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Title:
Genetic Algorithms, Pulsar Planets, and Ionized Interstellar Microturbulence
Authors:
Lazio, T. J.
Publication:
Publications of the Astronomical Society of the Pacific, v.109, p.1068 (PASP Homepage)
Publication Date:
09/1997
Origin:
PASP
DOI:
10.1086/133975
Bibliographic Code:
1997PASP..109.1068L

Abstract

We probe the intense microturbulence in the Galactic center and the radio-wave scattering it generates by analyzing observations of extragalactic sources, OH and H$_2$O masers, and free-free emission. The region responsible for the enhanced, anisotropic angular broadening of Sgr~A$^*$ and nearby OH masers is within 150~pc of the Galactic center and has an angular radius $\approx 1\arcdeg$. The enhanced scattering probably occurs in the interface regions between $10^7$~K gas and molecular clouds and is a manifestation of the energetic processes occurring in the Galactic center. Radio scattering measurements are also used to probe turbulent gas toward the Galactic anticenter. Ionized gas at Galactocentric distances $\sim 50$~kpc is suggested by absorption lines in quasar spectra, the appearance of the H 1 disks of nearby galaxies, and models for low-redshift quasar absorption systems and Galactic ``fountains.'' We conducted multifrequency, Very Long Baseline Array (VLBA) observations on twelve extragalactic sources in order to measure their scattering diameters. Seven sources are at $|b| < 1\arcdeg$ and their lines of sight potentially probe path lengths $\gtrsim 50$~kpc through the disk. We find that the ionized disk is unwarped, has an extent of $\approx 20$~kpc, and traces the extent of massive star formation in the outer Galaxy. Planetary companions to neutron stars are challenging to recognize amid the several processes that contribute to pulsar arrival time data. We use a genetic algorithm to search for planetary companions to pulsars. Genetic algorithms are an optimization method that uses biological-like concepts such as survival of the fittest, mutation, and chromosome exchange. The algorithm searches parameter space in the same way that life finds optimal niches in the biological environment---incremental rewarding of successful variations. Fitting for Keplerian orbits requires a search through four non-linear parameters per planet and is especially difficult if there is a large range of planetary masses and orbital periods. We find that the GA is more efficient and more accurate than the downhill simplex and simulated annealing algorithms. We confirm the presence of a second planetary companion to PSR~B0329$+$54 and identify possible companions to B1911$-$04 and B1929$+$10. (SECTION: Dissertation Summaries)

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