No abstract

The next generation of radio telescope interferometric arrays requires careful design of the array configuration to optimize the performance of the overall system. We have developed a framework, based on a genetic algorithm, for rapid exploration and optimization of the objective space pertaining to multiple objectives. We have evaluated a large space of possible designs for 27-, 60-, 100-, and 160-station arrays. The 27-station optimizations can be compared to the well-known VLA case, and the larger array designs apply to arrays currently under design such as LOFAR, ATA, and the SKA. In the initial implementation of our framework we evaluate designs with respect to two metrics, array imaging performance and the length of cable necessary to connect the stations. Imaging performance is measured by the degree to which the sampling of the uv plane is uniform. For the larger arrays we find that well-known geometric designs perform well and occupy the Pareto front of optimum solutions. For the 27-element case we find designs, combining features of the well-known designs, that are more 1 Research Assistant, Aeronautics/Astronautics 2 Professor of Physics 3 Assistant Professor of Aeronautics/Astronautics and Engineering Systems -2optimal as measured by these two metrics. The results obtained by the multiobjective genetic optimization are corroborated by simulated annealing, which also reveals the role of entropy in array optimization. Our framework is general, and may be applied to other design goals and issues, such as particular schemes for sampling the uv plane, array robustness, and phased deployment of arrays.

We present numerical hydrodynamical models of the effects of planets or brown dwarfs orbiting within the extended atmosphere and wind formation zone of Mira variables. We find time-dependent wake dynamics and episodic accretion phenomena which may give rise to observable optical events and affect SiO maser emission.Comment: 4 pages, 3 figures, follow-up to JAAVSO pape

We present numerical hydrodynamical modelling of the effects of a giant planet or brown‐dwarf companion orbiting within the extended atmosphere and wind formation zone of an approximately solar‐mass Mira variable star. The large‐scale, time‐dependent accretion flows within the radially oscillating and outflowing circumstellar gas around Miras are related to Bondi–Hoyle–Lyttleton flows, but have not, to our knowledge, been previously modelled. The new models presented in this paper illustrate the changes in accretion and wake dynamics as the companion mass is varied over a range from 10 to 50 Jupiter masses (MJ), and generalize the results of the single model we presented in an earlier paper. The character of the accretion on to the companion changes greatly as the companion mass is increased. At the lowest companion masses considered here, a low continuous rate of mass accretion is punctuated by large, nearly periodic bursts of accretion. When the companion mass is large, the mass accretion has both a continuous part and a rapidly varying, nearly stochastic part. Surprisingly, the angular momentum of the accreted gas shows an opposite trend with mass, varying nearly periodically at large companion masses and stochastically at low masses. These trends can be understood as the result of the interplay between the shocks and radial oscillations in the circumstellar gas and the wake flow behind the companion. Boundary conditions also affect the character of the accretion. The equation of state, however, is found to have little effect, at least for gamma‐law gases, with gamma in the range from 1 to 5/3. Models with accretion bursts may produce observable optical brightenings, and may affect SiO maser emission, as we suggested in previous papers. Interruptions of continuous accretion, or shadowing effects, could give rise to bursts of dimming in the optical. Such dimming effects are likely to be correlated with bursts and optical flashes, helping to explain some rather mysterious Hipparcos observations.