**FAINT AND FAR WITH FINESSE:**

***ASTRONOMY WITH SIM PLANETQUEST***

***FAINT:***

SIM PlanetQuest will be able to observe faint targets with exquisite
astrometric accuracy, and it's the only mission also capable of
observing targets as faint as V\~20. Some examples of SIM PlanetQuest
science that take advantage of SIM's high astrometric accuracy at faint
magnitudes include:

-   The Key Project *SIM's Dynamical Observations of Galaxies*
    (Edward J. Shaya, P.I.) will obtain proper motions for a sample of
    27 nearby galaxies (including Andromeda, M33, and Centaurus A).
    These will be the first optical proper motion measurements of
    galaxies beyond the satellite system of the Milky Way, and in
    conjunction with new gravitational flow models, will result in the
    first total mass measurements of individual galaxies. This SIM study
    will lead to vastly improved determinations of individual galaxy
    masses, halo sizes, and the spatial distribution of dark matter.

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-   The *Binary Black Holes, Accretion Disks and Relativistic Jets:
    Photocenters of Nearby Active Galactic Nuclei (AGN) and Quasars*
    (Ann E. Wehrle, P. I.) Key Project will use SIM's ability to observe
    faint sources (down to V=17) 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 astrometric signature of
    these systems to discover if the cores of galaxies harbor binary
    supermassive black holes remaining from galaxy mergers.

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-   The *Taking the Measure of the Milky Way* Key Project (Steven
    Majewski, P.I.) will use the three-dimensional motions of distant
    halo stars to measure the Milky Way\'s total mass and its
    distribution, probing the contribution of dark matter to the
    Galaxy\'s architecture. This project will also determine fundamental
    quantities such as the mass density of (and the dark matter
    contribution to) the Galactic disk, the solar rotation speed and the
    distance to the Galactic Centre. These measurements require high
    accuracy (10 μas/year) proper motions for faint (V=17 to 20) stars,
    observations that only SIM can provide..

-   The Key Project *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 a 5 μas parallax
    accuracy.

-   The Key Project *The Astrophysics of Reference Frame Tie Objects*
    (Kenneth L. Johnston, P. I.) will investigate the underlying physics
    of SIM grid objects and improve the absolute reference frame of SIM
    beyond the solution obtainable from the K-giant grid stars alone.
    SIM will observe 100 \"bright\" (15-16th mag) quasi-stellar objects
    (QSOs) at 10-20 μas single epoch positional accuracy, required to
    achive the 4 μas or better SIM absolute reference frame, which in
    turn will enable the science of many other Key Projects, which
    depend on absolute proper motions, parallaxes, and small systematic
    errors in the SIM frame.

> SIM's high single source positional precision is mandatory for this
> Key Project and will allow SIM to examine the physics of QSOs and
> identify μas \"anomalies\" from source to source, which is not
> possible with any other mission.

-   The Key Project *MASSIF (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
    accurate masses for the ubiquitous M dwarfs in essential clusters of
    various ages out to 1 kpc, allowing us to map their descent onto the
    main sequence.

***FAR:***

SIM's ability to observe faint objects at high accuracy allows the
mission to perform high-accuracy measurements over a larger volume of
space. For every factor of two in astrometric accuracy, SIM will reach a
factor of two further 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 PlanetQuest science that take
advantage of SIM's ability to reach further into the Galaxy include:

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

-   Tidal streams drawn from globular clusters and dwarf spheroidal
    satellites of the Milky Way offer very 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* Key
    Project. The orbital motions are highly sensitive to sub-structure
    in the halo, and therefore test galaxy formation theories,
    particularly those tied to cold dark matter cosmologies. SIM is the
    only instrument capable of obtaining the requisite observations,
    proper motions accurate to 10-15 μas/yr for targets with V=18-20.

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-   For compelling results in the *Population II/Globular Cluster* Key
    Project, distances accurate to 5% are required, which translates to
    5 μas parallax accuracy for clusters at 10 kpc with SIM. With 10 μas
    parallax errors, astronomers can measure a 5% distance to objects
    within 5 kpc of the Sun. There are only 5 globular clusters within 5
    kpc of the Sun which have relatively low reddening, and none of
    these globular clusters are very metal-poor. In contrast, SIM can
    survey a volume of space 8 times larger within which there are
    roughly 30 globular clusters with low reddening for which SIM could
    obtain distances accurate to 5%. This allows the Population II
    distance scale Key Project to select the 20 most scientifically
    interesting globular clusters for SIM observations, including 6
    globular clusters which are very metal-poor. 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 this key
    project to determine when the first stars were forming in the
    universe.

