Tobias J. Kreidl

Wide field camera observations of Baade's Window

We have observed a field in Baades Window using the Wide Field Camera of the Hubble Space Telescope (HST) and obtain V- and /-band photometry down to V~22.5. These data go several magnitudes fainter than previously obtained from the ground. The location of the break in the luminosity function suggests that there are a significant number of intermediate age ( < 10 Gyr) stars in the Galactic bulge. This conclusion rests on the assumptions that the extinction towards our field is similar to that seen in other parts of Baades Window, that the distance to the bulge is approximately 8 kpc, and that we can determine fairly accurate zero points for the HST photometry. Changes in any one of these assumptions could increase the inferred age, but a conspiracy oflower reddening, a shorter distance to the bulge, and/or photometric zero-point errors would be needed to imply a population entirely older than 10 Gyr. We infer an initial mass function slope for the main-sequence stars, and find that it is consistent with that measured in the solar neighborhood; unfortunately, the slope is poorly constrained because we sample only a narrow range of stellar mass and because of uncertainties in the observed luminosity function at the faint end.

Periodic photometric variability of the black hole binary V404 Cygni

Periodic variations of V404 Cyg having a period of 6.474 +/- 0.005 d with a full amplitude of 0.2 mag were discovered with CCD photometry, in the I band, of the compact star. The light curve consists of two maxima and two minima per cycle, and the shape suggests that it is due to ellipsoidal variations of the secondary star which is tidally distorted by the presence of the massive compact object. It is proposed that the secondary is a 1 solar mass K0 IV star with MV of about 2.5 mag at a distance of about 3.5 kpc. It is suggested that if the secondary star is nearly filling its Roche lobe, then the orbital inclination must be about 60 deg. If the inclination is as high as 80 deg, then the secondary star fills about 90 percent of the Roche lobe. These limits imply a compact star mass of 8-12 solar masses and thus strengthen the evidence that the compact star is extremely massive and possibly a black hole.

ROSAT observations of the black hole candidate V404 Cygni in quiescence

V404 Cyg was detected with the ROSAT PSPC in a 16 ks observation 1265 days after the 1989 May outburst. This is the first unambiguous detection of a soft X-ray transient or X-ray nova black hole candidate in quiescence. The observed X-ray spectrum is extremely soft and can be described equally well by either a blackbody, power-law, or a thermal bremsstrahlung continuum. The best-fit blackbody spectrum is characterized by a temperature of 0.2 keV. A significant amount of absorption, both along the line of sight and intrinsic to the V404 Cyg system, is required to understand the observations. The 0.2-2.4 keV light curve of V404 Cyg exhibits substantial variability on timescales of less than a day. Assuming a distance of 3.5 kpc, the quiescent X-ray luminosity of V404 Cyg is 8 x 10(exp 33) ergs/s, nearly two orders of magnitude larger than the upper such as Cen X-4. The lack of a significant hard X-ray luminosity in quiescence, the presence of a large and cold neutral region in the accretion disk, and a low-mass accretion rate suggests that an accretion disk instability might account for the outburst of V404 Cyg.

Optical identification of the X-ray source GS 2023+338 as V404 Cygni

On 1989 May 22 the Japanese Ginga Team reported the discovery of a bright new X-ray transient and cataloged the source as GS 2023+338. We have identified this X-ray source with V404 Cygni, an optical nova whose last reported major outburst was in 1938.

A seven-year northern sky survey of Ap stars for rapid variability

A high-speed photometric survey of 120 Ap stars in the northern sky, has been conducted, between 1985 and 1991, in order to search for rapid variability. Stars of spectral types, namely from B8 to F4, have been selected for the survey. The selected pulsational variable stars occupy the hotter regions of the instability strip of the Hertzsprung-Russel diagram. Noted is the absence of pulsations in the hotter B8-A3 Ap stars; this does not, however, preclude the existence of pulsations, since HD 218495 was recently discovered to be a rapidly oscillating Ap (roAp) star. The primary result of this study is that various combinations of photometric indices, while pointing towards roAp stars having the characteristic signatures of cool, SrCrEu stars, still fail to isolate the roAp phenomenon from similar nonpulsating Ap stars. Color-magnitude and color-color diagrams are presented in order to complete this survey.

