K. J. Mitchell

Statistical error analysis in CCD time-resolved photometry with applications to variable stars and quasars

Differential photometric time series obtained from CCD frames are tested for intrinsic variability using a newly developed analysis of variance technique. In general, the objects used for differential photometry will not all be of equal magnitude, so the techniques derived here explicitly correct for differences in the measured variances due to photon statistics. Other random-noise terms are also considered. The technique tests for the presence of intrinsic variability without regard to its random or periodic nature. It is then applied to observations of the variable stars ZZ Ceti and US 943 and the active extragalactic objects OQ 530, US 211, US 844, LB 9743, and OJ 287. 25 references.

A medium-bright quasar sample - New quasar surface densities in the magnitude range from 16.4 to 17.65 for B

A new medium-bright quasar sample (MBQS) is constructed from spectroscopic observations of 140 bright objects selected for varying degrees of blue and ultraviolet excess (B-UVX) in five Palomar 1.2 m Schmidt fields. The MBQS contains 32 quasars with B less than 17.65 mag. The new integral surface densities in the B range from 16.45 to 17.65 mag are approximately 40 percent (or more) higher than expected. The MBQS and its redshift distribution increase the area of the Hubble diagram covered by complete samples of quasars. The general spectroscopic results indicate that the three-color classification process used to catalog the spectroscopic candidates (1) has efficiently separated the intrinsically B-UVX stellar objects from the Population II subdwarfs and (2) has produced samples of B-UVX objects which are more complete than samples selected by (U - B) color alone.

IDENTIFICATION OF US 943 AS A HALO CATACLYSMIC VARIABLE.

US 943 has been shown to exhibit large (greater than about 5 mag) amplitude variations on time scales of one to two years. New time-resolved CCD photometry presented herein shows it also to be variable on time scales of 20 to 40 minutes with amplitude variation of up to 1 mag. Its membership as a cataclysmic variable is suggested. This assignment would place US 943 at a galactic distance of about 700 pc.

Radio observations of a new class of optically selected Quasi-Stellar Objects

Most QSOs found by low-resolution spectroscopy are known to be radio-quiet,, but QSOs selected by polarization, power-law continuum spectra, or optical variability might have different radio characteristics. We have observed a sample of 96 QSOs and other blue objects, chosen on the basis of optical variability, with the VLA at 4885 MHz. Only three were detected (LB 8755, LB 8956, and LB 9013), and two of these (LB 8755=OJ 180 and LB 9013=4C 17.46) had previously been found in radio-selected samples. For the rest, 5sigma upper limits of 2.0 mJy can be placed on their 4885 MHz flux densities. The radio-to-optical luminosity ratios of the variable components of these QSOs are more than two orders of magnitude lower than those of radio-selected QSOs. Although the detection rate is low, the detected QSOs are among the most variable in the sample observed. Future work in this field is suggested.

A New Sample of Faint Halo B Stars

This poster reports the isolation of a new, complete sample of 24 faint halo B stars. These B stars were selected as blue- and ultraviolet-excess objects in the US survey at high Galactic latitudes (Usher and Mitchell 1990), and were given preliminary classifications using low-resolution spectrophotometry. The new sample is complete over 206 square degrees of sky to faint magnitude completeness limits in the range B = 16.5 and B = 18.3.