-   The *MASSIF* Key Project 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 not possible with any other
mission. SIM is a nimble targeted mission that can make observations of
any target at any time (limited to solar exclusion constraints). GAIA is
a scanning mission with a fixed observing schedule for the full mission.
Specifically timed observations for planets, stars, and exotic objects
having short period or highly eccentric orbits, and for targets of
opportunity, such as microlensing events, are all possible with SIM.
Some examples of SIM PlanetQuest science that take advantage of SIM's
scheduling advantages include:

-   SIM PlanetQuest is able to discover terrestrial planets within their
    stellar habitable zones and, uniquely, to obtain unambiguous planet
    masses. Two SIM Key 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. If every
    nearby star has an "average-Earth" sized planet (0.5 to 2 Earth
    masses) in its habitable zone, then SIM will find 15 Earth-sized
    planets. If every nearby star has a "heavy-Earth" planet (2 to 5
    Earth masses) in its habitable zone, then SIM will find 220 heavy
    Earths.

-   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* Key Project
    (Charles Beichman, PI) will develop a dataset for comparison with
    the planetary systems found around mature stars discovered with
    other SIM Key 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 solar system (\<
    5AU, or 30 milliarcsec at Taurus) impossible even for advanced AO
    systems.

-   The SIM PlanetQuest Key Project *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) can obtain microarcsecond
    accuracy and can detect extra-solar planets, measure interacting
    binaries, and reveal other exciting objects with a few days of
    observation.

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

-   The primary goal of the *Stellar, Remnant, Planetary, and
    Dark-Object Masses from Astrometric Microlensing* (Andrew P.
    Gould, P. I.) Key Project is to make a census of the stellar
    population of the Galaxy, in particular "dark" objects lurking in
    the Galactic halo. The only way to examine the field population of
    such objects in the halo (red dwarfs, degenerate white dwarfs,
    neutron stars, and black holes) is through microlensing, when the
    deflection of light from a visible background star (the "screen") by
    an foreground object (dark or not) results in a brightening of the
    screen. The microlensing effect can be detected in two ways:
    photometrically, i.e. by measuring the magnification of the source
    by the lens, and astrometrically, i.e. by measuring the shift in the
    image centroid. The combination of SIM PlanetQuest and Earth
    observatories allows astronomers to break the microlensing
    degeneracy and allow measurement of the mass, distance, and velocity
    of individual microlensing events.

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-   Using SIM PlanetQuest, the *Masses and Luminosities of X-Ray
    Binaries* (Andreas Quirrenbach, P. I.) Key Project Project 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
    same project will use the wide-angle mode of SIM PlanetQuest to
    measure the distances to a large number of X-ray binaries, providing
    for the first time reliable luminosities of many systems.

-   *The Astrophysics of Reference Frame Tie Objects* Key Project (Ken
    Johnston, P.I.) 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.

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

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-   The goal of the *Open and Globular Cluster Distances for
    Extragalactic, Galactic, and Stellar Astrophysics* Key Project (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 to 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 Key Project, whose
    science is also mostly beyond what GAIA can accomplish.

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-   As mentioned, the *SIM's Dynamical Observations of Galaxies* Key
    Project will measure proper motions of 27 galaxies with SIM. GAIA
    can only measure proper motions for M31 and M33, an order of
    magnitude less than can be done with SIM, which will provide a
    robust map of the dynamical environment of the Local Group. Once
    these additional information are 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:***

-   Finally, SIM will demonstrate high-dynamic-range synthesis imaging
    at optical wavelengths in space, thereby showing the viability of
    this approach for the design of the next generation of UV/Optical/IR
    imaging telescopes for space astronomy. Targets of modest complexity
    but showing a wide range of surface brightness will be chosen for
    this demonstration, consistent with the limited number of physical
    baselines available in the SIM instrument. SIM's imaging PI (Ron
    Allen, STScI) 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; no other mission will explore these
    important issues.