**FAINT AND FAR WITH FINESSE:**

***ASTRONOMY WITH SIM PLANETQUEST***

*The SIM PlanetQuest key projects exploit the mission's unique
capability in numerous ways. This document presents planned science
programs that are unique to SIM.*

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height="3.2694444444444444in"}

*This plot shows the expected parallax mission performance, above and
left of the curves, for the SIM PlanetQuest (green) and GAIA (blue)
missions as a function of target magnitude. Black points represent
specific science targets requested by SIM Key Project P.I.s*

***FAINT:***

SIM PlanetQuest is the only mission, past or planned, that will achieve
unmatched astrometric accuracy for targets as faint as V\~20. Some
projects that take advantage of SIM's high astrometric accuracy at faint
magnitudes include:

-   *SIM's Dynamical Observations of Galaxies* (Edward J. Shaya, P.I.)
    will obtain proper motions for a sample of 27 nearby galaxies within
    5 Mpc from the Milky Way. These will be the first proper motion
    survey of galaxies, and, in conjunction with advanced gravitational
    flow models, will result in the first total mass measurements of
    individual galaxies. This SIM study will lead to determinations of
    galaxy halo sizes, and the overall spatial distribution of dark
    matter in the local vicinity.

-   *Taking the Measure of the Milky Way* (Steven Majewski, P.I.) will
    determine fundamental quantities such as the mass density of (and
    the dark matter contribution to) the Galactic disk, the Sun's
    angular velocity around the Galactic Center, and the Sun's distance
    to the Galactic Center. These measurements require high accuracy (10
    microarcseconds (μas)/year) proper motions for faint (V=17 to 20)
    stars, observations that only SIM can provide.

-   *Binary Black Holes, Accretion Disks and Relativistic Jets:
    Photocenters of Nearby Active Galactic Nuclei (AGN) and Quasars*
    (Ann E. Wehrle, P. I.) will use SIM's ability to observe faint
    sources to determine if the most compact optical emission from an
    AGN comes from an accretion disk or from a relativistic jet. SIM
    will also measure the precise location of the photocenters of these
    systems (down to V=17) to discover if the cores of galaxies harbor
    binary supermassive black holes remaining from galaxy mergers.

-   *Anchoring the Population II Distances and Ages of Globular
    Clusters* (Brian Chaboyer, P.I.) will significantly improve the
    Population II distance scale and significantly diminish the errors
    in the ages of metal-poor globular clusters. SIM will look at
    globular cluster stars as faint as V\~15 with five μas parallax
    accuracy. These improved distance measurements translate into more
    accurate determination of globular cluster luminosities and, hence,
    the mass (and age) of stars in the cluster turning off of the main
    sequence, thereby significantly reducing the uncertainty in the
    derived ages of metal-poor stars. Determining the distance scale to
    globular clusters will allow us to determine the ages of globular
    clusters and field halo stars with unprecedented accuracy.

-   *The Astrophysics of Reference Frame Tie Objects* (Kenneth L.
    Johnston, P. I.) will investigate the underlying physics of SIM
    global reference grid objects and improve the absolute reference
    frame of SIM beyond the solution obtainable from the K-giant grid
    constituent stars alone. SIM will observe 100 "bright" (15-16th mag)
    quasi-stellar objects (QSOs) at 10-20 μas single epoch positional
    accuracy. This is required to achieve the 4 μas or better SIM
    absolute reference frame, which in turn will enable the science of
    many other projects which depend on absolute proper motions,
    parallaxes, and small systematic errors in the SIM frame. SIM's high
    single source positional precision will allow SIM to examine the
    physics of QSOs and identify microarcsecond "anomalies" from source
    to source.

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-   *Masses and Luminosities of X-Ray Binaries* (Andreas Quirrenbach, P.
    I.) will use the wide angle mode of SIM to measure the distances and
    proper motions to a large number of X-ray binaries, providing
    reliable luminosities of many systems for the first time. The
    project has targeted twenty-seven targets for distance measurements;
    nineteen are 17^th^ magnitude or fainter.

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-   *Masses and Stellar Systems with Interferometry* (Todd Henry, P.I.)
    will determine accurate masses (with errors of 1%) for
    representative examples of nearly every type of star. In particular,
    only SIM will be capable of measuring high-precision masses for the
    ubiquitous M dwarfs in essential clusters of various ages out to 1
    kpc. Knowledge of individual masses of binaries of known age and
    metallicity will allow us to map their descent onto the main
    sequence. SIM will deconvolve the binaries directly, thereby
    providing luminosities for both components.

