Observations of Binary Stars with the Differential Speckle Survey Instrument
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The Astronomical Journal, 144:165 (10pp), 2012 December doi:10.1088/0004-6256/144/6/165 C 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A. OBSERVATIONS OF BINARY STARS WITH THE DIFFERENTIAL SPECKLE SURVEY INSTRUMENT. IV. OBSERVATIONS OF KEPLER, CoRoT, AND HIPPARCOS STARS FROM THE GEMINI NORTH TELESCOPE Elliott P. Horch1,5,6, Steve B. Howell2,5, Mark E. Everett3,5, and David R. Ciardi4,5 1 Department of Physics, Southern Connecticut State University, 501 Crescent Street, New Haven, CT 06515, USA; [email protected] 2 NASA Ames Research Center, Moffett Field, CA 94035, USA; [email protected] 3 National Optical Astronomy Observatory, 950 N. Cherry Ave, Tucson, AZ 85719, USA; [email protected] 4 NASA Exoplanet Science Institute, California Institute of Technology, 770 South Wilson Avenue, Mail Code 100-22, Pasadena, CA 91125, USA; [email protected] Received 2012 September 5; accepted 2012 October 2; published 2012 November 2 ABSTRACT We present the results of 71 speckle observations of binary and unresolved stars, most of which were observed with the DSSI speckle camera at the Gemini North Telescope in 2012 July. The main purpose of the run was to obtain diffraction-limited images of high-priority targets for the Kepler and CoRoT missions, but in addition, we observed a number of close binary stars where the resolution limit of Gemini was used to better determine orbital parameters and/or confirm results obtained at or below the diffraction limit of smaller telescopes. Five new binaries and one triple system were discovered, and first orbits are calculated for other two systems. Several systems are discussed in detail. Key words: astrometry – binaries: visual – techniques: high angular resolution – techniques: interferometric – techniques: photometric 1. INTRODUCTION detection (e.g., a background eclipsing binary blended with the target star). Ten hours of time were granted for this purpose. In Since 2008, the Differential Speckle Survey Instrument addition, engineering time was given to us in evening twilight, (DSSI) has been used for two main projects at the WIYN 3.5 m when the telescope could not be used by other instruments in Telescope7 at Kitt Peak: (1) to survey binary stars and suspected queue observing mode. binaries discovered by Hipparcos and (2) to provide diffraction- The installation of the camera went smoothly, so that we were limited imaging of targets of interest to the Kepler satellite mis- able to use almost all of the engineering time to obtain science sion. The instrument is described in Horch et al. (2009, here- observations of known binary stars and other targets. These after Paper I) and subsequent WIYN observations have been non-Kepler stars fulfilled four purposes: (1) two binaries with discussed in Howell et al. (2011), as well as Horch et al. (2011a, extremely well-known orbits were observed to help determine 2011b, hereafter Papers II and III, respectively). The last of these the scale and orientation of the pixel axes with respect to papers showed that, largely because of the fact that it records celestial coordinates; (2) a number of targets observed below speckle observations in two colors simultaneously, DSSI can the diffraction limit at WIYN were observed in order to confirm measure binary parameters below the diffraction limit at WIYN the WIYN results with a resolved observation at the larger under certain conditions. The most important of these is that the aperture and/or provide data that would lead to an immediate source must be brighter than about the 10th magnitude, in order orbit calculation; (3) four spectroscopic binaries thought to be to ensure a high signal-to-noise ratio in the observation. thick disk members based on their metallicity and high proper In 2012 July, we had the opportunity to use the instrument motion were observed to add data for eventual calibration of a at the Gemini North Observatory on Mauna Kea, HI. With a low-metallicity main-sequence mass–luminosity relation; and primary mirror of 8.1 m in diameter, Gemini has an aperture size (4) two objects observed were high-priority targets for the more than twice that of WIYN, so it offers the twin advantages CoRoT satellite mission. During the run, we also observed the of greater light gathering power and a smaller diffraction limit. Pluto–Charon system; these results are found in Howell et al. The time awarded at Gemini was for diffraction-limited imaging (2012). of targets of the highest priority to Kepler—that is, those stars In this paper, we detail the observations taken of the binary thought to harbor Earth-like planets based on Kepler transit and unresolved stars, and characterize the astrometric and data. DSSI speckle observations at Gemini are able to yield photometric performance