The PHASES Differential Astrometry Data Archive. I. Measurements And
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Draft version October 8, 2018 A Preprint typeset using LTEX style emulateapj v. 11/10/09 THE PHASES DIFFERENTIAL ASTROMETRY DATA ARCHIVE. I. MEASUREMENTS AND DESCRIPTION Matthew W. Muterspaugh1, 2, Benjamin F. Lane3, S. R. Kulkarni4, Maciej Konacki5, 6, Bernard F. Burke7, M. M. Colavita8, M. Shao8, Sloane J. Wiktorowicz9, J. O’Connell1 Draft version October 8, 2018 ABSTRACT The Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) monitored 51 sub-arcsecond binary systems to determine precision binary orbits, study the geometries of triple and quadruple star systems, and discover previously unknown faint astrometric companions as small as giant planets. PHASES measurements made with the Palomar Testbed Interferometer (PTI) from 2002 until PTI ceased normal operations in late 2008 are presented. Infrared differential photometry of several PHASES targets were measured with Keck Adaptive Optics and are presented. Subject headings: astrometry – binaries:close – binaries:visual – techniques:interferometric 1. INTRODUCTION physical properties such as component masses and sys- A technique has been developed to obtain high pre- tem distance when possible (Muterspaugh et al. 2010b). cision (35 µas) astrometry of close stellar pairs (sepa- The third paper presents limits on the existence of sub- ration less than 1 arcsec; Lane and Muterspaugh 2004) stellar tertiary companions orbiting either the primary or using long-baseline infrared interferometry at the Palo- secondary stars in those systems that are found to be con- mar Testbed Interferometer (PTI; Colavita et al. 1999). sistent with being simple binaries (Muterspaugh et al. This technique was applied to 51 binary systems as 2010c). Paper four presents orbital solutions to a known the Palomar High-precision Astrometric Search for Ex- triple star system (63 Gem A = HD 58728) and a oplanet Systems (PHASES) program during 2002-2008. newly discovered triple system (HR 2896 = HD 60318) PHASES science results included precision binary orbits (Muterspaugh et al. 2010a). Finally, paper five presents and component masses, studies of the geometries of and candidate substellar companions to PHASES binaries as physical properties of stars in triple and quadruple star detected by astrometry (Muterspaugh et al. 2010d). systems, and limits on the presence of giant planet com- Astrometric measurements were made at PTI, which panions to the binaries. was located on Palomar Mountain near San Diego, CA. This paper is the first in a series analyzing the final It was developed by the Jet Propulsion Laboratory, Cali- results of the PHASES project as of its completion in fornia Institute of Technology for NASA, as a testbed for late 2008. This paper describes the observing method, interferometric techniques applicable to the Keck Inter- sources of measurement uncertainties, limits of observ- ferometer and other missions such as the Space Interfer- ing precisions, derives empirical scaling rules to account ometry Mission (SIM). It operated in the J (1.2µm), H for noise sources beyond those predicted by the stan- (1.6µm), and K (2.2µm) bands, and combined starlight dard reduction algorithms, and presents the full catalog from two out of three available 40-cm apertures. The of astrometric measurements from PHASES. The second apertures formed a triangle with one 110 and two 87 me- paper combines PHASES astrometry, astrometric mea- ter baselines. PHASES observations began in 2002 and surements made by other methods, and radial velocity continued through 2008 November when PTI ceased rou- observations (where available) to determine orbital solu- tine operations. tions to several binaries’ Keplerian motions, determining 2. OBSERVATIONS AND DATA PROCESSING [email protected], [email protected], [email protected] The initial PHASES observing method and arXiv:1010.4041v1 [astro-ph.IM] 19 Oct 2010 1 Department of Mathematics and Physics, College of Arts data processing algorithm were presented by and Sciences, Tennessee State University, Boswell Science Hall, Nashville, TN 37209 Lane and Muterspaugh (2004). Incremental im- 2 Center of Excellence in Information Systems, Tennessee provements to these procedures were updated in papers State University, 3500 John A. Merritt Boulevard, Box No. 9501, from the PHASES science program. The final observing Nashville, TN 37209-1561 procedure and data processing algorithm is presented 3 Draper Laboratory, 555 Technology Square, Cambridge, MA 02139-3563 in complete form here. All astrometry measurements 4 Division of Physics, Mathematics and Astronomy, 105-24, were reprocessed using the final algorithm presented California Institute of Technology, Pasadena, CA 91125 here. Measurements taken with different instrumental 5 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Rabianska 8, 87-100 Torun, Poland configurations than the standard one presented here 6 Astronomical Observatory, Adam Mickiewicz University, (for example, those lacking longitudinal dispersion ul. Sloneczna 36, 60-286 Poznan, Poland compensation) are noted in Table 5. 