COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3693 Konkoly Observatory Budapest 2 January 1992 HU ISSN 0374 - 0676 TWO-SPOT MODELLING OF HD 106225 = HU Vir IN 1990 AND 1991 HD 106225 was found to be variable by Fekel, Hall, and Henry (1984) and later given the name HU Vir in the 68th Name List of Variable Stars. Their 1982 photometry showed a range of 0.25m in V and a period of 10.6d +-0.1d. As No. 82 in the Catalogue of Chromospherically Active Binaries (Strassmeier et al. 1988), it is a K0 III SB1 with an orbital period of 10.330d, a negligible eccentricity, and a V sin i of 25 km/sec. Differential photoelectric photometry obtained in the B and V bandpasses during 1990 and 1991 with the 16-inch automatic telescope described by Henry, Nagarajan, and Busby (1991) confirms the variability. The Julian date ranges covered were 2,447.987.8 to 2.448,092.7 and 2,448,292.9 to 2,448,437.7 and the comparison star was HD 105796, the same one used by Fekel, Hall, and Henry (1984). We fit the two years of data, separately, with the two-spot modelling procedure developed by Hall, Henry, and Sowell (1990). The resulting parameters are shown in Table 1 (for the V-band data) and Table 2 (for the B-band data). In both years there were two starspots on the K0 III star, one larger than the other. In both years and for both spots the light loss was a little greater in B than in V. The two spots in 1990 had rotation periods which were equal within their uncertain ties but the two spots in 1991 had significantly different rotation periods. The brightness at maximum, when both spots were turned away from view, increased by about 0.03m in V and 0.02m in B between 1990 and 1991. The rms deviation from the tour fits ranged from +-0.006 to +-0.013m. Figure 1 shows the V-band light curves in 1990 and 1991, with the individual observations as points and the parameters from Table 1 as solid curves. The four different spots show rotation periods which span n range of 2.3 percent. If this is presumed to result from differential rotation, then we can follow the procedure of Hall and Busby (1990, figure 2) to estimate the differential rotation coefficient k. With four spots, we have n = 4 and hence f = 0.45. The result is then k = 0.023/0.45 = 0.05. Note that this value fits almost perfectly on the k vs. P(rot.) relation established by Hall (1991, eqtn. 2) for a larger sample of spotted stars. For the Sun, by comparison, k = 0.19. This work was supported by N.A.S.A. research grant N.A.G. 8-111. [FIGURE 1] Figure 1. Light curve of HD 106225 = HU Vir in 1990 (top) and (1991) (bottom). Each point is differential magnitude in V with respect to HD 105796. The solid curves represent the two-spot model fits, taken from Table 1. Table 1. Parameters of the two-spot model fits - V bandpass epoch rms max. spot period JD(min.) ampl. 1990.39 +-0.006m 0.568m A 10.22d 7993.68d 0.142m +-.002 +-.01 +-.03 +-.004 B 10.18 7997.83 0.097 +-.02 +-.15 +-.004 1991.30 +-0.012 0.541 A 10.44 8365.80 0.232 +-.002 +-.01 +-.03 +-.004 B 10.27 8371.81 0.098 +-.04 +-.08 +-.009 Table 2. Parameters of the two-spot model fits - B bandpass epoch rms max. spot period JD(min.) ampl. 1990.34 +-0.011m 0.481m A 10.20d 7993.72d 0.168m +-.003 +-.02 +-.05 +-.007 B 10.20 7997.75 0.113 +-.03 +-.11 +-.006 1991.30 +-0.013 0.458 A 10.43 8365.81 0.264 +-.002 +-.01 +-.03 +-.009 B 10.27 8371.83 0.108 +-.02 +-.07 +-.005 DOUGLAS S. HALL Dyer Observatory Vanderbilt University Nashville, Tennessee 37235 U.S.A. GREGORY W. HENRY Center of Excellence in Information Systems Tennessee State University Nashville. Tennessee 37203 U.S.A. References: Fekel, F.C., Hall, D.S., and Henry, G.W. 1984, I.B.V.S. No. 2543. Hall, D.S. 1991, in The Sun and Cool Stars: Activity, Magnetism, Dynamos, edited by I. Tuominen, D. Moss, and G. Rudiger (Berlin: Sprinqer- Verlag), p. 353. [BIBCODE 1991LNP...380..353H ] Hall, D.S. and Busby, M.R. 1990, in Active Close Binaries, edited by C. Ibanoglu (Dordrecht: Kluwer), p. 377. [BIBCODE 1990acb..proc..377H ] Hall, D.S., Henry, G.W., and Sowell, J.R. 1990, A. J. 99. 396. [BIBCODE 1990AJ.....99..396H ] Henry, G.W., Nagarajan, R., and Busby, M.R. 1991, I.A.P.P.P. Comm. No. 45, 11. [BIBCODE 1991IAPPP..45...11H ] Strassmeier, K.G., Hall, D.S., Zeilik, M., Nelson, E., Eker, Z., and Fekel, F.C. 1988, Astr. Astrophys. Suppl. 72, 291. [BIBCODE 1988A&AS...72..291S ]