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: 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

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: 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

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

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