COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 2904 Konkoly Observatory Budapest 5 June 1986 HU ISSN 0374 - 0676 STROMGREN uvby-H_beta PHOTOMETRY OF THE SOLAR TYPE STAR HD 206860 An observational program is currently being developed to obtain accurate four color uvby-H_beta photometry of some selected Ca II emission solar type stars (Wilson 1978, Vaughan et al. 1981). They have been monitored to check the existence of correlation between the chromospheric Ca II variability and the optical output in the shorter time scale (stellar rotation). The existence of photometric variations in the group of stars given by Wilson have been reported by Dorren et al. (1982), Radick et al. (1982, 1983), showing the correlations between the chromospheric phenomena and the light fluctuations due the existence of spots associated to the activity centers on the stellar surface. In this note we present the photometric results of the Ca II variable star HD 206860 = HR 8314 that shows a well defined light curve with a photometric period equal to the previously reported Ca II period (Vaughan et al. 1981). The observations were carried out from July 21 to 31, 1985, using the 1.5m telescope at the Calar Alto Observatory (Almeria-Spain), located at 2165 m above the sea level. A multipurpose UBVRI single channel, pulse-counting photometer was used. A general description of this instrument is given by Lahulla (1982). Observations were made in each of the four Stromgren passbands and the Crawford H_beta filters with 16 integration time in each band, followed by sky-background deflections. The observing sequence was the familiar pattern of sky-comparison-variable-comparison-sky. The effects of differential atmospheric extinction were removed and the data reduced to differential magnitudes with respect to the comparison star HR 8313. Due to the close angular proximity of the comparison, the extinction effects were removed using the mean coefficients for each night. Only one comparison was used due to the nature of standard uvby of the HR 8313 (Crawford and Barnes 1970). The transformations to the Crawford-Barnes standard system have been determined for stars of spectral type from A2 to G0 and luminosity class V (Reglero et al. 1986) and they can be used to transform the differential photometry to the standard system. In this particular case, the standard transformation could be affected by non negligible errors due to the uncertainties associated with the spectral type and luminosity class of the comparison star HR 8313 G0 Ib. In order to preserve the intrinsic quality of the data, the results are given in the instrumental system. One hour of observations was typically obtained each night and normal points were formed by averaging the individual measurements. In Table I we list the nightly mean RMS dispersions (^sigma^) for the normal points in the y band and (b-y), m_1, c_1 indices. The night-to-night RMS dispersion (sigma) is also given. Table I y b-y m_1 c_1 H_beta ^sigma^ .005 .004 .006 .009 .008 sigma .015 .005 .005 .005 .006 The y instrumental magnitude differences of HD 206860 with respect to the HR 8313 is plotted in Figure 1. The plot shows a well defined sinusoidal light curve with an amplitude Delta_y=.04 mag., that corresponds to eight times the ^sigma^ confidence level for the y band. Period determinations give us a value of P=4.7 days equal to that previously obtained by Vaughan et al. (1981) with the Ca II emission-line measurements. This photometric period is in good accordance with the spectroscopically determined value of V sin i = 11 Km/s obtained by Kraft (1967). Small (b-y) color variations can be deduced from Figure 1, with an amplitude of Delta_(b-y)=.012 mag. This amplitude is correlated with the variations in y in the sense "redder when darker", that is consistent with the model of stellar activity due to the presence of dark spots on the stellar surface. No noticeable variations are found in the m_1 and c_1 indices below the 3sigma confidence level. [FIGURE 1] Figure 1: Photometric variations of HD 206860 vs phase. Simultaneous H_beta photometry was made to check the behaviour of the Balmer hydrogen beta-line against stellar rotation. The measurements were transformed to the Crawford standard system (Reglero et al. 1986). The plot H_beta versus orbital phase is shown in the lower curve in Figure 1. A mean value of H_beta=2.610 with a night-to-night dispersion a=.006 mag. is computed. Following the H_beta calibration from Crawford and Mander (1986), this value is consistent with the G0 V spectral type given by Kraft (1967) for HD 206860. Further observations of this star and others in Wilson's group are required to establish the nature of the photometric variations. This work was supported by the grant No. 3455/83 from the CAICYT. V. REGLERO and J. FABREGAT A. DE CASTRO Departamento de Astronomia Observatorio Astronomico de Madrid Universitat de Valencia Madrid, Spain Valencia, Spain References: Crawford, D. L., and Barnes J. V., 1970, A.J., 75, 978. [BIBCODE 1970AJ.....75..978C ] Crawford, D. L., and Mander, J., 1966, A.J. 71, 114. [BIBCODE 1966AJ.....71..114C ] Dorren, J. D., and Guinan, E. F., 1982, A.J., 87, 1546. [BIBCODE 1982AJ.....87.1546D ] Kraft, R. P., 1967, Ap. J., 150, 551. [BIBCODE 1967ApJ...150..551K ] Lahulla, J. F. 1982. Bol. Obs. Astron. de Madrid, Vol. X, 6. [BIBCODE 1981BAOM...10....3L ] Radick, R. R., Hartmann, L., Mihalas, D., Worden, S, Africano, J., Klinke, M., and Tyson, E., 1982, P.A.S.P., 94, 934. [BIBCODE 1982PASP...94..934R ] Radick, R. R., and Wilkerson, M. S., 1983, P.A.S.P., 95, 300. [BIBCODE 1983PASP...95..300R ] Reglero, V., Fabregat, J., De Castro, A., 1986, Bol. Obs. Astron. de Madrid (in press). [BIBCODE 1985BAOM...11....7R ] Reglero, V, Fabregat, J., De Castro, A., 1986, Bol. Obs. Astron. de Madrid (submitted). [BIBCODE 1986BAOM...11...81R ] Vaughan, A. H., Baliunas, S. L., Middelkoop, F., Hartmann, L.W., Mihalas, D., Noyes, R. W., and Preston, G.W., 1981, Ap. J. 250, 276. [BIBCODE 1981ApJ...250..276V ] Wilson, O. C., 1978, Ap. J. 226, 379. [BIBCODE 1978ApJ...226..379W ]