595 research outputs found
Pseudo Goldstone Bosons Phenomenology in Minimal Walking Technicolor
We construct the non-linear realized Lagrangian for the Goldstone Bosons
associated to the breaking pattern of SU(4) to SO(4). This pattern is expected
to occur in any Technicolor extension of the standard model featuring two Dirac
fermions transforming according to real representations of the underlying gauge
group. We concentrate on the Minimal Walking Technicolor quantum number
assignments with respect to the standard model symmetries. We demonstrate that
for, any choice of the quantum numbers, consistent with gauge and Witten
anomalies the spectrum of the pseudo Goldstone Bosons contains electrically
doubly charged states which can be discovered at the Large Hadron Collider.Comment: 25 pages, 5 figure
Predictions for Triple Stars with and without a Pulsar in Star Clusters
Though about 80 pulsar binaries have been detected in globular clusters so
far, no pulsar has been found in a triple system in which all three objects are
of comparable mass. Here we present predictions for the abundance of such
triple systems, and for the most likely characteristics of these systems. Our
predictions are based on an extensive set of more than 500 direct simulations
of star clusters with primordial binaries, and a number of additional runs
containing primordial triples. Our simulations employ a number N_{tot} of equal
mass stars from N_{tot}=512 to N_{tot}=19661 and a primordial binary fraction
from 0-50%. In addition, we validate our results against simulations with
N=19661 that include a mass spectrum with a turn-off mass at 0.8 M_{sun},
appropriate to describe the old stellar populations of galactic globular
clusters. Based on our simulations, we expect that typical triple abundances in
the core of a dense cluster are two orders of magnitude lower than the binary
abundances, which in itself already suggests that we don't have to wait too
long for the first comparable-mass triple with a pulsar to be detected.Comment: 11 pages, minor changes to match MNRAS accepted versio
Basic N-Body Modelling of the Evolution of Globular Clusters. I. Time Scaling
We consider the use of N-body simulations for studying the evolution of rich
star clusters (i.e. globular clusters). The dynamical processes included in
this study are restricted to gravitational (point-mass) interactions, the
steady tidal field of a galaxy, and instantaneous mass loss resulting from
stellar evolution. With evolution driven by these mechanisms, it is known that
clusters fall roughly into two broad classes: those that dissipate promptly in
the tidal field, as a result of mass loss, and those that survive long enough
for their evolution to become dominated by two-body relaxation.
The time scales of the processes we consider scale in different ways with the
number of stars in the simulation, and the main aim of the paper is to suggest
how the scaling of a simulation should be done so that the results are
representative of the evolution of a `real' cluster. We investigate three
different ways of scaling time. One of these is appropriate to the first type
of cluster, i.e. those that dissipate rapidly, and similarly a second scaling
is appropriate only to the second (relaxation-dominated) type. We also develop
a hybrid scaling which is a satisfactory compromise for both types of cluster.
Finally we present evidence that the widely used Fokker-Planck method produces
models which are in good agreement with N-body models of those clusters which
are relaxation dominated, at least for N-body models with several thousand
particles, but that the Fokker-Planck models evolve too fast for clusters which
dissipate promptly.Comment: 24 pages, 3 figures, MNRAS, in pres
The Promiscuous Nature of Stars in Clusters
The recent availability of special purpose computers designed for calculating
gravitational interactions of N-bodies at extremely high speed has provided the
means to model globular clusters on a star-by-star basis for the first time. By
endeavouring to make the N-body codes that operate on these machines as
realistic as possible, the addition of stellar evolution being one example,
much is being learnt about the interaction between the star cluster itself and
the stars it contains. A fascinating aspect of this research is the ability to
follow the orbits of individual stars in detail and to document the formation
of observed exotic systems. This has revealed that many stars within a star
cluster lead wildly promiscuous lives, interacting, often intimately and in
rapid succession, with a variety of neighbours.Comment: 15 pages, 1 figure, to appear in the Astrophysical Journa
The M/L ratio of massive young clusters
We point out a strong time-evolution of the mass-to-light conversion factor
\eta commonly used to estimate masses of dense star clusters from observed
cluster radii and stellar velocity dispersions. We use a gas-dynamical model
coupled with the Cambridge stellar evolution tracks to compute line-of-sight
velocity dispersions and half-light radii weighted by the luminosity. Stars at
birth are assumed to follow the Salpeter mass function in the range [0.15--17
M_\sun]. We find that , and hence the estimated cluster mass, increases
by factors as large as 3 over time-scales of 20 million years. Increasing the
upper mass limit to 50 M_\sun leads to a sharp rise of similar amplitude but
in as little as 10 million years.
