COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2372 Konkoly Observatory Budapest 22 July 1983 HU ISSN 0374-0676 PHOTOELECTRIC PHOTOMETRY OF Ap STARS IN IC 2602, NGC 6281 AND IN THE SCORPIO-CENTAURUS GROUP: PRELIMINARY RESULTS New photometric periods have been obtained for five Ap stars and for one He-weak star which are members of clusters and associations. Observations were carried out at the European Southern Observatory at La Silla (Chile) with the Swiss telescope, using the Geneva photometry, during April, May and June 1983. The measurements are absolute ones, i.e. no comparison stars were used. However, standard stars were measured sufficiently often and at about the same airmass (i.e. within a few hundredths), so that the V, [U-B] and [B-V] values of the variables could be estimated fairly accurately at the end of each night, with only a rough preliminary reduction procedure. The data were analyzed with three techniques of period determination: Deeming's (1975) method of discrete Fourier transform, Renson's (1978) Theta_1 test and Stellingwerf's (1978) phase dispersion minimization method. The lightcurves were fitted by a function of the type f(t)=A_0 + A_1 cos (2pi/p (t-t_0) + phi_1) + A_2 cos (4pi/p (t-t_0) + phi_2) which is generally sufficient. The stars HD 145102 (DM-26d11240 following the HD practice, Si) and HD 147105 (DM-25d11483, Sr), although measured 34 and 30 times respectively, do not show any really conclusive period because their amplitude is extremely small. HD 147105 might have a greater amplitude than HD 145102, but it is fainter (m_v=8.794), so that the signal-to-noise ratio is poorer. The very strange peculiar star HD 144667 (HR 6000), which seems extremely young (The and Tjin A Djie 1978) and is strongly deficient in silicium (Castelli et al., 1981) was measured six times and seems quite stable. The integration time was three times longer than usual, in order to have better precision. The possibility remains, however, that HR 6000 may vary on a very long timescale, since its projected rotational velocity is lower than 20 km/s (Castelli et al., 1981). The values are given in Table I. Table I Preliminary values of the colours and. of the magnitude of HD 144667 (HR 6000;. A more refined reduction procedure is being made at Geneva Observatory. Brackets mean that the indices are relevant to the Geneva system and not to Johnson's. The V band may be considered as equivalent to Johnson's. ---------------------------------------------------------------------- JD- [U-B] [V-B] V Remark 2440000 ---------------------------------------------------------------------- 5442.847 .879 1.054 6.629 5443.794 .881 1.054 6.630 5444.740 .880 1.055 6.635 5445.797 .877 1.054 6.629 5449.751 .880 1.053 5.639 V magnitude value slightly doubtful 5451.773 .884 1.053 5.632 ---------------------------------------------------------------------- The star NGC 6281-15 was detected as peculiar by the Geneva photometry alone, through the Delta(V1-G) and Z parameters (North and Cramer, 1981). A variation is undoubtedly present, although it is not possible at the present time to make a choice between two possible periods. Measurements are going on in order to get a better phase coverage, but it is already worth noticing that for the first time, a lightcurve is established for an Ap star known as such from photometry alone. The star IC 2602-17 rotates very rapidly, since it has one of the shortest periods known. HD 147890, on the contrary, has a rather long period, strengthening the impression that young Ap stars do not necessarily rotate much faster than older ones (North, 1982) HD 137193 seems to have a relatively long period too; another, shorter period could be possible, but gives a greater residual dispersion in both the B and V bands. [FIGURE 1] [FIGURE 2] [FIGURE 3] [FIGURE 4] [FIGURE 5] [FIGURE 6] Table II ----------------------------------------------------------------------------------------- Cluster or HD DM Period Resid.dispersion No.of Remark association Peculiarity [U] [B] V meas. ----------------------------------------------------------------------------------------- Upper Cen 137193 -39d09848 Si 4.907 .0054 .0041 .0027 40 0.8291 .0053 .0047 .0031 Less probable Upper Sco 142884 -23d12597 Si 0.8030 .0061 .0049 .0033 29 Upper Sco 144334 -23d12700 He wk 1.492 .0048 .0042 .0042 29 0.5982 .0047 .0055 .0047 Much less probable Upper Sco 147890 -29d12529 Si 4.331 .0055 .0048 .0037 20 IC 2602-17 92385 -64d1374 Si 0.5490 .0046 .0028 .0028 30 NGC 6281-15 153948 -37d11216 Delta(V1-G) 1.166 .0150 .0039 .0031 21 Ph.metric =.034 or AP star 8.119 .0120 .0051 .0035 ------------------------------------------------------------------------------------------ The results are summarized in Table II, where the standard mean deviations of the residuals are also given, and in Figures 1 to 6. P. NORTH C. WAELKENS Institut d'Astronomie de Astronomisch Instituut l'Universite de Lausanne et Naamsestraat 61 Observatoire de Geneve B-3000 LEUVEN CH-1290 Chavannes-des-Bois Belgium Switzerland References: Castelli, F., Cornachin, M. et al.: 1981, in Upper Main Sequence CP Stars, 23rd Liege Astrophys. Coll., p. 149 [BIBCODE 1981LIACo..23..149C ] Deeming, T.J.: 1975, Astrophys: Space Sci. 36, 137 [BIBCODE 1975Ap&SS..36..137D ] North, P.: 1982, Inf. Bull. Var. Stars No. 2208 North, P. and Cramer, N.: 1981, in Upper Main Sequence CP Stars, 23rd Liege Astrophys. Coll., p. 55 [BIBCODE 1981LIACo..23...55N ] Renson, P.: 1978, Astron. Astrophys. 63, 125 [BIBCODE 1978A&A....63..125R ] Stellingwerf, R.F.: 1978, Astrophys. J. 224, 953 [BIBCODE 1978ApJ...224..953S ] The, P.S., Tjin A Djie, H.R.E.: 1978, Astron. Astrophys. 62, 439 [BIBCODE 1978A&A....62..439T ]