of the DSSI at Gemini, insofar as high signal-to-noise diffraction-limited images in two colors to that is possible given the small number of observations that greatly reduce the parameter space of false positives for planet we have. We also discovered six previously unknown systems (two from the Kepler target list, two bright stars that we used 5 Visiting Astronomer, Gemini Observatory, National Optical Astronomy as point-source calibrators, and two others), and we determine Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of orbital elements for two other systems. Based on the information the Gemini partnership: the National Science Foundation (United States), the at hand, we discuss the future value of these and certain Science and Technology Facilities Council (United Kingdom), the National other objects for stringent tests of our understanding of the Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministerio´ da Ciencia,´ Tecnologia e Inovanao˜ (Brazil), mass–luminosity relation. and Ministerio de Ciencia, Tecnolog´ıa e Innovacion´ Productiva (Argentina). 6 Adjunct Astronomer, Lowell Observatory. 2. OBSERVATIONS AND DATA REDUCTION 7 The WIYN Observatory is a joint facility of the University of Wisconsin-Madison, Indiana University, Yale University, and the National The DSSI was installed on the telescope on 2012 July 25, Optical Astronomy Observatories. and one first-light observation was obtained in twilight on that 1 The Astronomical Journal, 144:165 (10pp), 2012 December Horch et al. evening (UT July 26) after the pointing and focus were obtained. Since we have used these two objects in the determination of The observations described here were obtained on UT July scale, we obviously do not report the relative astrometry for them 27 and 28, nominally comprising the first half of each night. in Section 3. However, we do report their relative photometry, Seeing on both nights was outstanding, usually between 0.4 and which was not used in any calibration procedure. 0.5 arcsec. For all of the observations discussed here, the filters used were 2.2. Reduction Method a 692 nm center-wavelength filter with a 40 nm width in Channel A of the instrument, and an 880 nm center-wavelength filter of The analysis of the data is essentially the same as has been de- width 50 nm in Channel B. While these filters are somewhat scribed in earlier papers in this series. We calculate the autocor- wider than might typically be chosen for speckle observations relation and triple correlation of each frame (see, e.g., Lohmann at such a large telescope, we observed mainly at low airmass, et al. 1983), and average these over the entire observation. In the where any residual atmospheric dispersion would be kept to Fourier domain, these functions are represented by the spatial a minimum. The filter width therefore allowed us to push the frequency power spectrum and bispectrum, respectively. From limiting magnitude as faint as possible, which was especially these image products and a power spectrum of a point source, an important for the Kepler science. estimate of the Fourier transform of the diffraction-limited im- For brighter targets (V<12), we typically took a sequence age can be obtained using the method of Meng et al. (1990), and of 1000 speckle frames in each filter, where each frame was a then filtered and inverse transformed to arrive at a reconstructed 256 × 256 subarray on each chip centered on the target. Such a image. For a binary star observation, we typically use this image sequence consisted of 60 ms frames and required approximately only to identify the secondary location (i.e., to make the quad- 3 minutes to obtain. For fainter objects (and nearly all of the rant determination) for use as the starting position for fitting the Kepler targets fell into this category), we took from three to eight fringes observed in the power spectrum. However, in the case such observations in sequence, depending on the faintness of the of unresolved objects, we do use the reconstructed images to source. As has been our pattern at WIYN, we also periodically estimate the detection limit as a function of separation from the took point-source observations, to gauge dispersion and to have primary star. This is discussed more fully in Section 3.4. a high-quality speckle transfer function available to pair with each observation. This is needed to create the reconstructed 3. RESULTS images, as the target power spectrum must be divided by the Our main results are presented in Tables 1 and 2. These speckle transfer function to yield the modulus of the object’s include results on targets from all three satellite missions Fourier transform. (Kepler, CoRoT, and Hipparcos). Table 1 contains the obser- 2.1. Pixel Scale and Orientation vations where the target was successfully resolved into two or more stars, and Table 2 contains high-quality non-detections.