740 research outputs found

### Exploring the Use of Virtual Worlds as a Scientific Research Platform: The Meta-Institute for Computational Astrophysics (MICA)

We describe the Meta-Institute for Computational Astrophysics (MICA), the
first professional scientific organization based exclusively in virtual worlds
(VWs). The goals of MICA are to explore the utility of the emerging VR and VWs
technologies for scientific and scholarly work in general, and to facilitate
and accelerate their adoption by the scientific research community. MICA itself
is an experiment in academic and scientific practices enabled by the immersive
VR technologies. We describe the current and planned activities and research
directions of MICA, and offer some thoughts as to what the future developments
in this arena may be.Comment: 15 pages, to appear in the refereed proceedings of "Facets of Virtual
Environments" (FaVE 2009), eds. F. Lehmann-Grube, J. Sablating, et al., ICST
Lecture Notes Ser., Berlin: Springer Verlag (2009); version with full
resolution color figures is available at
http://www.mica-vw.org/wiki/index.php/Publication

### Binary-Binary Interactions and the Formation of the PSR B1620-26 Triple System in M4

The hierarchical triple system containing the millisecond pulsar PSR B1620-26
in M4 is the first triple star system ever detected in a globular cluster. Such
systems should form in globular clusters as a result of dynamical interactions
between binaries. We propose that the triple system containing PSR B1620-26
formed through an exchange interaction between a wide primordial binary and a
{\it pre-existing\/} binary millisecond pulsar. This scenario would have the
advantage of reconciling the $\sim10^9\,$yr timing age of the pulsar with the
much shorter lifetime of the triple system in the core of M4.Comment: 13 pages, uuencoded compressed postscript with figures, IASSNS-AST
94/4

### Star Clusters with Primordial Binaries: I. Dynamical Evolution of Isolated Models

In order to interpret the results of complex realistic star cluster
simulations, which rely on many simplifying approximations and assumptions, it
is essential to study the behavior of even more idealized models, which can
highlight the essential physical effects and are amenable to more exact
methods. With this aim, we present the results of N-body calculations of the
evolution of equal-mass models, starting with primordial binary fractions of 0
- 100 %, with values of N ranging from 256 to 16384. This allows us to
extrapolate the main features of the evolution to systems comparable in
particle number with globular clusters. In this range, we find that the
steady-state `deuterium main sequence' is characterized by a ratio of the core
radius to half-mass radius that follows qualitatively the analytical estimate
by Vesperini & Chernoff (1994), although the N dependence is steeper than
expected. Interestingly, for an initial binary fraction f greater than 10%, the
binary heating in the core during the post collapse phase almost saturates
(becoming nearly independent of f), and so little variation in the structural
properties is observed. Thus, although we observe a significantly lower binary
abundance in the core with respect to the Fokker-Planck simulations by Gao et
al. (1991), this is of little dynamical consequence. At variance with the study
of Gao et al. (1991), we see no sign of gravothermal oscillations before 150
halfmass relaxation times. At later times, however, oscillations become
prominent. We demonstrate the gravothermal nature of these oscillations.Comment: 14 pages, 22 figures, MNRAS accepte

### Star Clusters with Primordial Binaries: II. Dynamical Evolution of Models in a Tidal Field

[abridged] We extend our analysis of the dynamical evolution of simple star
cluster models, in order to provide comparison standards that will aid in
interpreting the results of more complex realistic simulations. We augment our
previous primordial-binary simulations by introducing a tidal field, and
starting with King models of different central concentrations. We present the
results of N-body calculations of the evolution of equal-mass models, starting
with primordial binary fractions of 0 - 100 %, and N values from 512 to 16384.
We also attempt to extrapolate some of our results to the larger number of
particles that are necessary to model globular clusters. We characterize the
steady-state `deuterium main sequence' phase in which primordial binaries are
depleted in the core in the process of `gravitationally burning'. In this phase
we find that the ratio of the core to half-mass radius, r_c/r_h, is similar to
that measured for isolated systems. In addition to the generation of energy due
to hardening and depletion of the primordial binary population, the overall
evolution of the star clusters is driven by a competing process: the tidal
disruption of the system. We find that the depletion of primordial binaries
before tidal dissolution of the system is possible only if the initial number
is below 0.05 N, in the case of a King model with W_0=7 and N=4096 (which is
one of our longest living models). We compare our findings, obtained by means
of direct N-body simulations but scaled, where possible, to larger N, with
similar studies carried out by means of Monte Carlo methods.Comment: 15 pages, 18 figures, matches MNRAS accepted version, some sections
reorganized but no major change

### Time-Symmetrized Kustaanheimo-Stiefel Regularization

In this paper we describe a new algorithm for the long-term numerical
integration of the two-body problem, in which two particles interact under a
Newtonian gravitational potential. Although analytical solutions exist in the
unperturbed and weakly perturbed cases, numerical integration is necessary in
situations where the perturbation is relatively strong. Kustaanheimo--Stiefel
(KS) regularization is widely used to remove the singularity in the equations
of motion, making it possible to integrate orbits having very high
eccentricity. However, even with KS regularization, long-term integration is
difficult, simply because the required accuracy is usually very high. We
present a new time-integration algorithm which has no secular error in either
the binding energy or the eccentricity, while allowing variable stepsize. The
basic approach is to take a time-symmetric algorithm, then apply an implicit
criterion for the stepsize to ensure strict time reversibility. We describe the
algorithm in detail and present the results of numerical tests involving
long-term integration of binaries and hierarchical triples. In all cases
studied, we found no systematic error in either the energy or the angular
momentum. We also found that its calculation cost does not become higher than
those of existing algorithms. By contrast, the stabilization technique, which
has been widely used in the field of collisional stellar dynamics, conserves
energy very well but does not conserve angular momentum.Comment: figures are available at http://grape.c.u-tokyo.ac.jp/~funato/; To
appear in Astronomical Journal (July, 1996

