THE ABSOLUTE DIMENSIONS OF THE OVERCONTACT BINARY FI BOOTIS

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1 The Astronomical Journal, 132:1153Y1157, 2006 September # The American Astronomical Society. All rights reserved. Printed in U.S.A. A THE ABSOLUTE DIMENSIONS OF THE OVERCONTACT BINARY FI BOOTIS Dirk Terrell Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302; terrell@boulder.swri.edu Wayne Osborn and Jason Smolinski Department of Physics, Central Michigan University, Mount Pleasant, MI 48859; wayne.osborn@cmich.edu and John Gross Sonoita Research Observatory, Box 131, Sonoita, AZ 85637; johngross3@msn.com Received 2006 March 30; accepted 2006 June 2 ABSTRACT UBVR C I C photometry of the W UMa eclipsing binary FI Bootis has been obtained and analyzed simultaneously with previously published photometry and radial velocities. The analysis of the light and radial velocity curves shows that the system is an A-type W UMa system consisting of stars with masses 0.82 and 0.31 M. Key words: binaries: close binaries: eclipsing stars: individual (FI Bootis) Online material: machine-readable tables 1. INTRODUCTION The variability of FI Bootis (HIP 75203, HD ; J2000:0 ¼ 15 h 22 m 05: s 9669, J2000:0 ¼þ B329) was revealed by photometric observations with the Hipparcos satellite. The EW nature of the light curve was reported by Duerbeck (1997). The close positional agreement with RX J suggests that it has the X-ray emission common in overcontact systems. Spectroscopic observations by Lu et al. (2001) showed FI Boo to be a double-line spectroscopic binary with a period of 0.39 days. They published well-sampled radial velocity curves for both components and gave a spectral type of G3 V for the system. They concluded that the system was of the W type with a mass ratio of q ¼ 0:372 0:021, in which the less massive star is eclipsed at primary minimum. Selam (2004) analyzed the Hipparcos photometry using the method of Rucinski (1993). He derived a mass ratio of 0.35 and an orbital inclination of 45. The existence of radial velocities for both components gave FI Boo a high priority in our observing program of overcontact binaries. We have obtained UBVR C I C light curves of FI Boo and analyzed these with the previous radial velocity and photometric data to determine the absolute dimensions of the system. 2. OBSERVATIONS We initially obtained UBVR C I C measures on five nights in 2004 at Lowell Observatory. These were followed by a comprehensive set of BVI C observations on 16 nights in 2005 obtained at the Sonoita Research Observatory (SRO) located in Sonoita, Arizona. The Lowell observations were obtained with the National Undergraduate Research Observatory (NURO) 0.75 m f /16 Cassegrain reflector in 2004 May. These data suffered from two shortcomings. First, with the small 4 0 ; 4 0 field of view of the CCD camera, the only comparison star that could be placed on the images of FI Boo was GSC , which is about 4 mag fainter. This made the U observations unreliable and 1153 produced larger than desired errors in the B photometry. We consequently also took frequent images of an alternate comparison star of similar brightness and color to FI Boo, HD FI Boo differential magnitudes were determined relative to both GSC and HD A second difficulty with the Lowell photometry was that clouds interfered with the observations on four of the five nights. While full phase coverage was achieved, the number of data points in each passband was rather sparse. The clouds also significantly affected some of the differential measures based on the alternate comparison star. The problems with the Lowell data led us to obtain a second, more comprehensive set of observations. FI Boo was observed on 16 nights in 2005 April and May with the SRO 0.35 m telescope. The larger field of view (20 0 ; 20 0 ) enabled us to use brighter comparison and check stars than possible with the NURO telescope. The images were centered to have a suitable comparison star, GSC , and check star, GSC , as well as FI Boo in the field. No variability in the comparison or check star was detected. The reductions were done using the IMRED and APPHOT packages in IRAF. 1 Zero and flat-field corrections and exposure time normalization were applied to the images, after which magnitudes were determined by aperture photometry. The Lowell data are given in Table 1. The differential magnitudes computed relative to GSC are denoted as B 1, V 1, R 1, and I 1; U 2, B2, V 2, R 2, and I 2 refer to magnitudes relative to the interpolated brightness of HD at the time of exposure of the FI Boo image. The SRO observations are denoted B3, V 3, and I 3andare givenintable2. 3. DATA ANALYSIS We performed a simultaneous analysis of our photometry, the Hipparcos photometry, and the Lu et al. (2001) radial velocities 1 IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

2 TABLE 1 Lowell Observations of FI Boo HJD a U 2 b HJD B1 B 2 HJD V 1 V 2 HJD R1 R2 HJD I 1 I Note. Table 1 is published in its entirety in the electronic edition of the Astronomical Journal. A portion is shown here for guidance regarding its form and content. a Heliocentric Julian Date minus 2,453,000. b Comparison stars for the different data sets are discussed in the text.