Fitting truncated isothermal (Michie-King) models to the projected light
profile leads to over-estimates of the concentration par ameter c of compared to the same functional fit applied to the proj ected
mass density.Comment: Draft version of an ApJ lette
Distant Companions and Planets around Millisecond Pulsars
We present a general method for determining the masses and orbital parameters
of binary millisecond pulsars with long orbital periods (P_orb >> 1 yr), using
timing data in the form of pulse frequency derivatives. We apply our method to
analyze the properties of the second companion in the PSR B1620-26 triple
system. We use the latest timing data for this system to constrain the mass and
orbital parameters of the second companion. We find that all possible solutions
have a mass m_2 in the range 2.4 10^-4 M_sun <= m_2 sin i_2 <= 1.2 10^-2 M_sun,
i.e., almost certainly excluding a second companion of stellar mass and
suggesting instead that the system contains a planet or a brown dwarf. Using
Monte-Carlo realizations of the triple configuration in three dimensions we
find the most probable value of m_2 to be 0.010(5) M_sun, corresponding to a
distance of 38(6) AU from the center of mass of the inner binary (the errors
indicate 80% confidence intervals). We also apply our method to analyze the
planetary system around PSR B1257+12, where a distant, giant planet may be
present in addition to the three well-established Earth-mass planets. We find
that the simplest interpretation of the frequency derivatives implies the
presence of a fourth planet with a mass of ~100 M_earth in a circular orbit of
radius ~40 AU.Comment: 30 pages, Latex, 10 Postscript figures, uses aaspp4.sty. ApJ
submitted. Also available at http://ensor.mit.edu/~rasi
Models of core reconstruction for the 90-degree partial dislocation in semiconductors
We compare the models that have been proposed in the literature for the
atomic structure of the 90-degree partial dislocation in the homopolar
semiconductors, silicon, diamond, and germanium. In particular, we examine the
traditional single-period and our recently proposed double-period core
structures. Ab-initio and tight-binding results on the core energies are
discussed, and the geometries are compared in light of the available
experimental information about dislocations in these systems. The double-period
geometry is found to be the ground-state structure in all three materials. We
address boundary-conditions issues that have been recently raised about these
results. The structures of point excitations (kinks, solitons, and kink-soliton
complexes) in the two geometries are also reviewed.Comment: 9 pages, with 3 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/rn_eds/index.htm
Monte Carlo Simulations of Globular Cluster Evolution. III. Primordial Binary Interactions
We study the dynamical evolution of globular clusters using our 2D Monte
Carlo code with the inclusion of primordial binary interactions for equal-mass
stars. We use approximate analytical cross sections for energy generation from
binary-binary and binary-single interactions. After a brief period of slight
contraction or expansion of the core over the first few relaxation times, all
clusters enter a much longer phase of stable "binary burning" lasting many tens
of relaxation times. The structural parameters of our models during this phase
match well those of most observed globular clusters. At the end of this phase,
clusters that have survived tidal disruption undergo deep core collapse,
followed by gravothermal oscillations. Our results clearly show that the
presence of even a small fraction of binaries in a cluster is sufficient to
support the core against collapse significantly beyond the normal core collapse
time predicted without the presence of binaries. For tidally truncated systems,
collapse is easily delayed sufficiently that the cluster will undergo complete
tidal disruption before core collapse. As a first step toward the eventual goal
of computing all interactions exactly using dynamical three- and four-body
integration, we have incorporated an exact treatment of binary-single
interactions in our code. We show that results using analytical cross sections
are in good agreement with those using exact three-body integration, even for
small binary fractions where binary-single interactions are energetically most
important.Comment: Accepted for publication in ApJ. Minor changes to reflect accepted
version. 28 pages, 17 figures; some figures low resolution. Full resolution
paper available at http://www.mit.edu/~fregeau/paper3.pd
High Orbital Eccentricities of Extrasolar Planets Induced by the Kozai Mechanism
One of the most remarkable properties of extrasolar planets is their high
orbital eccentricities. Observations have shown that at least 20% of these
planets, including some with particularly high eccentricities, are orbiting a
component of a wide binary star system. The presence of a distant binary
companion can cause significant secular perturbations to the orbit of a planet.
In particular, at high relative inclinations, a planet can undergo a
large-amplitude eccentricity oscillation. This so-called "Kozai mechanism" is
effective at a very long range, and its amplitude is purely dependent on the
relative orbital inclination. In this paper, we address the following simple
question: assuming that every host star with a detected giant planet also has a
(possibly unseen, e.g., substellar) distant companion, with reasonable
distributions of orbital parameters and masses, how well could secular
perturbations reproduce the observed eccentricity distribution of planets? Our
calculations show that the Kozai mechanism consistently produces an excess of
planets with very high (e >0.6) and very low (e < 0.1) eccentricities. The
paucity of near-circular orbits in the observed sample cannot be explained
solely by the Kozai mechanism, because, even with high enough inclinations, the
Kozai mechanism often fails to produce significant eccentricity perturbations
when there are other competing sources of orbital perturbations on secular
timescales, such as general relativity. On the other hand, the Kozai mechanism
can produce many highly eccentric orbits. Indeed the overproduction of high
eccentricities observed in our models could be combined with plausible
circularizing mechanisms (e.g., friction from residual gas) to create more
intermediate eccentricities (e=0.1-0.6).Comment: 24 pages, 6 figures, ApJ, in press, minor changes to reflect the
accepted versio
- …