COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2148 Konkoly Observatory Budapest 1982 May 19 HU ISSN 0374 - 0676 PHOTOMETRY OF THE SHELL STAR BU Tau (PLEIONE) 1980 - 1982 This note continues our report on the photometric behaviour of the shell star BU Tau (Hopp, Witzigmann, 1980). BV measurements were collected during 12 nights with the 75 cm telescope of the Wilhelm Foerster Sternwarte, Berlin in the same manner as described in our previous note. In addition, one of us (UH) made UBV photometry with the 36 Cassegrain telescope of the Observatorium Hoher List during 7 nights. This telescope is equipped with an uncooled 1P21 photomultiplier, a conventional DC amplifier, standard UBV filters, two small band interference filters centered on 403 nm and 419 nm, respectively, and one broad band filter combination centered on 470 nm (G of the RGU system). As comparison stars served some bright Pleiades stars, especially 21, 22, and 27 Tau were always used. Magnitudes and colours of the stars are taken from Johnson and Mitchell (1958). Our new values are given in the table, n is the number of individual measurements per colour of BU Tau. The B lightcurve of Sharov and Lyuty (1976), updated by our own measurements and by new observations of these authors (1981) is shown in Figure 1. [FIGURE 1] Fig. 1: The B lightcurve of BU Tau from 1980 to 1982. Small dots are photographic, large dots are photoelectric observations. Table: UBV values of BU Tau 1980 - 1982 Jul. Date V B-V U-B n 244 4490d 5.21m -0.03m -0.14m 2 4489 5.21 -0.05 -0.16 3 4544 5.19 -0.02 2 4570 5.22 -0.05 3 4590 5.20 -0.07 2 4602 5.26: -0.12: -0.12 1 4644 5.20 -0.04 2 4647 5.20 -0.04 2 4662 5.21 -0.05 3 4852 5.19 -0.01 -0.14 2 4853 5.23 -0.04 -0.03 1 4872 5.16 -0.02 -0.14 2 4906 5.20 -0.01 3 4968 5.11 -0.05 -0.06: 5 5001 5.13 -0.04 5 5020 5.19 -0.04 1 5022 5.23 -0.03 3 5036 5.20 -0.03 4 5037 5.19 -0.04 4 Accuracy: V +- 0.01m, B-V +- 0.01m, U-B +- 0.02m During September 1980 and 1981, we used also the additional three filters at the 36 cm telescope to make a multicolour photometry of BU Tau and some bright Pleiades members relative to the star 19 Tau. As example, we show in Figure 2 the measurements of Sept. 7, 1980. From these measurements, we conclude that BU Tau has an unusual great Balmer jump compared to stars of the same spectral type and luminosity class. If we use 16 or 18 Tau as standard, BU Tau has a deficit of about 0.27 in U. On Sept. 6, 1980, also. a digital spectrogram was obtained with the grating spectrograph at the 106 cm Cassegrain telescope of the Observatorium Hoher List by means of a two dimensional SIT- vidicon detector system (EG & G-Instruments Optical Multichannel Analyzer OMA 2). For details of the OMA 2 system and the data reduction, see e.g. Geyer (1981). [FIGURE 2] Fig. 2: Relative spectrophotometry of BU Tau and seven bright Pleiades members in comparison to 19 Tau. Star numbers, spectral types and luminosity classes are indicated. 19 Tau is of type B6V. Figures 3a, b show the background but not flatfield corrected blue (lambdalambda376 to 492 nm) and yellow-red (lambdalambda550 to 660 nm) spectra of BU Tau with a total observing time of 14 sec in comparison to a 1.5 sec blue spectrum of alpha Lyrae taken with the same equipment under identical conditions but with nearly closed spectrograph slit. The spectral resolution of these spectra is roughly 0.3 nm. Besides a number of absorption lines of HeI, NaI, CaII, FeII, SiII, TiII and CrII which can be identified, we see only Halpha in emission (W_lambda= 10.7 A), H_beta in absorption with imbedded, redshifted emission feature and Hgamma - H_10 in pure absorption. In Figure 4 enlarged portions of the H_3 to H_5 lines of BU Tau are shown. From all the observations presented here, we conclude that the BU Tau shell episode which started in 1973 still goes on. [FIGURE 3] Fig. 3a,b: Blue and yellow-red spectral features of BU Tau and alpha Lyrae. [FIGURE 4] Fig. 4: Enlarged portions of the emission/absorption H3 to H5 - lines of BU Tau and alpha Lyrae. Acknowledgement: E.H.Ge. acknowledges support for the OMA detector system by grant Ge 209/11-2 of the Deutsche Forschungsgemeinschaft. U. HOPP Institut fur Astronomie und Astrophysik Technische Universitat Berlin Ernst Reuter Platz 10 D 1000 Berlin 10, FRG S. WITZIGMANN Wilhelm Foerster Sternwarte Munesterdamm 90 D 1000 Berlin 41, RG E. H. GEYER Observatorium Hoher List D 5568 Daun / Eifel,FRG References: Geyer, E.H.: 1981, Mitt. A.G. 54, 228 [BIBCODE 1981MitAG..54..228G ] Hopp, U., Witzigmann, S.: 1980, Inf.Bull.Variable Stars 1782 Johnson, H.L., Mitchell, R.I.: 1958, Astrophys. J. 128, 31 [BIBCODE 1958ApJ...128...31J ] Sharow, A.S., Lyuty, V.M.: 1976, in IAU Symp. 70, 105, ed. A. Slettebak, D. Reidel Publ. Co., Dordrecht [BIBCODE 1976IAUS...70..105S ] Sharow, A.S., Lyuty, V.M.: 1980, Astron. Tsirk 1119, 1 [BIBCODE 1980ATsir1119....1S ]