Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975natur.258..586s&link_type=abstract
Nature, vol. 258, Dec. 18, 1975, p. 586, 587. Research supported by the Science Research Council.
Astronomy and Astrophysics
Astronomy
4
Quasars, Spatial Distribution, Cepheid Variables, Contaminants, Cosmology, Galactic Structure, Light Curve, Luminous Intensity, M Stars, Radio Sources (Astronomy), Red Shift, Stellar Luminosity
Scientific paper
A method for selection of radio-quiet quasars and BL Lac-type objects based on their optical variability is described, with consideration of a sample of 13 objects reported by van der Bergh et al. (1973). The number of luminous variable stars present in the sample is minimized by selection of directions well out of the galactic plane, and is further reduced by a close study of the light curves of objects in the sample. The contaminating effects of late M-type flare stars and N-type galaxies are mitigated by the introduction of further weak color selection. The number-magnitude relation based on quasars detected by the ultraviolet excess method of selection (Setti and Woltjer, 1973) was used to calculate the number of quasars in the van der Bergh BHP sample; the numbers of contaminants were estimated on the basis of variable-star surveys. Results, expressed as a function of magnitude and compared with the total number of objects in the sample, indicate that the BHP sample contains 65% quasars; half the number of quasars found by ultraviolet selection techniques are detected without the accompanying redshift-dependent selection effects inherent in these methods.
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