blue straggler distribution, the evolution of blue stragglers is
apparently not dominated by the effects of rotation.Comment: 36 pages, including 1 table and 7 postscript figures (LaTeX2e). Also
avaliable at http://astrowww.phys.uvic.ca/~ouellet/ . Accepted for
publication in A
Composition Mixing during Blue Straggler Formation and Evolution
We use smoothed-particle hydrodynamics to examine differences between direct
collisions of single stars and binary star mergers in their roles as possible
blue straggler star formation mechanisms. We find in all cases that core helium
in the progenitor stars is largely retained in the core of the remnant, almost
independent of the type of interaction or the central concentration of the
progenitor stars.
We have also modelled the subsequent evolution of the hydrostatic remnants,
including mass loss and energy input from the hydrodynamical interaction. The
combination of the hydrodynamical and hydrostatic models enables us to predict
that little mixing will occur during the merger of two globular cluster stars
of equal mass. In contrast to the results of Proctor Sills, Bailyn, & Demarque
(1995), we find that neither completely mixed nor unmixed models can match the
absolute colors of observed blue stragglers in NGC 6397 at all luminosity
levels. We also find that the color distribution is probably the crucial test
for explanations of BSS formation - if stellar collisions or mergers are the
correct mechanisms, a large fraction of the lifetime of the straggler must be
spent away from the main sequence. This constraint appears to rule out the
possibility of completely mixed models. For NGC 6397, unmixed models predict
blue straggler lifetimes ranging from about 0.1 to 4 Gyr, while completely
mixed models predict a range from about 0.6 to 4 Gyr.Comment: AASTeX, 28 pg., accepted for ApJ, also available at
http://ucowww.ucsc.edu/~erics/bspaper.htm
Phase instabilities in hexagonal patterns
The general form of the amplitude equations for a hexagonal pattern including
spatial terms is discussed. At the lowest order we obtain the phase equation
for such patterns. The general expression of the diffusion coefficients is
given and the contributions of the new spatial terms are analysed in this
paper. From these coefficients the phase stability regions in a hexagonal
pattern are determined. In the case of Benard-Marangoni instability our results
agree qualitatively with numerical simulations performed recently.Comment: 6 pages, 6 figures, to appear in Europhys. Let
Cataclysmic Variables and a New Class of Faint UV Stars in the Globular Cluster NGC 6397
We present evidence that the globular cluster NGC 6397 contains two distinct
classes of centrally-concentrated UV-bright stars. Color-magnitude diagrams
constructed from U, B, V, and I data obtained with the HST/WFPC2 reveal seven
UV-bright stars fainter than the main-sequence turnoff, three of which had
previously been identified as cataclysmic variables (CVs). Lightcurves of these
stars show the characteristic ``flicker'' of CVs, as well as longer-term
variability. A fourth star is identified as a CV candidate on the basis of its
variability and UV excess. Three additional UV-bright stars show no photometric
variability and have broad-band colors characteristic of B stars. These
non-flickering UV stars are too faint to be extended horizontal branch stars.
We suggest that they could be low-mass helium white dwarfs, formed when the
evolution of a red giant is interrupted, due either to Roche-lobe overflow onto
a binary companion, or to envelope ejection following a common-envelope phase
in a tidal-capture binary. Alternatively, they could be very-low-mass
core-He-burning stars. Both the CVs and the new class of faint UV stars are
strongly concentrated toward the cluster center, to the extent that mass
segregation from 2-body relaxation alone may be unable to explain their
distribution.Comment: 11 pages plus 3 eps figures; LaTeX using aaspp4.sty; to appear in The
Astrophysical Journal Letter
Millisecond Pulsars as Probes of Mass Segregation in the Galactic Center
We propose a simple test for the existence of a cluster of black hole
remnants around Sgr A* that is based on a small sample of any type of Galactic
Center objects, provided they are substantially less massive than the black
holes and constitute part of an old (> 1 Gyr) population. The test relies on
the fact that, under the presence of such a cluster of heavy remnants and
because of energy equipartition, lower mass objects would be expelled from the
central regions and settle into a distribution very different than the cusp
expected to be induced by the supermassive black hole alone. We show that with
a sample of just 50 objects and using only their angular positions on the sky
relative to Sgr A* it is possible to clearly differentiate between a
distribution consistent with the presence of the cluster of black holes and a
power-law cusp distribution. We argue that millisecond pulsars might currently
be the best candidate to perform this test, because of the large uncertainties
involved in the age determination of less exotic objects. In addition, by
measuring their first and second period derivatives, millisecond pulsars offer