3 ABSOLUTE DIMENSIONS OF FI BOO 1155 TABLE 2 Sonoita Observations of FI Boo HJD a B 3 b HJD V 3 HJD I Note. Table 2 is published in its entirety in the electronic edition of the Astronomical Journal. A portion is shown here for guidance regarding its form and content. a Heliocentric Julian Date minus 2,450,000. b The comparison star was GSC using the 2003 version of the Wilson-Devinney program ( Wilson & Devinney 1971, hereafter WD; Wilson 1979, 1990). Of the Lowell data sets, we used the U2, B1, V1, R1, and I1 data in the analysis. The analysis was performed in WD mode 3, appropriate for overcontact binaries. The adjusted parameters were the semimajor axis of the relative orbit (a), binary center-of-mass radial velocity (V ), orbital inclination (i), secondary mean effective temperature (T 2 ), modified surface potential of the primary ( 1 ), mass ratio (q), and bandpass-specific luminosity of the primary (L 1 ). Certain parameters, such as the bolometric albedos and gravity-brightening exponents, were held fixed at their expected theoretical values for stars with convective envelopes. The logarithmic limb-darkening law was used with coefficients from Van Hamme (1993). On one photometric night at SRO we observed FI Boo and several Landolt standard fields and measured a B V color of 0:71 0:02 for FI Boo. This agrees with the B V that is derived from Hipparcos photometry using the relations given by Bessell (2000): B V ¼ 0:71 0:03. Adopting this value and using the calibration tables of Flower (1996), we set the mean effective temperature of the primary to 5528 K. The results of the simultaneous analysis show that the more massive star is eclipsed at primary minimum, making FI Boo an A-type W UMa system. Figure 1 shows the SRO data and the fits to those data using the parameters in Table 3. Figures 2 and 3 show the fits to the Lowell and Hipparcos photometry, respectively. The fits to the Lu et al. (2001) radial velocities are shown in Figure 4. The fits to some of the light curves indicate that the system probably has spots, but the low inclination of the system makes it very difficult to estimate the spot parameters from the photometry with any reasonable accuracy. Since we used time rather than phase as the independent variable, we adjusted the reference time of minimum (HJD 0 )and the period (P). Attempts to adjust the time derivative of the period (Ṗ) did not result in values statistically different from zero. The resulting ephemeris is Min: I ¼ 2;453;142:0857(3) þ 0: (8)E; where the errors in the last digits of the parameters are given in parentheses. Note that this ephemeris reverses the identities of the primary and secondary eclipses used in previous studies. TABLE 3 Parameters of FI Boo Parameter Value a Fig. 1. SRO observations and computed curves. The light curves have been shifted vertically for clarity. a (R )... 2:34 0:04 V (km s 1 )... 30:4 0:1 i (deg)... 43:1 0:9 T 1 ( K) T 2 ( K) :626 0:007 q... 0:382 0:002 HJD ; 453; 142:0857 0:0003 P (days)... 0: : L 1 /(L 1 þ L 2 ) U... 0:837 0:013 L 1 /(L 1 þ L 2 ) B... 0:802 0:009 L 1 /(L 1 þ L 2 ) V... 0:782 0:007 L 1 /(L 1 þ L 2 ) RC... 0:771 0:008 L 1 /(L 1 þ L 2 ) IC... 0:762 0:006 L 1 /(L 1 þ L 2 ) HP... 0:788 0:009 R 1 (R )... 1:10 0:02 R 2 (R )... 0:71 0:01 M 1 (M )... 0:82 0:04 M 2 (M )... 0:31 0:02 L 1 (L )... 1:02 0:04 L 2 (L )... 0:31 0:04 a Quoted errors are 1 errors.

4 1156 TERRELL ET AL. Vol. 132 Fig. 2. Lowell observations and computed curves. The light curves have been shifted vertically for clarity. The primary component has a mass of 0:82 0:04 M, and its radius is 1:10 0:02 R. The zero-age radius of a star of that mass would be approximately 0.88 R, indicating that the system is somewhat evolved, consistent with the A-type classification (Wilson 1978). The rather large radius and luminosity also indicate that appreciable mass loss from the system must have occurred, because a 0.82 M star would require more than the Hubble time to evolve to those values. Evolutionary models incorporating mass and angular momentum loss, such as those of Yakut & Eggleton (2005), might provide insight into the origin of the FI Boo system. Fig. 4. Radial velocities from Lu et al. (2001) and the computed curves. agreement with the Hipparcos value. The large values of the primary s radius and luminosity for its mass indicate that appreciable mass loss from the system must have occurred, making FI Boo an interesting system for testing models of mass loss in W UMa systems. 4. CONCLUSIONS Simultaneous analysis of new UBVR C I C photometry, the Hipparcos photometry, and radial velocity data from Lu et al. (2001) shows that FI Boo is an A-type W UMa system with grazing eclipses, as illustrated in Figure 5. The larger than zero-age radius of the primary is consistent with the A-type classification. The Hipparcos parallax yields a distance of pc. Using the bolometric corrections of Flower (1996), our simultaneous light and velocity solution gives a distance of 95 2 pc, in good Fig. 3. Hipparcos observations and computed curve. Fig. 5. Grazing nature of the primary eclipse of FI Boo.

5 No. 3, 2006 ABSOLUTE DIMENSIONS OF FI BOO 1157 Former Central Michigan University students J. Beningo, M. Curtis, and H. Hoehn helped obtain the Lowell observations. This research made use of NASA s Astrophysics Data System and the SIMBAD database, operated by CDS, Strasbourg, France. Bessell, M. S. 2000, PASP, 112, 961 Duerbeck, H. W. 1997, Inf. Bull. Variable Stars, 4513, 1 Flower, P. J. 1996, ApJ, 469, 355 Lu, W., Rucinski, S. M., & Ogloza, W. 2001, AJ, 122, 402 Rucinski, S. M. 1993, PASP, 105, 1433 Selam, S. O. 2004, A&A, 416, 1097 REFERENCES Van Hamme, W. 1993, AJ, 106, 2096 Wilson, R. E. 1978, ApJ, 224, , ApJ, 234, , ApJ, 356, 613 Wilson, R. E., & Devinney, E. J. 1971, ApJ, 166, 605 ( WD) Yakut, K., & Eggleton, P. P. 2005, ApJ, 629, 1055