A complete sample of white dwarfs, hot subdwarfs, and quasars

Citation

Green, Richard Frederick (1977) A complete sample of white dwarfs, hot subdwarfs, and quasars. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8Q1N-ZW17. https://resolver.caltech.edu/CaltechETD:etd-09252008-085810

Abstract

An analysis is made of a complete sample of hot white dwarfs, identified spectroscopically from candidates selected for ultraviolet excess without regard to proper motion. The luminosity function and local space density of hot white dwarfs are derived, giving 1.43 ± .28 per 1000 cubic parsecs for M[subscript v] < 12.75. A model of the local rate of star formation is constructed, which, when combined with white dwarf cooling theory, satisfactorily reproduces the observed luminosity function. The predicted densities at fainter absolute magnitudes also agree with the observations although uncertainties in the data do not allow a determination of the change in star formation rate with time. The model predicts a range of scale heights for hot white dwarfs of 220 to 270 pc, and a total local density of degenerate stars of at least 20 per 1000 cubic parsecs. The assumption of a single population of DA white dwarfs with identical composition is not adequate to explain the observed color-color diagram.

A breakdown by spectral type of the entire complete pie of ultraviolet excess objects shows that the hot white dwarfs comprise 70% of the faint blue stars to a mean limiting magnitude of B = 15.7. The mean local density of subdwarf B stars is found to be (1.4 ± .5) x 10[superscript -9] pc[superscript -3], representing only .0005 of the number of stars on the halo horizontal branch, and thereby explaining their near absence in globular clusters. The mean local density of subdwarf O stars of (3.4 ± 1.2) x 10[superscript -9] pc[superscript-3] makes unlikely the possibility that these stars have any direct evolutionary connection with the observed Population I hot white dwarfs or planetary nebulae. The local space density of quasars is derived on the assumption of cosmological redshifts, and provides no contradiction to the hypothesis of a smooth transition between the phenomena of Seyfert galaxy nuclei and quasars. Both the space and surface densities are consistent with the luminosity function and density evolution laws found by Schmidt (1972); the statistical uncertainties in the present case are too great to determine the choice of density laws.

The validity of these statistical studies depends upon the objective selection of a sample within well-defined limits of magnitude and color. To that end, computer programs have been developed for use in collecting and processing data from a PDS scanning microdensitometer. The goal is to obtain fast and simple algorithms for handling an entire astronomical photograph with one-pass digitization. This capability is realized by a real-time detection scheme that provides a data compression of a factor of 100, and a processing program that produces a catalogue of magnitudes colors, and positions for up to 90,000 multicolor stellar images.

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