On the Self-Consistent Response of Stellar Systems to Gravitational Shocks

We study the reaction of a globular star cluster to a time-varying tidal perturbation (gravitational shock) using self-consistent N-body simulations and address two questions. First, to what extent is the cluster interior protected by adiabatic invariants. Second, how much further energy change does the postshock evolution of the cluster potential produce and how much does it affect the dispersion of stellar energies. We introduce the adiabatic correction} as ratio of the energy change, , to its value in the impulse approximation. When the potential is kept fixed, the numerical results for the adiabatic correction for stars with orbital frequency \omega can be approximated as (1 + \omega^2 \tau^2)^{-\gamma}. For shocks with the characteristic duration of the order the half-mass dynamical time of the cluster, \tau < t_{dyn,h}, the exponent \gamma = 5/2. For more prolonged shocks, \tau > 4 t_{dyn,h}, the adiabatic correction is shallower, \gamma = 3/2. When we allow for self-gravity and potential oscillations which follow the shock, the energy of stars in the core changes significantly, while the total energy of the system is conserved. Paradoxically, the postshock potential fluctuations reduce the total amount of energy dispersion, . The effect is small but real and is due to the postshock energy change being statistically anti-correlated with the shock induced heating. These results are to be applied to Fokker-Planck models of the evolution of globular clusters.Comment: 20 pages; ApJ 513 (in press

Dimensional Dependence of the Hydrodynamics of Core-Collapse Supernovae

The multidimensional character of the hydrodynamics in core-collapse supernova (CCSN) cores is a key facilitator of explosions. Unfortunately, much of this work has necessarily been performed assuming axisymmetry and it remains unclear whether or not this compromises those results. In this work, we present analyses of simplified two- and three-dimensional CCSN models with the goal of comparing the multidimensional hydrodynamics in setups that differ only in dimension. Not surprisingly, we find many differences between 2D and 3D models. While some differences are subtle and perhaps not crucial to understanding the explosion mechanism, others are quite dramatic and make interpreting 2D CCSN models problematic. In particular, we find that imposing axisymmetry artificially produces excess power at the largest spatial scales, power that has been deemed critical in the success of previous explosion models and has been attributed solely to the standing accretion shock instability. Nevertheless, our 3D models, which have an order of magnitude less power on large scales compared to 2D models, explode earlier. Since we see explosions earlier in 3D than in 2D, the vigorous sloshing associated with the large scale power in 2D models is either not critical in any dimension or the explosion mechanism operates differently in 2D and 3D. Possibly related to the earlier explosions in 3D, we find that about 25% of the accreted material spends more time in the gain region in 3D than in 2D, being exposed to more integrated heating and reaching higher peak entropies, an effect we associate with the differing characters of turbulence in 2D and 3D. Finally, we discuss a simple model for the runaway growth of buoyant bubbles that is able to quantitatively account for the growth of the shock radius and predicts a critical luminosity relation.Comment: Submitted to the Astrophysical Journa

Cooling flows and quasars: different aspects of the same phenomenon? I. Concepts

We present a new class of solutions for the gas flows in elliptical galaxies containing massive central black holes (BH). Modified King model galaxies are assumed. Two source terms operate: mass loss from evolving stars, and a secularly declining heating by SNIa. Relevant atomic physical processes are modeled in detail. Like the previous models investigated by Ciotti et al. (1991), these new models first evolve through three consecutive evolutionary stages: wind, outflow, and inflow. At this point the presence of the BH alters dramatically the subsequent evolution, because the energy emitted by the BH can heat the surrounding gas to above virial temperatures, causing the formation of a hot expanding central bubble. Short and strong nuclear bursts of radiation are followed by longer periods during which the X-ray galaxy emission comes from the coronal gas (Lx). The range and approximate distribution spanned by Lx are found to be in accordance with observations of X-ray early type galaxies. Moreover, although high accretion rates occur during bursting phases when the central BH has a luminosity characteristic of QSOs, the total mass accreted is very small when compared to that predicted by stationary cooling-flow solutions and computed masses are in accord with putative BH nuclear masses. In the bursting phases Lx is low and the surface brightness profile is very low compared to pre-burst or to cooling flow models. We propose that these new models, while solving some long-standing problems of the cooling flow scenario, can provide a unified description of QSO-like objects and X-ray emitting elliptical galaxies, these being the same objects observed at two different evolutionary phases.Comment: 10 pages, ApJ LaTeX, plus 5 .eps figures and TeX-macro aasms4.sty - revised version - in press on ApJ Letter

Modeling and Simulation of Damage In The Brazilian Indirect Tension Test Using The Finite-Discrete Element Method

The Brazilian indirect tension test is used to investigate possible correlations between progressive damage and associated permeability changes. The test offers ease of replicability with a damage behavior known to lead to fracture openings as a tool for interpreting the indirect tensile strength of concrete and rocks. This behavior can lend insight into the nature of tensile damage and fracture progression in association with changes in permeability. The nature of these brittle materials is known to exhibit rapid failures in the Brazilian indirect tension test and require a method to retard the progression of damage for the possibility of acquiring permeability measurements. Experimental results are replicated and investigated using the finite-discrete element method, which allows for the replication of both the elastic and post-peak fractured behaviors seen within the test. The model results are used to interpret the progression of damage and its type. Investigations of a sample pre-damaged with a drop tower is also made to observe a differing damage mechanism. The model validates the methodology behind what is called the stiff Brazilian indirect tension test and results from the models indicate that damage progression initiates near the contact points as shear separations, quickly followed by tensile separations along the line of fracture. The FDEM code used is observed to be a useful method for continued investigations into future modifications to the Brazilian indirect tension test for a broader damage and permeability correlation objective

Dynamic Micromechanical Fabry-Perot Cavity Sensors Fabricated by Multiphoton Absorption Onto Optical Fiber Tips

This research leveraged two-photon polymerization microfabrication to integrate dynamic mechanical components with Fabry-Perot resonators onto the ends of low-loss optical fibers to prototype 3 micro-optic devices. The first device featured a multi-positional mirror that enabled thin-film deposition onto cavities of any length with mirrors of significant curvature, for refractive index sensing. The second device combined an FP cavity with a spring body featuring easily scalable stiffness for pressure sensing. The third device presented a high-speed rotating micro-anemometer for measuring a wide range of gas flows. All devices represent a significant reduction in size and weight over commercially available devices