1 Introduction

This paper discusses single-channel differential Strömgren uvby observations of the magnetic Chemically Peculiar (mCP) stars 20 Eri, HR 1297, 12 CMa, and HD 134214 obtained with the Four College Automated Photoelectric Telescope (FCAPT). As both 20 Eri and HR 1297 were found to possibly exhibit changes in the shapes of their light curves by Adelman & Boyce (1995) and by Adelman & Brunhouse (1998), respectively, new sets of observations were made to further investigate this possibility for these stars. 12 CMa is a mCP star whose period was found from magnetic field measures, but lacked photometry around its cycle of variability while HD 134214 is a rapidly oscillating Ap star.

The emergent energy distributions, photospheric abundances, and magnetic field strengths of mCP stars depend upon photospheric position. A distant observer observes photometric, spectrum, and/or magnetic variability due to rotation when the geometry is favorable and the surface ranges in abundance and in magnetic field strength are sufficient since the magnetic and rotational axes often make a large angle with each other. Diffusion and gravitational settling and other hydrodynamical processes in radiative atmospheres and envelopes which have strong magnetic fields are thought to produce their anomalous photospheric abundances which depend both on the local magnetic field strength and the time since the star was on the ZAMS (Michaud & Proffitt 1993 and references therein). Photometric studies using data from the FCAPT have both improved periods and better defined the shapes of their light curves (see, e.g. Adelman et al. 1999). If spectra of these stars are obtained as a function of rotational phase, the distribution of surface abundances can be derived for stars which show modest to moderate rotation.

Since September 1996 the FCAPT has been at Washington Camp, AZ, after previously operating from Mt. Hopkins, AZ. It first measures the dark count for each group of variable, check, and comparison star. In each filter it then 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. The photometer uses a thermoelectrically cooled GaAs Hammatsu photomultiplier (Genet et al. 1987). Table 1 contains group information (Hoffleit 1982, SIMBAD database). In the magnitudes discussed in this paper 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. Later Adelman et al. (1998) checked their stability using Hipparcos photometry (ESA 1997). The Scargle periodogram (Scargle 1982; Horne & Baliunas 1986) and the clean algorithm (Roberts et al. 1987) are used to help find the rotational periods.

 

 

Table 1: Photometric groups

HD Number	Star Name	Type	V	Spectral Type
22470	20 Eri	v	5.23	B9p
23055	HR 1128	c	6.59	A3V
23754	$\tau$6 Eri	ch	4.23	F3III
26571	HR 1297	v	6.12	B9IIIp:Si:
27176	51 Tau	c	5.65	F0V
24740	32 Tau	ch	5.63	F2IV
49333	12 CMa	v	6.08	B7IIIn
47827	HR 2452	c	6.05	A0
46933	$\xi$2 CMa	ch	4.54	A0V
134214	BD $-13{^\circ}$ 4081	v	7.7	Ap
132230	17 Lib	c	6.60	A1V
137052	$\epsilon$ Lib	ch	4.94	F5IV