COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 3202 Konkoly Observatory Budapest 14 June 1988 HU ISSN 0374 - 0676 AG VIRGINIS: NEW TIMES OF MINIMA AND PERIOD STUDY Photoelectric observations of the eclipsing binary star AG Virginis (BD+13d2481) were made on seven nights (Februrary, March and April 1987) using the one meter telescope at the Stephen F. Austin Observatory. The photometer was a Thorn EMI Gencom, Inc. "Starlight-1R" photon counting system equipped with an uncooled EMI 9798A S-20 response tube. Telescope positioning, photometer operation, and data logging was controlled automatically by a Commodore 64 microcomputer (Markworth and Rafert, 1985). Using the comparison star BD+13d 2485, 2,209 observations were obtained in the natural Blue, Visual, and Red bandpasses. These observations resulted in nearly complete BVR light curves and 15 new times of minimum light (12 primary and 3 secondary). The differential magnitudes were normalized and the resulting visual light curve is presented in Figure 1. Determining accurate times of minimum light for AG Virginis is complicated by the asymmetries found in the eclipse branches and the distorted primary minimum. The light curve presented in Figure 1 shows a much smaller distortion in the primary minimum and a more symmetrical secondary eclipse than reported by past observers (Wood, 1946, Binnendijk, 1969, and Blanco and Catalano, 1970). Nightly variations at the bottom of primary eclipse were also evident in the new observations. TABLE 1 JD HEL. MIN O-C OBSERVER 2429329.8510 II +.0144 WOOD (1946) 9334.9930 II +.0152 WOOD (1966) 9335.9560 I +.0142 WOOD (1946) 9337.8840 I +.0142 WOOD (1946) 9338.8510 II +.0172 WOOD (1946) 9339.8110 I +.0133 WOOD (1946) 9346.8790 I +.0121 WOOD (1946) 9359.7340 I +.0141 WOOD (1946) 9363.9100 II +.0129 WOOD (1946) 9368.7320 I +.0150 WOOD (1946) 2433387.8540 I -.0004 NASON AND MOORE (1951) 4086.4195 I +.0039 KWEE (1958) 4120.4787 I +.0026 KWEE (1958) 4455.2919 I -.0052 SZCZEPANOWSKA (1958) 4458.5090 I -.0014 SZCZEPANOWSKA (1958) 4487.4297 I +.0000 SZCZEPANOWSKA (1958) 4776.6215 I -.0010 SZCZEPANOWSKA (1958) 5197.5551 I -.0036 SZCZEPANOWSKA (1958) 5198.5286 II +.0059 SZCZEPANOWSKA (1958) 5219.4146 I +.0058 SZCZEPANOWSKA (1958) 5561.2979 I -.0016 SZCZEPANOWSKA (1958) 5562.2619 II -.0010 SZCZEPANOWSKA (1958) 5848.5649 I +.0011 SZCZEPANOWSKA (1958) 7028.4755 I +.0050 PURGATHOFER AND WIDORN (t9641 8846.5350 I +.0058 BLANCO AND CATALANO (1970) 9587.5065 I +.0010 BLANCO AND CATALANO (1970) 9596.5010 I +.0014 BLANCO AND CATALANO (1970) 9618.3520 I -.0007 BLANCO AND CATALANO (1970) 9643.4142 I -.0019 BLANCO AND CATALANO (1970) 9943.8593 II +.0040 BINNENDIJK (1969) 9944.8191 I -.0002 BINNENDIJK (1969) 9946.7472 I +.0000 BINNENDIJK (1969) 9948.6755 I +.0003 BINNENDIJK (1969) 2441391.4270 I +.0011 KIZILIRMAK AND POHL (1974) 2451.4800 II +.0018 POHL AND KIZILIRMAK (1977) 2892.6620 I +.0041 MALLAMA et. al. (1982) 4709.4356 I +.0042 POHL et. al. (1977) 5741.2071 II +.0000 KALUZNY (1987) 6855.8809 I -.0038 MICHAELS 6855.8821 I -.0026 MICHAELS 6855.8835 I -.0012 MICHAELS 6859.7363 I -.0043 MICHAELS 6859.7381 I -.0025 MICHAELS 6859.7389 I -.0017 MICHAELS 6860.7096 II +.0051 MICHAELS 6860.7103 II +.0058 MICHAELS 6860.7120 II +.0075 MICHAELS 6875.8047 I -.0021 MICHAELS 6875.8051 I -.0017 MICHAELS 6875.8059 I -.0009 MICHAELS 6911.7914 I -.0039 MICHAELS 6911.7924 I -.0029 MICHAELS 6911.7935 I -.0018 MICHAELS [FIGURE 1] Figure 1 The normalized visual differential magnitudes of AG Virginis in the sense variable-comparison. TABLE 2 OBS* TIME N O-C N O-C DIFF. SECONDARY (PRI) (PRI) (SEC) (SEC) DISPLACEMENT WO MAR 1919-APR 1939 6 .0138 4 .0149 -.0011 1.6 MIN. LATE SZ APR 1955-MAR 1956 3 .0002 2 .0025 -.0023 3.3 MIN. LATE BI MAR 1968-APR 1968 3 .0000 1 .0040 -.0040 5.8 MIN. LATE MI MAR 1987-APR 1987 12 -.0025 3 .0061 -.0061 12.4 MIN. LATE * WO = Wood (1946); SZ = Szczepanowska (1958); BI = Binnendijk (1969); Mi = Michaels [FIGURE 2] Figure 2 The O-C diagram fro AG Virginis using only photoelectric primary minima. Line Segment 1 and Segment 2 illustrate a possible period change. Photoelectric minima for this star typically show a significantly smaller scatter in the O-C diagram compared to those determined from visual and photographic observations. Only photoelectric minima are, therefore, compiled in Table 1 and used in this period study. The new times of minimum in this paper were determined by using a FORTRAN program