Engineering topics

Howell, Steve B.

Publications and source records attributed to Howell, Steve B..

26 records · Page 2

NASA NTRS1992

TT Crateris - A long-period, double-lined dwarf nova

Time-resolved CCD photometry and spectroscopy of the dwarf nova TT Crt during quiescence are presented. The spectra reveal Balmer emission as well as absorption features of a K5-M0 secondary. The B, V, R light curves are dominated by a periodic variation of full amplitude 0.35 mag in V, which are interpreted as the ellipsoidal variation of the secondary star. The remarkably large amplitude indicates a high inclination (50-65 deg) and a massive white dwarf M greater than 0.8 solar mass. The ratio of the radial velocity semiamplitudes of the primary to the secondary is 0.5-1.4. The best period that can be determined from the observations is 438.17 minutes, with a possible alias at 445 minutes.

Szkody, Paula↗

NASA NTRS1992

Astronomical CCD observing and reduction techniques

CCD instrumentation and techniques in observational astronomy are surveyed. The general topics addressed include: history of large array scientific CCD imagers; noise sources and reduction processes; basic photometry techniques; introduction to differential time-series astronomical photometry using CCDs; 2D imagery; point source spectroscopy; extended object spectrophotometry; introduction to CCD astrometry; solar system applications for CCDs; CCD data; observing with infrared arrays; image processing, data analysis software, and computer systems for CCD data reduction and analysis. (No individual items are abstracted in this volume)

Howell, Steve B.↗

NASA NTRS1991

CCD time-resolved photometry of faint cataclysmic variables. IV

Time-resolved CCD photometry in V, B, and the near-IR has been obtained, with average time-series length of 3 hours, for 15 certified or candidate cataclysmic-variable faint stars. Orbital periods are found in three of the stars, and nine others are noted to exhibit evidence leading toward confirmation of cataclysmic-variable status. The characteristics of PG 0917+342 and PG 2240+193 are as yet unclear.

Howell, Steve B.↗

NASA NTRS1990

CCD time-resolved photometry of faint cataclysmic variables. III

CCD time-resolved photometry in V, B, and near-IR for 17 faint cataclysmic variables (CVs) is presented and analyzed. The data are obtained at Kitt Peak National Observatory, the Perkins reflector, Lowell Observatory, and the Observatorio del Roque de los Muchachos from April-June 1989. The degree of variability and periodicities for the CVs are examined. It is observed that the variability of most of the stars is consistent with CV class behavior. Orbital periods for five CVs are determined, and three potential eclipsing systems are detected.

Howell, Steve B.↗

NASA NTRS1990

High galactic latitude cataclysmic variables

A sample of stars which are either known or are good candidates for being halo cataclysmic variables has been isolated. This sample consists of 84 stars of which 57 are dwarf novae (two of these are probable DQ Her systems), 12 are novalike stars, seven are AM Her objects, and eight are classical novae. Periodicities are known for 27 of the systems. If these stars are indeed halo members, then their distances range from 350 pc to 8 kpc and they represent objects that should be significantly older than their disk counterparts. If they do not represent true halo members, then they are intrinsically very faint systems (Mv greater than +10). Comparing the gross properties of this sample to those for the disk cataclysmic variables, it is found that below the period gap, the mean outburst amplitude for the halo dwarf novae is 3 mag greater than for the disk dwarf novae. For systems with periods less than 4 h, the halo cataclysmic variables have a higher percentage of systems below the period gap than the disk cataclysmic variables. The implications of these findings are discussed, as well as possible sample biases and selection effects.

Howell, Steve B.↗

NASA NTRS1988

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.

Howell, Steve B.↗

NASA NTRS1987

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.

Howell, Steve B.↗

NASA NTRS1986

X-ray, UV, and optical observations of the dwarf nova BV Puppis

Exosat, IUE, optical spectroscopic, and broad-band photometric data on BV Pup are presented and analyzed in order to study the system characteristics and their relation to the high excitation of the emission lines. It is observed that: (1) at quiescence and outburst, the He II 4686 A/H-beta ratio for the star is larger than for other observed dwarf novae; (2) the X-ray flux is high for a system at outbursts and the X-ray flux correlates with an increased He II flux over the quiescent values; (3) the flat slope of the flux distribution at quiescence implies relatively low accretion rate; and (4) the abnormal UV line spectrum suggests a different geometry/opacity for the line emission zone compared to normal dwarf novae.

Szkody, Paula↗