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Astron. Astrophys. 357, 548-552 (2000)

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1. Introduction

Single-channel differential Strömgren uvby observations of the magnetic Chemically Peculiar (mCP) stars HD 36668, 36 Lyn, HD 86592, and HR 8861 obtained with the Four College Automated Photoelectric Telescope (FCAPT) are investigated. As these stars are photometric, spectrum, and magnetic variables, their emergent energy distributions, photospheric abundances, and magnetic field strengths depend upon photospheric location. Since their magnetic and rotational axes are not usually aligned, as the stars rotate a distant observer will see variability. Hydrodynamical processes including radiative diffusion and gravitational settling in radiative envelopes which have strong magnetic fields are thought to produce their anomalous photospheric abundances which depend on the local magnetic field strength and the time since the star was on the ZAMS (Michaud & Proffitt 1993 and references therein). FCAPT studies have both improved periods and better defined the shapes of their light curves (see, e.g. Adelman et al. 1999). Using these results astronomers can better relate observations taken at different times, detect variable light curves, and study the period distribution of mCP stars. Using spectra in addition, maps of abundances over the surface can be derived. These results serve as tests for theories of the production of their anomalous abundances.

The FCAPT has operated at Washington Camp, AZ since September 1996. Before that it was on Mt. Hopkins, AZ. For each group of variable, check, and comparison stars, the telescope first measures the dark count. Then in each filter it observes the sky-ch-c-v-c-v-c-v-c-ch-sky where sky is a reading of the sky, ch that of the check star, c that of the comparison star, and v that of the variable star. Table 1 contains group information (Hoffleit 1982, Hoffleit et al. 1983, SIMBAD database). Corrections were not made for neutral density filter differences among the stars of each group. The comparison and check stars were chosen from supposedly non-variable stars near the variable on the sky that had similar V magnitudes and B-V colors. Adelman et al. (1998) checked their stability using Hipparcos photometry (ESA 1997). To help find periods the Scargle periodogram (Scargle 1982, Horne & Baliunas 1986) and the clean algorithm (Roberts et al. 1987) were used.


Table 1. Photometric groups

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000