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

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5. HR 8861

The well studied mCP star HR 8861 (= HD 219749 = ET And) is a single-line spectroscopic binary with a very eccentric orbit (e = 0.5) and an orbital period of 48.3 days (Ouhrabka & Gygra 1979). Leroy (1995) found its polarization is non-variable. Its rotational period is well determined from its photometry (Hildebrandt & Hempelmann 1981):

JD (minimum light) = 2441204.54 + 1.61887 E.

Piskunov et al. (1994) used this ephemeris and spectrograms to map HR 8861's Si and He surface abundances. Much of its photometry is in UBV system from Postdam (Scholz et al. 1985; Hildebrandt et al. 1985). Fig. 4 shows the V photometry from Hildebrandt et al. (1985) (+'s) and Scholz et al. (1985) (o's) and FCAPT u , v , b , and y photometry (filled squares) (also given in Table 5). The u light curve shows a weak secondary minimum (phase 0.6) and a strong primary minimum (phase 0.0) along with primary (phase 0.3) and secondary maxima (phase 0.7). The v , b , and y light curves are somewhat different. They have their primary maxima at u secondary minimum and their minima at phase 0.0. The amplitudes of variations are 0.08 mag. for u and 0.025 mag. for v , b , and y . The V data of Hildebrandt et al. (1985) and Scholz et al. (1985) as renormalized to y overlay the y data quite well and thus improving the ephemeris is not possible at this time.

[FIGURE] Fig. 4. Differential FCAPT uvby photometry of HR 8861 (filled squares) plotted with the ephemeris JD (minimum light) = 2441204.54 + 1.61887E. V photometry normalized to the y zero point are shown as +'s for values from Hildebrandt et al. (1985) and as o's from Scholz et al. (1985).

The model of Piskunov et al. (1994) indicates that the minimum at phase 0.0 coincides with the two largest regions of silicon underabundance crossing the stellar disk. At phase 0.6 the region of greatest silicon overabundance crosses the center of the stellar disk. There are no obvious correlations with the helium abundance variations.

Weiss et al. (1998) recently found a period of 1.618875 days which makes little difference in the photometric results. Further HD 219891, which was used a a comparison star, is a pulsational variable, mostly likely a [FORMULA] Scuti star with a frequency of 10.0816 d-1 and a semi-amplitude of 2.5 mmag. This adds a small source of noise to the v-c data.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000