COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1592 Konkoly Observatory Budapest 1979 April 27 OBSERVATIONS OF TWO LOW-AMPLITUDE DELTA SCUTI STARS: HD 23156 AND HD 73763 We have recently begun a new observational programme at the Konkoly Observatory with the aim of studying low-amplitude short-periodic variables (Delta Scuti and magnetic stars). In the frame of this programme we observed HD 23156 and HD 73763 on January 4, 5 and 6, 1979 (UT) by a one-channel uncooled EMI 9502S photomultiplier attached to the 100 cm Ritchey-Chretien telescope at the mountain station on Piszkesteto. The observational process was controlled by a TPA/i computer. All the observations were made only in yellow light close to the V band of the UBV system. Though the differential magnitudes were left in the instrumental system, the average deviation of these from the standard V values should not be greater than the accuracy of the individual observations, which is about 0.001-0.003 magnitudes. The observed light-curves are shown in Figure 1. HD 23156 was discovered and classified as a Delta Scuti star by Breger (1972). According to him, it has the shortest period among the Delta Scuti variables. Recently Seeds and Stephens (1977) observed this star and found a period of 0.0205 days against Breger's period of 0.024 days. Using the technique of Fourier analysis of unequally spaced data as developed by Deeming (1975) we did not find any sign of long lived oscillation which would be present throughout the three nights (see Figure 2.). The frequencies and amplitudes of the most prominent oscillations are given in Table 1. Breger (1969) also reported HD 73763 as a Delta Scuti star with a period of 0.038 days and an amplitude of 0.012 magnitudes in V. The power spectra for the individual nights of observation display a great similarity (see Figure 3.). The data were prewhitened by subtracting out the sinusoid corresponding to the frequency of the main peak. The spectra of the prewhitened data of the individual nights are shown in Figure 4. Altogether five [FIGURE 1] Figure 1: Differential magnitudes (m(var.)-m(comp.) in the instrumental system for HD 73763 (Run 1, 2, 3, comparison star HD 73890) and HD 23156 (Run 4, 5, 6, comparison star HD 23246). The following run numbers were used: 1, 4 for January 4, 1979 (UT), 2, 5 for January 5, 1979 (UT) and 3, 6 for January 6, 1979 (UT). [FIGURE 2] Figure 2: Power spectra of HD 23156 for the individual nights of observation. The power was normalized in each spectrum according to the main peak of the given spectra. [FIGURE 3] Figure 3: Power spectra of HD 73763 for the individual nights of observation. The power was normalized in each spectrum according to the main peak of the given spectra. [FIGURE 4] Figure 4: Power spectra of the prewhitened data of HD 73763 for the individual nights of observation. The ordinate is the power relative to the main peak of the spectra of the given original data. Table 1 Table 2 Frequencies for HD 23156. The Frequencies for HD 73763. frequencies at which the power was lower than one half of the Run Frequency Amplitude power at the main peak in the (C/D) (mag.) given spectra, were omitted. Original data ------------------------------------------------------------ Run Frequency Amplitude 1. 25.92 0.0072 (C/D) (mag.) 2. 26.01 0.0055 35.71 0.0024 3. 25.355 0.0092 4. 56.18 0.0026 Whole 25.7596 0.0071 70.42 0.0026 data 14.29 0.0040 Prewhitened data 5. 33.44 0.0030 11.75 0.0033 79.37 0.0029 1. 38.14 0.0032 50.70 0.0013 6. 29.01 0.0017 8.850 0.0018 10.70 0.0011 Whole 18.920 0.0014 2. 30.80 0.0031 data 30.758 0.0014 40.03 0.0028 35.593 0.0013 49.26 0.0011 12.70 0.0019 29.30 0.0016 3. 35.65 0.0017 46.60 0.0017 13.2050 0.0016 Whole 31.4475 0.0011 data 38.8140 0.0018 46.7270 0.0013 ------------------------------------------------------------ frequencies were identified occuring nearly at the same places in the individual spectra. The frequencies and the amplitudes of these waves are listed in Table 2., together with the frequencies obtained by the analysis of the whole data. In addition we performed a MEM analysis using Burg's (1975) and Ulrych and Clayton's (1976) algorithm, and a least-squares fit of two sinusoids with arbitrary frequencies. Because of the high noise level both of these methods gave very questionable results, so we leave these frequencies out of consideration. Finally, for checking the stability of the main period of 0.0388205 days we ordered Breger's and our observations in phase according to this period. Breger's observations fitted very well to our data, showing the stability of this mode. The multiple periods can not be interpreted as being periods of different modes of radial pulsation. More observations should be made at low noise level in order to test the stability of the frequencies we have found. G. KOVACS B. VETO Konkoly Observatory Budapest, Hungary References: Breger, M., 1969, Ap.J. Suppl., 19,79 [BIBCODE 1969ApJS...19...79B ] Breger, M., 1972, Ap.J., 176, 367 [BIBCODE 1972ApJ...176..367B ] Burg, J.P., 1975. Maximum entropy spectral analysis. Ph.D. Thesis, Stanford University, Palo Alto.,Calif., 123 pp. Deeming, T.J., 1975, Astrophys. Space Sci., 36, 137 [BIBCODE 1975Ap&SS..36..137D ] Seeds, M.A. and Stephens, C.J., 1977, IBVS, No. 1273 Ulrych, T.J. and Clayton, R.W., 1976, Phys. Earth Planet. Inter., 12, 188 [BIBCODE 1976PEPI...12..188U ]