uvby photometry of the mCP stars HD 35298, 19 Lyrae, HD 192678, and HR 8216

Astron. Astrophys. Suppl. Ser. uvby photometry of the mCP stars HD 35298, 19 Lyrae, HD 192678, and HR 8216? S.J. Adelman 0 R.H. Rice 0 0 Department of Physics , The Citadel, 171 Moultrie Street, Charleston SC 29409 , U.S.A Di erential Str¨omgren uvby observations from the Four College Automated Photoelectric Telescope are presented for the mCP stars HD 35298, 19 Lyr, HD 192678, and HR 8216. The period for HD 35298 of 1.85457 days is a revision of North's value while that for 19 Lyr of 7.0980 days is alias of that found by Winzer. HD 192678 is found to be a small amplitude photometric variable with the 6.4186 day period proposed by Leroy from polarization measurements. For HR 8216, observations taken between 1995 and 1998 con rm that the star has remained constant at least since 1990. stars; individual; HD 35298; 19 Lyr; HD 192678; HR 8216 | stars; chemically peculiar - 1. Introduction This paper presents di erential Stro¨mgren uvby observations of four magnetic Chemically Peculiar (mCP) stars HD 35298, 19 Lyr, HD 192678, and HR 8216. Studies of the mCP stars using data from the Four College Automated Photoelectric Telescope (FCAPT) have both improved our knowledge of their rotational periods and better de ned the shapes of their light curves (see, e.g. Adelman & Brunhouse 1998 and references therein) . These results can be used to better relate observations taken at di erent times and to study the period distribution of mCP stars. Their variable light curves provide information concerning the uniformity of the surface abundances. Hydrodynamical processes including di usion and gravitational settling in radiative atmospheres with strong Send o print requests to: S.J. Adelman ? Tables 2, 3, 4 and 6 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.ustrasbg.fr/Abstract.html v c ch v c ch v c ch v c ch magnetic elds most likely produce the anomalous photospheric abundances of the mCP stars. Their abundance distributions are patchy and a ect the emergent flux distribution. As their magnetic and rotational axes are not aligned, a distant observer will see photometric, magnetic, and spectrum variability as the stars rotate (Michaud & Pro tt 1993 and references therein). The FCAPT operated on Mt. Hopkins, AZ between September 1990 and July 1996 and since then on nearby Washington Camp, AZ. After the dark count, the telescope measures the sky{ch{c{v{c{v{c{v{c{ch{sky in each lter 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 (a variable along with two supposedly non-variable stars, the comparison and check, against which the brightness of the variable is compared) information (Ho eit 1982; Ho eit et al. 1993) . Corrections were not made for neutral density lter differences among the stars of each group. The comparison and check stars were chosen from supposedly non-variable stars in the vicinity of the variable on the sky that had 0.79 0.80 0.81 0.82 -)c 0.83 v (u 0.84 0.85 0.86 0.87 0.88 1.15 1.16 1.17 1.19 1.20 1.21 -)c 1.18 v ( v 0.92 0.93 0.94 similar V magnitudes and B − V colors. Adelman (1998) checked their stability using Hipparcos photometry (ESA 1997). We used the Scargle periodogram (Scargle 1982; Horne & Baliunas 1986) and the clean algorithm (Roberts et al. 1987) in nding periods. North (1984) derived a period of 1.85336 0.00116 days for the Helium weak star HD 35298 (= V1156 Ori = BD +1 996), a member of the Ori OB1 association. This star exhibits a double-wave light curve with all colors varying in phase. The amplitude is as large as 0.07 mag in the Geneva photometric system. Borra (1981) measured the magnetic eld with essentially this period. We have 100 di erential uvby observations, 56 from the 1995-96 and 44 from the 1996-97 observing seasons. A periodogram analysis resulted in a period close to one-half of North's period. But when we plotted the observations, there were two values for each phase indicating that the period was twice that. To have North's V values properly rezeroed best overlay our y values we had to adjust his period slightly. The zero epoch is better taken for maximum light in u. Thus HJD (umax) = 2444973.886 0.006 + (1.85457 0.00001) E. 0.79 ) -cv 0.80 ( u Figure 1 shows our FCAPT photometry (+ signs) and North's V values (solid diamonds) on our y magnitude scale plotted with our ephemeris. The amplitudes are 0.08 mag for u, 0.05 mag for v and b, and 0.037 mag for y. The light curves are in phase with two minima and a primary and secondary maxima. Some light curves especially u and v show additional structure as the star begins the primary minimum. The relative depths of the two minima are more discrepant for u than for v, b, and y. We are probably seeing most of the photosphere including both polar regions