Photometric calibration of the HST wide-field/planetary camera. I, Ground-based observations of standard stars

This paper describes the observation and analysis of field stars, undertaken as a step toward understanding the photometric properties of the filters and CCDs aboard the Wide-Field/Planetary Camera of the Hubble Space Telescope. Ground-based CCD observations have been made simulating 15 of the most important WF/PC passbands. Data are presented here for an equatorial network of stars including many UBVRI standards and some spectrophotometric standards. They serve to establish a photometric system that will be used in the calibration of in-flight data and that is useful for the calibration of other ground-based data. Transformations between WF/PC and other photometric systems are discussed, as well as the effects of interstellar absorption and atmospheric extinction on data in the WF/PC system. Synthetic photometry has been used as an aid in the data analysis and is described.

Narrow-band photometry and the evolution of galaxies. III, The intermediate-redshift sample

A near-UV and blue rest frame narrow-band filter system is used to investigate the spectrophotometric evolution of elliptical galaxies. Data are presented for mean colors of ellipticals in six distant clusters (z = 0.37-0.46) for comparison with the present-day ellipticals and models of color evolution. Good agreement is found with the UV-cold models of Guiderdoni B. and Rocca-Volmerangre B. (1987, Astron. Astrophys., 186), particularly with the confirmation of a strong post-MS light contribution at z = 0.4. The fraction of starforming to nonstarforming objects (Butcher-Oemler effect) is calculated from rest frame colors.

ANALYSIS OF CCD PHOTOMETRY FOR THE VERY SHORT PERIOD ECLIPSING BINARY BM URSAE MAJORIS

CCD images of the fourteenth magnitude variable BM Ursae Majoris were taken in the spring of 1991. The observations were taken at Lowell Observatory. An RCA CCD camera system and a standard PMT were used. Four times of minimum light were determined and improved linear and quadratic ephemerides were calculated. A period study, spanning over a thirty year interval, shows that the system is undergoing a small but continuous period decrease of ~5 X 10-8 d/yr. This may be due to angular momentum loss (AML) caused by stellar winds. The V, R, I light curves formed from the present precision observations, show that BM UMa is a W-type W UMa system. The first synthetic light curve solution of BM UMa is also presented. This solution reveals that BM UMa is a contact binary consisting of two early K spectral type components with a fill out of ~20% and a mass ratio of ~0.5. A subtantial temperature difference of ~400 K was determined. Both unspotted and spotted models were calculated. The spotted model indicates the presence of a region of enhanced birghtness in the neck of the secondary component. This may be attributed to fluid dynamics of mass in transit rather than to magnetic activity.

Near-infrared CCD photometry of Cygnus X-3

The first photometric observations capable of resolving the 4.8-hr period at near-infrared wavelengths of the extraordinary X-ray source Cygnus X-3 have been obtained using a CCD at the Perkins 1.8-m telescope. The data show that the 4.8-hr light curve at I band exhibits a variety of forms ranging from being relatively quiescent within the photometric errors to showing fluctuations exceeding 1 mag on time scales of tens of minutes to several hours. There is a suggestion that the variations may be correlated with phase of the 4.8-hr infrared and X-ray period, but the pulsed fraction of the I band fluctuations is about 90 percent compared to 10-25 percent for the infrared and X-ray variations. If confirmed, these vairations may represent important new information concerning the origin of the 4.8-hr cycle and of the nature of the binary system. The mean I-magnitude of Cygnus X-3 is found to be 20.0 + or - 0.2. 23 refs.

CCD PHOTOMETRY AND PRELIMINARY ANALYSIS OF V865 CYGNI

CCD images of the thirteenth magnitude variable V865 Cygni were taken on four consecutive nights in the summer of 1990. The 1.1-m Hall reflector telescope at Lowell Observatory was used in conjunction with an RCA CCD camera system. Four times of minimum light were determined and improved linear and quadratic ephemerides were calculated. A period study, spanning over a forty year interval, may indicate that the system is undergoing a continuous period increase of 5.4 X 10-8 d/yr. In a conservative scenario, this would be an indication of a mass accretion by the primary, more massive, component. The V, R, I light curves formed by the first precision photometry to be obtained on V865 Cygni, show that it is an A-type W UMa system. However, the curves are somewhat incomplete since the maximum which follows the secondary eclipse was not covered. The first synthetic light curve solution of V865 Cygni is also presented. This solution reveals that V865 Cygni is a contact binary consisting of two G spectral type components with a fillout of 18% and a mass ratio of 0.45. The component temperature difference is only 110K. A substantial hot spot region of 34 degrees diameter is proposed to model the asymmetry in the light curves following the observed maximum. We suggest that the origins of this region may be attributed to fluid dynamics rather than to the usual magnetic activity which is prevalent in late-type systems like V865 Cygni.