Accepted for publication in PASP, January 2008 issue A Preprint typeset using LTEX style emulateapj v. 08/22/09 TAKING THE MEASURE OF THE UNIVERSE: PRECISION ASTROMETRY WITH SIM PLANETQUEST Stephen C. Unwin1, Michael Shao2, Angelle M. Tanner2, Ronald J. Allen3, Charles A. Beichman4, David Boboltz5, Joseph H. Catanzarite2, Brian C. Chaboyer6, David R. Ciardi4, Stephen J. Edberg2, Alan L. Fey5, Debra A. Fischer7, Christopher R. Gelino8, Andrew P. Gould9, Carl Grillmair8, Todd J. Henry10, Kathryn V. Johnston11,12, Kenneth J. Johnston5, Dayton L. Jones2, Shrinivas R. Kulkarni4, Nicholas M. Law4, Steven R. Majewski13, Valeri V. Makarov2, Geoffrey W. Marcy14, David L. Meier2, Rob P. Olling15, Xiaopei Pan2, Richard J. Patterson13, Jo Eliza Pitesky2, Andreas Quirrenbach16, Stuart B. Shaklan2, Edward J. Shaya15, Louis E. Strigari17, John A. Tomsick18,19, Ann E. Wehrle20, and Guy Worthey21 Accepted for publication in PASP, January 2008 issue ABSTRACT Precision astrometry at microarcsecond accuracy has application to a wide range of astrophysical problems. This paper is a study of the science questions that can be addressed using an instrument with flexible scheduling that delivers parallaxes at about 4 microarcsec (µas) on targets as faint as V = 20, and differential accuracy of 0.6 µas on bright targets. The science topics are drawn primarily from the Team Key Projects, selected in 2000, for the Space Interferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilities of this mission to illustrate the importance of the next level of astrometric precision in modern astrophysics. SIM PlanetQuest is currently in the detailed design phase, having completed in 2005 all of the enabling technologies needed for the flight instrument. It will be the first space-based long baseline Michelson interferometer designed for precision astrometry. SIM will contribute strongly to many astronomical fields including stellar and galactic astrophysics, planetary systems around nearby stars, and the study of quasar and AGN nuclei. Using differential astrometry SIM will search for planets with masses as small as an Earth orbiting in the ‘habitable zone’ around the nearest stars, and could discover many dozen if Earth-like planets are common. It will characterize the multiple-planet systems that are now known to exist, and it will be able to search for terrestrial planets around all of the candidate target stars in the Terrestrial Planet Finder and Darwin mission lists. It will be capable of detecting planets around young stars, thereby providing insights into how planetary systems are born and how they evolve with time. Precision astrometry allows the measurement of accurate dynamical masses for stars in binary systems. SIM will observe significant numbers of very high- and low-mass stars, providing stellar masses to 1%, the accuracy needed to challenge physical models. Using precision proper motion measurements, SIM will probe the Galactic mass distribution, and through studies of tidal tails, the formation and evolution of the Galactic halo. SIM will contribute to cosmology through improved accuracy of the Hubble Constant. With repeated astrometric measurements of the nuclei of active galaxies, SIM will probe the dynamics of accretion disks around supermassive black holes, and the relativistic jets that emerge from them. Subject headings: Extrasolar Planets, Stars, Galaxies, Quasars and Active Galactic Nuclei, Astronom- ical Instrumentation arXiv:0708.3953v2 [astro-ph] 7 Nov 2007 1 Jet Propulsion Laboratory, California Institute of Tech- CT 06459 nology, 4800 Oak Grove Drive, Pasadena, CA 91109; 12 Columbia University, Pupin Physics Laboratories, 550 West [email protected] 2 120th Street, New York, NY 10027 Jet Propulsion Laboratory, California Institute of Technology, 13 University of Virginia, Department of Astronomy, P.O. Box 4800 Oak Grove Drive, Pasadena, CA 91109 3 400325, Charlottesville, VA 22904-4325 Space Telescope Science Institute, 3700 San Martin Drive, 14 University of California, Berkeley, 417 Campbell Hall, Berke- Baltimore, MD 21218 4 ley, CA 94720 Michelson Science Center, California Institute of Technology, 15 University of Maryland, Astronomy Department, 0227 CSS 770 S. Wilson Ave., Pasadena, CA 91125 5 College Park, MD 20742 United States Naval Observatory, 3450 Massachusetts Avenue 16 Landessternwarte Koenigstuhl 12, 69117 Heidelberg, Ger- NW, Washington DC 20392 6 many Department of Physics and Astronomy, 6127 Wilder Labora- 17 University of California at Irvine, Department of Physics and tory, Dartmouth College, Hanover, NH 03755 7 Astronomy, Irvine, CA 92697 Department of Physics and Astronomy, San Francisco State 18 University of California at San Diego, Center for Astrophysics University San Francisco, CA 95064 8 and Space Sciences, 9500 Gilman Drive, La Jolla, CA 92093 Spitzer Science Center, 1200 E. California Blvd., Pasadena, 19 Space Sciences Laboratory, 7 Gauss Way, University of CA 91125 9 California, Berkeley, CA 94720-7450 Department of Astronomy, The Ohio State University, 140 W. 20 Space Science Institute, 4750 Walnut Street, Suite 205, 18th Avenue, Columbus, OH 43210 10 Boulder, CO 80301 Department of Physics and Astronomy, Georgia State Uni- 21 Department of Physics and Astronomy, Washington State versity, Atlanta, GA 30303 11 University, Department of Physics and Astronomy, Webster Hall Van Vleck Observatory, Wesleyan University, Middletown, 1245, Pullman, WA 99164-2814 2 Unwin et al. paper. 