that applied a parabolic least squares fit to the branches of each eclipse (the lower part of primary minimum was not used because it was clearly distorted). The new times of minima are contained in Table 1. Another interesting feature of the light curve is the displaced secondary eclipse, which was reported in previous studies (Binnendijk, 1969, Blanco and Catalano, 1970). Since a displaced secondary minimum will affect the accuracy of a period study, an effort was made to determine whether the displacement is a permanent feature of the light curve. A good method for measuring a change in the phase of the secondary minimum would be to difference the observed minus the calculated (O-C) times of the primary eclipse with the observed minus the calculated (O-C) times of the secondary eclipse. By computing the O-C's at several epochs (using the same orbital period) any change in secondary displacement would become apparent. For a circular orbit where primary eclipse occurs at 0.0 phase and secondary at 0.5 phase the difference should be zero. In Table 2 the minima of four different observers were used in the procedure outlined above. TIME is the calendar dates for each set of observations. (O-C)pri and (O-C)sec are the average residuals (O-Cs) for primary and secondary eclipse respectively. N_pri and N_sec are the number of minima used to compute each average O-C. The period used to form the O-C's was computed using Binnendijk's ephemeris (1969) Hel. JD(Min) = 2439946.7472 + 0.64265068d E. The difference in primary and secondary O-Cs indicates the displacement of the secondary eclipse has been increasing and is presently (April 1987) occuring 12.4 minutes late! The displaced secondary minimum is most likely the result of inaccurate times of minimum light due to distortions in the light curve. The distortions may affect both primary and secondary eclipses. A period study of AG Virginis should, therefore, use either primary minima or secondary minima but not both. Table 1 lists the Heliocentric Julian Date of each minimum, the O-C in days using Binnendijk's (1969) linear ephemeris, the type of minimum (primary or secondary) and the reference. Figure 2 is the O-C diagram using only the primary minima from Table 1. Despite the large scatter in the residuals it seems likely that one period change may have occured as first reported by Binnendijk (1969). The two line segments in Figure 2 represent the best linear fits before and after the period change. Using the primary minima only a least squares solution for the initial epoch and the period for each segment is given by, SEGMENT 1 Hel. JD(Min) = 2429335.9547 + 0.642649295d E .0015 .000000211 SEGMENT 2 Hel. JD(Min) = 2433387.8556 + 0.642650549d E .0008 .000000059 The period increase amounted to only 0.11 seconds. Segment 2 gives the best current light elements for AG Virginis. Another period study by Blanco and Catalano (1970) suggests the orbital period of AG Virginis undergoes a slow variation with a period of about 40 years. Considering the scatter in the O-C diagram (Figure 2) this conclusion is far from secure. A period variation on this time scale is unlikely given the short orbital period and the lack of any observed periodic displacement of the secondary minimum about 0.5 phase which would suggest apsidal motion. A complete discussion and light curve analysis of AG Virginis will be published elsewhere. E.J. MICHAELS Stephen F. Austin State University Department of Physics and Astronomy Box 13044 Nacogdoches, TX 75962 USA References: Binnendijk, L., 1969, A.J. 74, 1024. [BIBCODE 1969AJ.....74.1024B ] Blanco, C. and Catalano, F., 1970, Mem.S.A.I. 41, 343. [BIBCODE 1970MmSAI..41..343B ] Kaluzny, J., 1987, Preprint. Kizilirmak, A. and Pohl, E., 1974, I.B.V.S. No. 937. Kwee, K.K., 1958, Bull. Astron. Inst. Neth. 14, 131. [BIBCODE 1958BAN....14..131K ] Mallama, A.D., Skillman, D.R., Pinto, P.A., Krobousek, B.A., 1977, I.B.V.S. No. 1249. Markworth, N.L. and Rafert, J.B., Microcomputers In Astronomy, Fairborn Press, 1985. Nason, M.E. and Moore, R.C., 1951, A.J. 56, 183. [BIBCODE 1951AJ.....56..182N ] Pohl, E., Evren, S., Tumer, O., Sezer, C., 1982, I.B.V.S. No. 2189. Pohl, E. and Kizilirmak, A., 1976, I.B.V.S. No. 1163. Purgathofer, A. and Widorn, T., 1964, Mitt. Wien 12, 31. Szczepanowska, A., 1958, Acta Astron. 8, 36. [BIBCODE 1958AcA.....8...36S ] Wood, F.B., 1946, Contrib. Princeton Univ. Obs., No. 21, 4. [BIBCODE 1946CoPri..21....1W ]