### Star Clusters with Primordial Binaries: III. Dynamical Interaction between Binaries and an Intermediate Mass Black Hole

We present the first study of the dynamical evolution of an isolated star
cluster that combines a significant population of primordial binaries with the
presence of a central black hole. We use equal-mass direct N-body simulations,
with N ranging from 4096 to 16384 and a primordial binary ratio of 0-10%; the
black hole mass is about one percent of the total mass of the cluster. The
evolution of the binary population is strongly influenced by the presence of
the black hole, which gives the cluster a large core with a central density
cusp. Starting from a variety of initial conditions (Plummer and King models),
we first encounter a phase, that last approximately 10 half-mass relaxation
times, in which binaries are disrupted faster compared to analogous simulations
without a black hole. Subsequently, however, binary disruption slows down
significantly, due to the large core size. The dynamical interplay between the
primordial binaries and the black hole thus introduces new features with
respect to the scenarios investigated so far, where the influence of the black
hole and of the binaries have been considered separately. A large core to half
mass radius ratio appears to be a promising indirect evidence for the presence
of a intermediate-mass black hole in old globular clusters.Comment: 11 pages, 11 figures, accepted for publication in MNRA

### X-ray Emission from the Weak-lined T Tauri Binary System KH 15D

The unique eclipsing, weak-lined T Tauri star KH 15D has been detected as an
X-ray source in a 95.7 ks exposure from the Chandra X-ray Observatory archives.
A maximum X-ray luminosity of 1.5 x 10^{29} erg s$^{-1}$ is derived in the
0.5--8 keV band, corresponding to L_{X}/L_bol = 7.5 x 10^{-5}. Comparison with
samples of stars of similar effective temperature in NGC 2264 and in the Orion
Nebula Cluster shows that this is about an order of magnitude low for a typical
star of its mass and age. We argue that the relatively low luminosity cannot be
attributed to absorption along the line of sight but implies a real deficiency
in X-ray production. Possible causes for this are considered in the context of
a recently proposed eccentric binary model for KH 15D. In particular, we note
that the visible component rotates rather slowly for a weak-lined T Tauri star
and has possibly been pseudosynchronized by tidal interaction with the primary
near periastron

### Differential rotation enhanced dissipation of tides in the PSR J0045-7319 Binary

Recent observations of PSR J0045-7319, a radio pulsar in a close eccentric
orbit with a massive B-star companion, indicate that the system's orbital
period is decreasing on a timescale of $\approx 5 \times10^{5}$ years, which is
much shorter than the timescale of $\approx$ 10^9 years given by the standard
theory of tidal dissipation in radiative stars. Observations also provide
strong evidence that the B-star is rotating rapidly, perhaps at nearly its
break up speed. We show that the dissipation of the dynamical tide in a star
rotating in the same direction as the orbital motion of its companion (prograde
rotation) with a speed greater than the orbital angular speed of the star at
periastron results in an increase in the orbital period of the binary system
with time. Thus, since the observed time derivative of the orbital period is
large and negative, the B-star in the PSR J0045-7319 binary must have
retrograde rotation if tidal effects are to account for the orbital decay. We
also show that the time scale for the synchronization of the B-star's spin with
the orbital angular speed of the star at periastron is comparable to the
orbital evolution time. From the work of Goldreich and Nicholson (1989) we
therefore expect that the B-star should be rotating differentially, with the
outer layers rotating more slowly than the interior. We show that the
dissipation of the dynamical tide in such a differentially rotating B-star is
enhanced by almost three orders of magnitude leading to an orbital evolution
time for the PSR J0045-7319 Binary that is consistent with the observations.Comment: 8 pages, tex. Submitted to Ap

### Collisional Hardening of Compact Binaries in Globular Clusters

We consider essential mechanisms for orbit-shrinkage or "hardening" of
compact binaries in globular clusters to the point of Roche-lobe contact and
X-ray emission phase, focussing on the process of collisional hardening due to
encounters between binaries and single stars in the cluster core. The interplay
between this kind of hardening and that due to emission of gravitational
radiation produces a characteristic scaling of the orbit-shrinkage time with
the single-star binary encounter rate $\gamma$ in the cluster which we
introduce, clarify, and explore. We investigate possible effects of this
scaling on populations of X-ray binaries in globular clusters within the
framework of a simple "toy" scheme for describing the evolution of pre-X-ray
binaries in globular clusters. We find the expected qualitative trends
sufficiently supported by data on X-ray binaries in galactic globular clusters
to encourage us toward a more quantitative study.Comment: 8 pages, 4 figures. Accepted for publication in MNRA

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