1. INTRODUCTION Recommended by the 1990 NRC Decadal Survey Astrometry is perhaps the most fundamental, and old- (Bahcall 1990), SIM PlanetQuest entered its Formulation est of all areas in astronomy, and it remains a corner- Phase (Phase A) in October 1997 and was approved to stone of the field for the twenty-first century. Accurate enter Phase B in August 2003. SIM was again endorsed distances to astronomical objects are essential for de- by the 2000 NRC Decadal Survey (McKee & Taylor riving fundamental quantities like mass and luminosity. 2000) wherein it was assumed that SIM would be com- Photographic astrometry in the nineteenth and twen- pleted, making it unnecessary to rank it against new mis- tieth centuries laid the foundation for our understand- sion recommendations in that report. Technology devel- ing of local stellar populations by identifying the inhab- opment was completed in July 2005 and formally signed itants of the solar neighborhood (Gliese 1969; Luyten off by NASA Headquarters in March 2006 after extensive 1979). The Second US Naval Observatory CCD Astro- external independent review. Having completed nearly graph Catalog (UCAC2) is a CCD-based survey covering all of the Formulation Phase (Phase A/B), SIM is ready most of the sky, with accuracies of 15 70 milliarcsec to enter the Implementation Phase, with mature designs, (mas), depending on brightness (Zacharias− et al. 2004), well understood schedule and cost, and low technical and and utilizing the Hipparcos and Tycho-2 reference frame. cost risk. Unfortunately, there is no official launch date, Recent CCD based astrometry over narrow fields has since budget pressures on NASA’s Science Mission Direc- achieved an accuracy of less than 1 mas in a single mea- torate have resulted in NASA delaying the Implementa- surement (Pravdo et al. 2005). On still smaller scales, tion Phase. Lane & Muterspaugh (2004) have demonstrated 16 This paper covers the expected science contributions microarcsec measurements between the components≃ of a of SIM but does not describe any of the technical details 0.25-arcsecond binary, using the Palomar Testbed Inter- of the instrument or mission. Brief descriptions of the ferometer at 2 µm. CCD parallaxes now achieve typical instrument itself and the supporting technologies may errors of 0.5 mas (Harris et al. 2005). Wide-angle as- be found in several technical papers (Laskin 2006; Marr trometry using ground-based optical and near-IR inter- 2006; Shao 2006). A companion paper (Shao & Nemati, ferometers now reaches 20 mas (Hummel et al. 1994). In in preparation) explains the SIM instrument design, op- the radio range, very long baseline interferometry (VLBI) eration, performance and calibration in more detail. The astrometry of quasars has allowed the creation of a quasi- astrometric performance of SIM is based on an hierar- inertial reference frame, the ICRF (International Celes- chical error budget with more than 1000 terms, and with tial Reference Frame; Ma et al. 1998) with wide-angle key sets of parameters verified in a series of testbeds de- accuracy 0.25 mas. veloped during Formulation Phase. Quoted performance Space-based astrometry has brought about a renais- numbers are current best estimates from the error budget sance in the field. The ESA Hipparcos mission, which and detailed instrument design. operated from 1989-1993, yielded an astrometric cata- The acronym SIM stands for Space Interferometry Mis- log of 118,000 stars down to 12.5 magnitude, with posi- sion. SIM will be the first space-based Michelson inter- tional accuracy of 1 mas for stars brighter than V = 11. ferometer for astrometry. The instrument will operate The European Space Agency (ESA) is now developing in the optical waveband using a 9-m baseline between the Gaia mission as a next generation astrometric survey the apertures. With a global astrometry accuracy of mission (Perryman et al. 2001; Perryman 2002), which is 3 µas for stars brighter than V = 20, it will measure parallaxes and proper motions of stars throughout the expected to develop a catalog