***FAR:***

SIM's ability to observe faint objects at high accuracy allows the
mission to perform high-accuracy measurements within a volume of space
larger than for any other mission past or planned. For every factor of
two in astrometric accuracy, SIM will reach a factor of two farther in
distance and a factor of eight larger in volume, thereby opening up
large swaths of the Galaxy for high-precision astrometric work. Some
examples of SIM science that take advantage of SIM's ability to reach
further into the Galaxy and beyond include:

-   *SIM's Dynamical Observations of Galaxies* will reach beyond the
    immediate neighborhood of the Milky Way, LMC, and SMC out to 5 Mpc.

-   Tidal streams drawn from globular clusters and dwarf spheroidal
    galaxy satellites of the Milky Way offer sensitive probes of the
    symmetry and uniformity of the dark matter halo. These systems will
    be observed as part of the *Taking the Measure of the Milky Way*
    project. The orbital motions are highly sensitive to sub-structure
    in the halo, and therefore test merger theories in cold dark matter
    cosmologies. SIM is the only instrument capable of obtaining the
    requisite observations: proper motions translating to better than
    0.5 km/s accuracy in transverse velocities at 10 kpc for targets as
    faint as 20th magnitude.

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-   The most metal-poor globular clusters are among the first objects
    formed in the Milky Way, and an accurate determination of their age,
    coupled with the WMAP estimates of the age of the universe, will
    allow the *Population II/Globular Cluster* project to determine when
    the first stars were forming in the universe. SIM can obtain
    distances accurate to 5% (corresponding to age errors of 5-8%, and
    age difference errors of 5%) out to 10 kpc from the Sun,
    encompassing roughly 30 globular clusters with low reddening and of
    varying metallicity. The project will select the 20 most
    scientifically interesting globular clusters for SIM observations,
    including 6 globular clusters which are very metal-poor.

-   *Masses and Stellar Systems with Interferometry* will make accurate
    measurements of stars in the Orion star formation complex and the
    M67 star cluster as part of the detailed mapping of the stellar
    mass-luminosity relationship. To reach 1% mass errors in these (and
    other) clusters, high astrometric accuracy is of paramount
    importance to map the orbits of binary stars.

***FINESSE:***

As a pointed rather than a survey instrument, SIM will maintain high
astrometric accuracy to magnitude V=16 to18, opening up many
opportunities for scientific breakthroughs. SIM is a targeted mission
that can selectively schedule observations of varying duration for any
target, limited by solar exclusion constraints. High-accuracy
measurements for planets, stars, and exotic objects having short period
or highly eccentric orbits, as well as for targets of opportunity, such
as microlensing events, are all possible with SIM. Some examples of SIM
science that take advantage of the mission's astrometric accuracy and
scheduling advantages include:

-   SIM is able to discover terrestrial planets within their stellar
    habitable zones and, uniquely, to obtain unambiguous planet masses.
    Two SIM projects---*Discovery of Planetary Systems* (Geoffrey Marcy,
    P.I.) and *Extrasolar Planets Interferometric Survey* (Michael Shao,
    P.I.)---will use \~1 μas astrometric precision to detect terrestrial
    planets. This extraordinary precision will reveal the complex
    architecture of multi-planet systems and enable unprecedented
    precision in the measurement of key orbital parameters. In addition,
    SIM's pointed capability allows scheduled observations that maximize
    detection efficiency. In a survey of the best 120 candidate stars
    for hosting such planets, SIM PlanetQuest can find planets in the
    habitable zone as light as one Earth mass or smaller at six stars.
    Within this same group of stars, SIM's sensitivity permits it to
    detect planets of twice Earth's mass (or smaller) around 30 stars.
    SIM has the sensitivity to find planets of roughly triple Earth's
    mass (or less) around every star in this group.

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-   By detecting Jupiter- and Saturn-mass planets orbiting young stars
    in the critical orbital range of 1 to 5 AU, the *Search for Young
    Planetary Systems and the Evolution of Young Stars* project (Charles
    Beichman, P. I.) will develop a dataset for comparison with the
    planetary systems found around mature stars discovered with other
    SIM projects and with planets in distant orbits (50-100 AU) found
    around young stars (\<10 Myr) through AO imaging. Only astrometry
    with SIM can find these planets because (a) photospheric
    oscillations limit radial velocity measurements to \~100 times worse
    sensitivity than for mature stars; and (b) the distances to these
    stars (25-150 pc) makes direct imaging of the inner planetary system
    (\< 5AU, or 30 milliarcsec at Taurus) impossible even for advanced
    AO systems.

-   *A New Approach to Microarcsecond Astrometry with SIM Allowing Early
    Mission Narrow Angle Measurements of Compelling Astronomical
    Targets* (Stuart Shaklan, P. I.) demonstrates a technique for narrow
    angle astrometry that does not rely on the measurement of grid
    stars. This technique, called Gridless Narrow Angle Astrometry
    (GNAA) will obtain microarcsecond accuracy and can detect
    extra-solar planets, measure interacting binaries, and reveal other
    exciting objects with a few days of observation. The microarcsecond
    astrometry is processed within days of making the
    observations---there is no five year waiting period for a global
    grid to be finalized. This approach will return the first accurate
    mass measurements of known extrasolar planets and can be used to
    quickly measure relative astrometric motion of any time-critical
    target.

