Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989phdt........10f&link_type=abstract
Ph.D. Thesis Toronto Univ. (Ontario). Dept. of Astronomy.
Astronomy and Astrophysics
Astronomy
6
H Lines, Hydrogen Clouds, Interstellar Gas, Pulsars, Absorption Spectra, Data Acquisition, Distance, Electron Density (Concentration), Free Electrons, Hydrogen Atoms, Telescopes
Scientific paper
Neutral hydrogen absorption measurements at 21 cm have been made toward 20 pulsars with the Arecibo 305 m telescope in Arecibo, Puerto Rico and the 27 element interferometer (VLA) in Socorro, New Mexico. Two novel data acquisition methods were developed to collect this data. The Arecibo system is a general purpose pulsar observing system with asynchronous sampling for full resolution over a pulsar period. The single gating window design developed at the VLA is the first of its kind to make pulsar observations in spectral line mode using a connected element interferometer. Distances are derived using the 21 cm absorption and emission spectra toward each pulsar in conjunction with a Galactic rotation curve. The pulsars were chosen to be especially well suited to studying the electron density at low galactocentric radii. Models for the distribution of the free electron density are poorly determined in this region. The data show a clear rise in the electron density in the inner Galaxy, to about three times the local average. The rise is consistent with O stars being the principal ionizing agent of the diffuse interstellar medium. Monte-Carlo simulations were performed on the complete dataset in order to determine model parameters for the electron distribution. Smooth exponential disk models were found to be only partially successful in fitting the data. An observational selection effect and a large scatter inherent in the data limit the usefulness of this approach. Nearby HII regions are responsible for some of the observed scatter in the electron density measurements. However, a non-uniform, clumpy ionized medium is required to explain the remaining observations.
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