William H. Wehlau

The William-Wehlau Spectropolarimeter: Observing Hot Stars in All Four Stokes Parameters

We introduce a new polarimeter unit, which, mounted at the Cassegrain focus of any telescope and fiber-connected to a fixed CCD spectrograph, is able to measure all Stokes parameters I, Q, U, and V across spectral Lines of bright stellar targets and other

A SEARCH FOR RAPID VARIABILITY AMONG FOUR NORTHERN AP STARS.

Four Ap stars were monitored for rapid variability in two colors during six nights in March 1984. The photometric data were searched for periodicities by Fourier analysis. One star - 49 Cam - was apparently varying at low amplitude (ΔB≡3 mmag) with a period of approximately 61.6 minutes. No periodic variations were detected in HR 4330, HR 4369, or 53 Cam. The null result for 53 Cam is particularly significant, given previous reports in the literature of short time-scale variability in that star.

A search for rapidly oscillating Ap stars. I: The Ap-rich open cluster NGC 2516. II: Evidence for oscillations in the field star HD 116763

Four stars in the southern open cluster NGC 2516, suspected by Antonello and Mantegazza (1986) of being peculiar A stars and also lying at the red edge of the lower instability strip, were monitored photoelectrically for rapid variability in broad-band light during February/March 1987. No oscillations were detected, with upper limits of roughly 0.001 magnitude in amplitude; however, in light of the amplitude modulation which is observed in the known rapidly oscillating Ap (roAp) stars, these stars cannot yet be rejected as candidates. We also obtained rapid photometry of the field Ap star HD 116763. Frequency analysis of nine hours of data taken on four nights indicates the presence of coherent oscillations with periods of about 7.1 and 19.8 minutes. We urge further rapid photometry of this star by observers in the Southern Hemisphere to establish whether HD 116763 is in fact the 13th member of the class of roAp stars to be detected.

SURFACE DISTRIBUTION OF IRON AND CHROMIUM ON 84 UMA

High resolution spectra of 84 UMa have been used to map the surface distributions of iron and chromium. The abundances of iron and chromium on the surface varied by a factor of 15. Iron is less abundant than in the solar atmophere, by about a factor of 0.6 in the maximum areas, while chromium is more abundant than in the sun, by about a factor of 600 in the minimum areas; the abundances are compared with those for three other CP stars previously studied. The best fit of the computed line profiles to the observed ones was obtained using a microturbulent velocity of 4 km s-1; such broadening may be due to the Zeeman effect of an undetected magnetic field. Although the surface distributions show some regular structure, no symmetries similar to those found for other stars with dipolar magnetic fields are apparent.

NARROW-BAND PHOTOMETRY OF COMET BENNETT (1969 I)

The results of photographic photometry of Comet Bennett on several nights with narrow- band interference filters are presented. The filters, centered at 3878 A, 4870 A, and 5117 A served to photograph the comet in light from CN, the continuum of reflected sunlight, and C2 respectively. The extension of the reflected sunlight into the tail, compared with the light from CN and C2,is given quantitatively. Key words: comets - cometary photometry - photographic photometry

An Empirical T – τ Curve for the Roap Star HR 3831: Atmospheric Structure from Pulsation Amplitudes

The atmospheric structures of magnetic CP2 (Ap) stars are notoriously difficult to model: line blanketing is severe, surface gravities are extremely uncertain, and the surface abundance inhomogeneities lead to different atmospheric properties as a function of position on the star. Seismology of the p-modes of rapidly oscillating Ap (roAp) stars (Kurtz 1990), which vary with periods of a few minutes and amplitudes below 0.01 mag and 1 km/s in light and velocity, has already helped constrain the luminosities and hence, the log</ values of some cool CP2 stars (Kurtz 1992, these proceedings). We show here that the pulsations of an roAp star can also directly probe the temperature structure of a CP2 atmosphere. Available multicolour photometry of roAp stars (e.g., Weiss & Schneider 1984) had revealed that their pulsation amplitudes decline more rapidly with increasing wavelength than other known pulsators. Matthews et al. (1990) showed that this can be explained by the wavelength dependence of limb darkening and its weighting effect on the integrated amplitude of an (£, m) = (1,0) mode. This dipole mode which appears to dominate roAp stars; in particular, HR 3831 has two special characteristics:

The Rich p-Mode Pulsation Spectrum of HD 60435

Twelve cool Ap stars are known at present to undergo rapid light oscillations at low amplitude (ΔB < 0.m.012) and with periods of minutes. These rapidly oscillating Ap (roAp) stars also exhibit amplitude modulation with timescales of days, which correspond to the magnetic/rotation periods (when those have been measured). In at least one case (HR 3831), 180° phase shifts in the dominant oscillation are seen twice per modulation cycle.

Frequency Analysis of the Rapidly Oscillating Ap Star HD 60435

The cool Ap star HD 60435 was monitored in a programme of rapid B photometry during 18 nights in January/February 1984, from two stations widely spaced in longitude (the University of Toronto 0.6-m telescope at the Carnegie Southern Observatory (CARSO) on Las Campanas, Chile, and the 0.5-m telescope of the South African Astronomical Observatory (SAAO)). On six of those nights, contiguous light curves from both sites were obtained.

NARROW-BAND ISOPHOTES OF COMETS TAGO-SATO-KOSAKA AND BENNETT

Isophotes of Comets Tago-Sato-Kosaka and Bennett as observed through interference filters are presented for several nights. The filters show the comets in the light of the continuum, theCNbandat3883AandtheC2 band at 5165 A. Key words: comets - cometary photometry - photographic photometry

LIGHT VARIABILITY OF HD 173650

amplitude. Two comparison stars were used, HD 175427 (AO) and HD 171948 (B9). Since the light variability was expected to be small, the comparison stars were observed alternately during each night to eliminate the possibility of small light variations of the comparison stars. Each night nine observations of HD 173650 and five observations of each of the comparison stars were made. Extinction coefficients were determined nightly except for four nights when the range in zenith distance was small. Since no evidence for any variation in the comparison stars was found, the mean difference in magnitude between the comparison stars was applied to each of the observations of HD 171948 to reduce it to the equivalent values for HD 175427. Satisfactory observations were obtained on 17 nights in the blue and the yellow. The blue filters used were Schott GG 13 plus Corning 5030; the yellow filter was Schott GG 11. The approximate conversion from the magnitude and color system used in this paper to that of Johnson and Morgan1 is given by