during the cycle. North's U light curve has an amplitude similar to ours, but some of the details near phase 0.5 are not seen probably due to a lesser number of values. Likewise his B light curve is similar to our b. period. Winzer's zero epoch appeared to be satisfactory. Thus HJD (light maximum) = 2441449.99 0.01 + (7.0980 0.0001) E. Figure 2 shows the FCAPT values as + signs and Winzer's and Burke & Barr's V data rezeroed to our y scale as solid diamonds and circles, respectively. The variations in u, v, b, and y are in phase. The curves suggest possible incipient structure. The amplitudes are 0.045 mag, 0.025 mag, 0.030 mag, and 0.025 mag for u, v, b, and y, respectively. There is a sharp primary maximum with a weaker secondary maximum whose prominence decreases with increasing wavelength. Two approximately equal strength minima are symmetrically located with respect to the secondary maximum. 3. 19 Lyr 4. HD 192678 Winzer (1974) discovered the light variations of 19 Lyr (= HR 7283 = HD 179527 = V471 Lyrae) of 0.04 mag in Leroy (1995) found the time variations of the Stokes paU with smaller B and V amplitudes. His tentative period rameters of HD 192678 (= V1372 Cyg = BD +53 2368) of 1.1608 days was based upon very limited data. Burke could be described as & Barr (1981) obtained additional U BV photometry and JD (magnetic maximum) = 2444890.17 + 6.4186 E. con rmed Winzer's period. Mathys & Lanz (1992) demonstrated that the determi We obtained di erential uvby observations of 19 Lyr: nation of its period by Stepien (1968) of order 18 days 18 in the 1995-96 observing season, 40 in 1996-97, and was incorrect. Wade et al. (1996) presented a magnetic 66 in 1997-98. The periodogram contains several possible model consisting of an oblique rotating dipole with modiperiods of which one near 7.10 days, which is an alias of ed eld line inclinations. The dipole has = 173 and = Winzer's period, is the most likely and consistent with the 120 . Their ephemeris uses the same period, but a slightly v sin i value of Abt & Morrell (1995) , 20 km s−1. Using di erent zero point. Essentially the surface magnetic eld the V data of Winzer and of Burke & Barr, we re ned the is strongest near phase 0.00 and weakest near phase 0.50. We obtained 77 di erential uvby observations of 1991-94 vs. 1996-97 show small o sets in u, v, b, and y, reHD 192678, 45 during the 1995-96 and 32 during the 1996- spectively of 0.004, 0.000, 0.007, and 0.004 mag. Although 97 observing season (Table 4). As our analyses revealed a these di erences are suggestive of possible long term variperiod close to that of Leroy's, we adopted his ephemeris ability, observations in additional years are needed for and found that it was adequate to describe our obser- con rmation. vations. To improve the period requires photometry for Acknowledgements. This work was supported in part by NSF this star over a longer period of time. Figure 3 shows the grants AST-9115114 and 9528506 and in part by grants from uvby photometry as a function of phase. HD 192678 is The Citadel Development Foundation. We appreciate the conbrightest in u when the surface magnetic eld strength is tinuing e orts of Louis J. Boyd, Robert J., Dukes Jr., and weakest. The amplitude in u is 0.017 mag and in b is about George P. McCook to keep the FCAPT operating properly. 0.010 mag while this star is essentially constant in v and y. The light curves for u and b are shifted by about 0.15 in phase with u reaching its maximum before b. This be- References havior is consistent with the need for a complex magnetic model such as that of Wade et al. (1996) . Adelman et al. (1994) presented di erential uvby photometry of the mCP ultra-sharp lined star HR 8216 (= HD 204411) obtained between 1990 and 1994. The data for the three observing seasons with many observations showed that this star was constant and values in the literature indicate that this has been the case for many additional years. Its constancy has been con rmed also by Hipparcos photometry (ESA 1997) . Any evidence of variability can lead to an estimate of its rotational period. Table 5 summarizes the FCAPT photometry. For three of the eight observing seasons, there are only a few observations and di erences in ch − c values between these and other years are probably due to their poorer statistics. The 1996-97 and 1997-98 seasons have su cient observations to clearly show that HR 8216 was constant (Table 6). The telescope being moved just before the start of observing in the fall of 1996 and the order 0.01 mag changes in ch − c values seen between the last two and rst 6 observing seasons might simply reflect small errors in the extinction. The o set seen in the ch − c values of b is currently unexplained. 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