Jian-Yang Li, University of MarylandMarc Kuchner, NASA Goddard Space Flight CenterRon Allen, Space Telescope Science InstituteScott Sheppard, Carnegie Institution of Washington, DTM

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• Precision astrometry on stars to V=20.• Optical interferometer on a 6 m structure.

– One science interferometer.– One guide interferometer (4.2 m baseline) and one precision

guide telescope to stabilize the fringes.• Global astrometric accuracy: 4 µas.

– At end of 5 year mission lifetime.• Narrow-field astrometric accuracy: 1 µas, in a single measurement.

– Current state of the art is HST/FGS at ~500 µas.– Ground-based differential astrometry will reach ~20 µas.

• Typical observations take about 1 minute; ~ 5 million observations in 5 years.

RV will press on icy planets and close-in planets.

Transit & microlensing will provide statistical census of rocky planets (e.g., Kepler @ 1 kpc, and microlensing @ 5 kpc.

Together with some other instruments, SIM probes 1-10 MEarth(0.4 - 6.0 AU for nearby stars). It provides orbital parameters and masses for RV planets.

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The outer solar system objects (KBOs and Centaurs) will help us understand the characteristics and formation of big planets far away from the star

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D=34.3 mas

D=40.4 mas

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Brown et al. 2006

Shape+PeriodDensity

T

Radius

Size Escape Velocity

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Density+shape => internal structure

Iapetus: Brightness variations

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109 KBOs + 26 Centaurs in total:

20 KBOs and 18 Centaurs are identified to be observable for SIM

SIM can measure the:•Size•Shape•Rotational status

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2 Jinc(2πR)

Three visibilities, solve for two axes and one angle

B1

B3

B2

B1

B2 B3

Inverse jinc function to get the size of a circular disk with uniform brightness distribution

Rotation period 3.92 hr (Rabinowitz et al., 2006) Jacobi ellipsoid

Axial ratio b/a=0.86, c/a=0.54 (Lacerda and Jewitt, 2007)

Density 2551 (+115, -10) km m-3 (Lacerda and Jewitt, 2007)

Mass from two satellites Size 1960-2500 km (Rabinowitz et al., 2006) Albedo ~0.6 Dark red spots from photometric lightcurves (Lacerda

et al., 2008) Almost pure water ice on the surface (Trujillo et al.,

2007) Strong limb-darkening, k=0.85

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• 1% uncertainty of visibility from SIM• Size can be determined to better than 5% uncertainty

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SIM can achieve ~10 µas in visibility phase

SIM tracks the photocenter of the object precisely.•Rotational period•Pole orientation•Sense of rotation

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Complicated pattern for a surface with albedo features:•Not a problem for period determination•Should not be a problem for pole orientation determination•May be used to track surface features

k=0.4

k=0.5

k=0.8

Low albedo objectsSSA < 0.2, Moon

Minnaert limb-darkening model:I/F = A cosk (i) cos(k-1) (e)

High albedo, high roughness objectsSSA = 0.99, Θ=50 deg, g=0

High albedo, low roughness objectsTriton: SSA=0.99, Θ=14 deg, g=-0.38

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• Stellar limb-darkening from simultaneously visibility measurement at multiple λ/B (e.g. Quirrenbach et al., 1996)

• Limb-darkening from k=0.5 (uniform disk) to k=1 (strongest limb-darkening)

• Visibility uncertainty 1%

• Simultaneously measurement of visibility at multiple wavelengths from 0.4-0.9 µm

• Guide interferometer can be used to extend the λ/B range

Outer solar system object help us understand planetary system formation at large distances from the star.

Fundamental properties of KBOs and Centarus, such as their sizes, shapes, and rotations, can only be measured to be good at 20% level from radiometry.

SIM can observe about 40 KBOs and Centaurs Better than a few percent in size Determine 3-D shape Constrain rotational status, including pole orientation

and sense of rotation Limb-darkening can be measured by SIM through

simultaneously visibility measurements at multiple λ/B.

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