7 MIT Kavli Institute for Astrophysics and Space Research, MIT Department of Physics, 70 Vassar Street, Cambridge, MA 2.1. 02139 Astrometric Observation Method 8 Jet Propulsion Laboratory, California Institute of Technol- 2.1.1. Optical Interferometers ogy, 4800 Oak Grove Drive, Pasadena, CA 91109 9 Department of Astronomy, University of California, Mail In an optical interferometer, light is collected at two or Code 3411, Berkeley, CA 94720, USA more apertures and brought to a central location where 2 the beams are combined and a fringe pattern produced fed 40 cm beam compressing telescopes. These col- on a detector (at PTI, the detectors were NICMOS and limated the 40 cm input into a 7.5 cm beam. HAWAII infrared arrays, of which only a few pixels were used). For a broadband source of central wavelength λ 2. At the siderostat enclosure, a fast steering mir- and optical bandwidth ∆λ (for PTI ∆λ = 0.4µm), the ror (FSM) provided tip-tilt correction for low-order fringe pattern is limited in extent and appears only when adaptive optics improvement. The feedback sensor the optical paths through the arms of the interferometer for the tip-tilt system was located in the beam com- are equalized to within a coherence length (Λ = λ2/∆λ). bining facility (see step (9)), to include both atmo- For a two-aperture interferometer, neglecting dispersion, spheric and instrumental sources of tip-tilt varia- the intensity measured at one of the combined beams is tions. given by 3. The collimated beams propagated through pipes sin (πx/Λ) to the central beam combining building. For the I(x)= I 1+ V sin (2πx/λ) (1) 0 πx/Λ north and south siderostats, the pipes were held at vacuum; for the west siderostat, the pipe was filled where V is the fringe contrast or “visibility”, which can with air. be related to the morphology of the source, and x is the optical path difference between arms of the interferom- 4. Each collimated beam was directed to a long delay eter. More detailed analysis of the operation of optical line (DL) which tracks the sidereal delay rate and interferometers can be found in Principles of Long Base- receives feedback from the fringe tracking beam line Stellar Interferometry (Lawson 2000). combiner for removing some of the atmospheric turbulent delay variations. There were roughly 2.1.2. Interferometric Astrometry 38 m of optical delay available in the long DLs. The location of the resulting interference fringes is re- ± lated to the position of the target star and the observing 5. The collimated beams were recompressed to 2.5 cm geometry via collimated beams. 6. Each beam passed through a pair of matched d = −→B −→S + δa −→S,t + c (2) · prisms. In one arm, one prism was on a linear stage where d is the optical path length one must introduce be- to vary the total glass thickness the starlight passed tween the two arms of the interferometer to find fringes. through. The positions of these prisms were cali- brated using fringes from an internal light source This quantity is often called the “delay.” −→B is the to minimize the combined longitudinal dispersion baseline—the vector connecting the two apertures. −→S of air path and glass. This calibration also deter- is the unit vector in the source direction, and c is a con- mined the glass-to-air dispersion correction rate. stant additional scalar delay introduced by the instru- The linear stage was operated in open-loop mode ment. The term δa −→S,t is related to the differential to minimize longitudinal dispersion even as the air path was varied by the long DLs. Some early mea- amount of path introduced by the atmosphere over each surements were made before this dispersion com- telescope due to variations in refractive index. For a 100- pensator was available, as marked in Table 5. m baseline interferometer an astrometric precision of 10 µas corresponds to knowing d to 5 nm; while difficult, 7. The 2.5 cm collimated beams were split 70/30 this is achievable for all terms except that related to by plate beam splitters. The reflected ∼30% of the atmospheric delay. Atmospheric turbulence, which the light was directed to the “secondary”∼ beam changes over distances of tens of centimeters and on mil- combiner which made the astrometric science mea- lisecond timescales, forces one to use very short exposures surement. Red He-Ne LASER metrology signals (to maintain fringe contrast) and limits the sensitivity of were injected at these beam splitters and propa- the instrument. It also severely limits the astrometric gated through both beam combiners before being accuracy of a simple interferometer, at least over large extracted just before the infrared array detectors sky-angles. to monitor path variations. However, in narrow-angle astrometry one is concerned with a close pair of stars, and the observable is a differ- 8. The 70% transmitted light from each beam was ential astrometric measurement, i.e. one is interested in directed∼ through a short DL.