the rare opportunity of determining the complete phase space information of the
objects. We show that this extra information improves the detection of mass
segregation by about 30%.Comment: 16 pages, 1 Postscript figure; version accepted for publication in
Ap
Time Series Photometry of Variable Stars in the Globular Cluster NGC 6397
Time series BVI photometry is presented for 16 short-period variables located
in the central region of the globular cluster NGC 6397. The sample includes 9
newly detected variables. The light curve of cataclysmic variable CV6 shows
variability with a period of 0.2356 days. We confirm an earlier reported period
of 0.472 days for cataclysmic variable CV1. Phased light curves of both CVs
exhibit sine-like light curves, with two minima occurring during each orbital
cycle. The secondary component of CV1 has a low average density of 0.83
g/cm^{3} indicating that it cannot be a normal main sequence star. Variables
among the cluster blue stragglers include a likely detached eclipsing binary
with orbital period of 0.787 days, three new SX Phe stars (one of which has the
extremely short period of 0.0215 days), and three low amplitude variables which
are possible gamma Doradus variables.Comment: 28 pages, 13 figure
Defect Chaos of Oscillating Hexagons in Rotating Convection
Using coupled Ginzburg-Landau equations, the dynamics of hexagonal patterns
with broken chiral symmetry are investigated, as they appear in rotating
non-Boussinesq or surface-tension-driven convection. We find that close to the
secondary Hopf bifurcation to oscillating hexagons the dynamics are well
described by a single complex Ginzburg-Landau equation (CGLE) coupled to the
phases of the hexagonal pattern. At the bandcenter these equations reduce to
the usual CGLE and the system exhibits defect chaos. Away from the bandcenter a
transition to a frozen vortex state is found.Comment: 4 pages, 6 figures. Fig. 3a with lower resolution no
Evolution of Stellar Collision Products in Globular Clusters - II. Off-axis Collision
We continue our exploration of collisionally merged stars in the blue
straggler region of the color-magnitude diagram. We report the results of new
SPH calculations of parabolic collisions between two main-sequence stars, with
the initial structure and composition profiles of the parent stars having been
determined from stellar evolution calculations. Parallelization of the SPH code
has permitted much higher numerical resolution of the hydrodynamics. We also
present evolutionary tracks for the resulting collision products, which emerge
as rapidly rotating blue stragglers. The rotating collision products are
brighter, bluer and remain on the main sequence longer than their non-rotating
counterparts. In addition, they retain their rapid rotation rates throughout
their main sequence lifetime. Rotationally-induced mixing strongly affects the
evolution of the collision products, although it is not sufficient to mix the
entire star. We discuss the implications of these results for studies of blue
straggler populations in clusters. This work shows that off-axis collision
products cannot become blue stragglers unless they lose a large fraction of
their initial angular momentum. The mechanism for this loss is not apparent,
although some possibilities are discussed.Comment: 25 pages incl. 9 figures (one in colour). Submitted to Ap
Self-organized Vortex State in Two-dimensional Dictyostelium Dynamics
We present results of experiments on the dynamics of Dictyostelium discoideum
in a novel set-up which constraints cell motion to a plane. After aggregation,
the amoebae collect into round ''pancake" structures in which the cells rotate
around the center of the pancake. This vortex state persists for many hours and
we have explicitly verified that the motion is not due to rotating waves of
cAMP. To provide an alternative mechanism for the self-organization of the
Dictyostelium cells, we have developed a new model of the dynamics of
self-propelled deformable objects. In this model, we show that cohesive energy
between the cells, together with a coupling between the self-generated
propulsive force and the cell's configuration produces a self-organized vortex
state. The angular velocity profiles of the experiment and of the model are
qualitatively similar. The mechanism for self-organization reported here can
possibly explain similar vortex states in other biological systems.Comment: submitted to PRL; revised version dated 3/8/9
Reconstruction of cellular variability from spatiotemporal patterns of Dictyostelium discoideum
Variability in cell properties can be an important driving mechanism behind spatiotemporal patterns in biological systems, as the degree of cell-to-cell differences determines the capacity of cells to locally synchronize and, consequently, form patterns on a larger spatial scale. In principle, certain features of spatial patterns emerging with time may be regulated by variability or, more specifically, by certain constellations of cell-to-cell differences. Similarly, measuring variability in a system (i.e. the spatial distribution of cell-cell differences) may help predict properties of later-stage patterns