Michael Hobart

Character of the O–C Variation of Four W UMa Type Stars: AW UMa, AP Leo, AG Vir and AH Vir

An analysis was carried out of the secular period variation of four WU Ma type stars (AW UMa, AP Leo, AG Vir and AH Vir) using some newly determined times of minima as well as others reported in the literature. It was found that two stars show period increases, one a period decrease and the remaining shows no variation in period.

Do the Pre-Main Sequence δ Scuti Stars Really Exist?

Recently, new views of the current status of δ Scuti stars have been developed by Rodriguez & Breger (2001) who carried out an excellent review, listing 8 pre-main-sequence (PMS) δ Scuti candidates and examined the possibility of the existence of PMS δ Scuti variable stars. Here we discuss the nature of 4 stars from their list: 2 said to be members of NGC 2264 and 2 of NGC 6823.

Multiperiodicity in the Delta Scuti Star HR 4047

HR 4047 (HD 89343, EN UMa, V = 5 92, A7Vn) was found to be variable (Plume & Percy, 1988) with an amplitude of hundredths of a magnitude. Since then, this star has been observed by HIPPARCOS. Koen (2001), analysing those data, obtained two frequencies 6.43185 c d -1 and 4.31308 c d -1 . Our observations were made at the Observatorio de Sierra Nevada in Spain in 1997 and 1998 with the telescope of 0.9m. The comparisons stars for 1997 were C1 = HD 88983 (V = 5 77, A8III) and C2 = HD 88815 (V = 6 42, F2V), while for 1998 we add the check star C3 = HD 90838 (V = 7.30, F2). The photometric data obtained were analyzed using the Fourier Transform method in the Stromgren v filter. The periodogram of the v band data is shown in Fig. 1. The periodogram presents six clear frequencies and after prewhitening these there is still more power, but the frequencies found are not consistent between the different bands. Applying these frequencies to the data of the 1997 and 1998 separately, we obtain the results shown in Table 1. Note that the amplitudes have changed from one year to another. The results of 1998 agree with the two frequencies found by Koen (2001). Fig. 1 shows the light curves of the observed data for both campaigns in the v filter. The points represent the observations while the solid lines represent the fit for the frequencies listed in Table 1. Further observations are needed to obtain a more detailed pulsational behaviour of this star.

Physical Parameter Determination for the δ Scuti Star HD 200925

HD 200925 is an unusual star: the period is stable on a time scale of many years, yet it shows variations which are either large and irregular, or small, suggesting a possible Blazhko effect (Poretti 1984; Joner & Johnson 1985); it has larger values of T eff and log g derived from uvbyβ than those of dwarf Cepheids of similar period; and it has a somewhat higher metal abundance, [Fe/H], than the Hyades. Furthermore, in conflict to what Joner and McNamara (1983) determined – that m 1 index shows no variation with temperature – HD 200925 shows a definite variation, unlike that predicted in Crawford (1979). No explanation of this has yet been found. Johnson & Joner (1986) also derived an unusual and unexpected value of [Fe/H]. Finally, with respect to the pulsation periods of this star, Mantegazza & Poretti (1986) determined the existence of two periods: one definite, 0.26730 d and a possible second one of 0.2138 d which yield a ratio of P 2 / P 1 = 0.800, the theoretical ratio expected between the first and the second overtones, found in other pulsating variables. All of these facts make HD 200925 an interesting star, motivating the present study.