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-   The proposed observations in the *Masses and Stellar Systems with
    Interferometry* project will target 100 or more relatively close
    binary systems, separated by less than a few AU. In particular, SIM
    will be able to point at each binary at specified intervals to
    provide tailored orbital phase coverage.

-   The primary goal of *Stellar, Remnant, Planetary, and Dark-Object
    Masses from Astrometric Microlensing* (Andrew P. Gould, P. I.) is to
    make a census of the stellar-mass objects of the Galaxy, in
    particular "dark" objects like brown dwarfs and black holes, which
    cannot be detected *and* characterized by any other method. Current
    microlensing experiments routinely detect such dark "lenses" by the
    magnification they induce on background stars ("sources"), but these
    experiments are unable to measure the lens mass and distance, and
    thus do not distinguish them from ordinary stars. By measuring the
    100 μas astrometric deflection that the lens induces on the source
    (and by comparing its photometry with that obtained from Earth) SIM
    will break the mass/distance degeneracy of ground-based experiments,
    and permit individual identification and mass measurements of these
    dark populations for the first time.

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-   *Masses and Luminosities of X-Ray Binaries* will perform
    narrow-angle observations of several X-ray binaries to determine
    their orbits. Dynamical masses derived from these orbits provide
    fundamental information that cannot be obtained with any other
    mission. Precise mass determinations of neutron star systems such as
    Vela X-1 can address the question of whether neutron stars can be
    significantly more massive than 1.4 solar masses, which would
    eliminate soft models of the neutron star equations of state.

-   *The Astrophysics of Reference Frame Tie Objects* will also use
    SIM\'s flexibility in time resolution to observe non-thermal and
    chromospherically active "radio" stars. These RS CVn and Algol-type
    binaries have orbital periods on the order of a few days. SIM will
    determine accurate orbits for these binaries, and in conjunction
    with ground-based radio observations, determine the location of the
    radio emission relative to the two stars.

-   *Binary Black Holes, Accretion Disks and Relativistic Jets:
    Photocenters of Nearby Active Galactic Nuclei (AGN) and Quasars*
    will observe faint targets at high accuracy (12 μas single
    measurement accuracy). It will also examine if the optical
    photocenters of the quasars used for the reference frame tie shift
    on the timescales of their photometric variability at a level of a
    few microarcseconds.

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-   The goal of *Open and Globular Cluster Distances for Extragalactic,
    Galactic, and Stellar Astrophysics* (Guy Worthey, P.I.) is better
    than 1% distances to the "stellar population cluster network" at
    each point in age-metallicity space. GAIA can obtain this target
    precision for only three clusters, with little or no sampling of the
    metallicity axis. SIM will be able to measure an additional 8
    clusters to the requisite accuracy, with much greater coverage of
    the age-metallicity space. This project also inherits key distance
    information for metal-poor globular clusters from the *Population
    II/Globular Cluster* project, whose science is also mostly beyond
    what GAIA can accomplish.

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-   As mentioned, the *SIM's Dynamical Observations of Galaxies* project
    will measure proper motions of 27 galaxies with SIM, an order of
    magnitude greater than can be done with any other past or planned
    mission. These results will provide a robust map of the dynamical
    environment of the Local Group. Once this additional information is
    in hand, the orbits of hundreds of nearby galaxies since their
    formation until now will be calculable and the history of the nearby
    groups, clouds, and the Local Supercluster plane will be revealed.

> ***FUTURE OF ASTRONOMICAL IMAGING IN SPACE:***

-   SIM will demonstrate high-dynamic-range synthesis imaging at optical
    wavelengths in space, showing the viability of this approach for the
    next generation of UV/Optical/IR imaging telescopes. The
    demonstration will focus on targets of modest complexity but with a
    wide range of surface brightness. SIM's imaging P. I. (Ron Allen)
    will push this capability to the limit by imaging dense stellar
    systems, such as the cores of Galactic globular clusters and the
    nuclear regions of the nearest galaxies. By measuring positions,
    proper motions, and even accelerations of many individual stars in a
    crowded field (dozens of stars in the SIM aperture) over the mission
    lifetime, SIM will be able to detect massive dark objects in these
    regions.

-   SIM is in a unique position to have a profound influence on the
    architecture of future UV/Optical/IR imaging space astrophysics
    missions. Imaging synthesis provides for the first time the
    possibility of separately choosing the resolution and the collecting
    area of space telescopes in order to provide a more cost-effective
    match to the specific astrophysical problems to be addressed. Such
    flexibility is essential for the future if instruments are to
    provide ever-increasing angular resolution and still be affordable.
    SIM will break new ground by demonstrating these imaging techniques
    at optical wavelengths in space.