The FourierKelvin Stellar Interferometer:

1
The Fourier-Kelvin Stellar Interferometer A
Concept for an Interferometric Mission for
Discovering and Investigating Extrasolar Giant
Planets and a Progress Report R.K. Barry (1),
W.C. Danchi (1), J. Rajagopal (6), V. J. Chambers
(1), L.J. Richardson (7), A.J. Martino (1), R.J.
Allen (2), D. Deming(1), M. Kuchner(1), D.T.
Leisawitz(1), R. Linfield(4), J.D. Monnier(5),
L.G. Mundy(7), C. Noecker(4), S. Seager(7), W.
Traub (3) ____________________________ (1) NASA
Goddard Space Flight Center, (2) Space Telescope
Science Institute, (3) Harvard-Smithsonian Center
for Astrophysics, (4) Ball Aerospace, (5)
University of Michigan, (6) University of
Maryland, (7) Carnegie Institution of Washington
The Fourier-Kelvin Stellar Interferometer (FKSI)
is a mission concept for a nulling interferometer
for the near-to-mid-infrared spectral region (3-8
microns). FKSI is conceived as a scientific and
technological precursor to TPF. The scientific
emphasis of the mission is on the evolution of
protostellar systems, from just after the
collapse of the precursor molecular cloud core,
through the formation of the disk surrounding the
protostar, the formation of planets in the disk,
and eventual dispersal of the disk material.
We present preliminary results of a detailed
design study of the FKSI. Using a nulling
interferometer configuration, the optical system
consists of two 0.5m telescopes on a 12.5m boom
feeding a Mach-Zehnder beam combiner with a fiber
wavefront error reducer to produce a 0.01 null
of the central starlight. With this system,
planets around nearby stars can be detected and
characterized using a combination of spectral and
spatial resolution.
FKSI will answer key questions about extrasolar
planets What are the characteristics of the
known extrasolar giant planets? What are the
characteristics of the extrasolar zodiacal clouds
around nearby stars? Are there giant planets
around classes of stars other than those already
studied?
The Role of FKSI The Fourier-Kelvin Stellar
Interferometer Is a practical interferometric
system for discovering/investigating extrasolar
giant planets Represents a significant near
term step in planet finding characterization
Supports NASA Strategic Plan objectives Serves
as a pathfinder for TPF/Darwin missions To
answer key questions about extrasolar giant
planets, FKSI will Detect gt25 EGPs
Characterize their atmospheres with R20
spectroscopy Observe secular changes in
spectrum Observe orbit of the planet Estimate
density of planet determine if rocky or
gaseous Determine main constituents of
atmospheres Search for exoplanets around nearby
stars not planned for observations with
TPF Survey nearby M dwarfs (gt30 such stars) at
distances within 10 pc Survey nearby F,G,K
giants and subgiants, luminosity classes III and
IV (gt50 stars) within 30 pc Study circumstellar
material Exozodiacal measurements of nearby
stars Search for companions Debris disks,
looking for dust wakes due to planets See star
formation at high angular resolution Evolution
of circumstellar disks Characterize morphology,
gaps, rings, etc.
Engineering Realization of FKSI
Finding Planets with FKSI FKSIs sensitivity is
sufficient to detect and characterize extrasolar
giant planets within 30pc. FKSI is comparable
to other missions in the 5µm range (SIRTF,
JWST), and superior to ground-based facilities.
This is illustrated in the figure below (Seager,
2004 unpublished).
Below is given the predicted flux density of a
giant planet as a function of distance from the
star, in the near-infrared, at a distance of
20pc. FKSI can detect at least 25 planets.

.05 AU .10 AU .50 AU
Simulated FKSI planet detection Below are
simulations of FKSI response as it rotates about
its axis. At a single wavelength, the intensity
varies as a function of this rotation angle.
Ratios of various wavelengths produce the
spectrum of the star, unmodulated, and the
planet, modulated.
FKSI Mission Concept FKSI is a two-element
nulling interferometer. The two telescopes,
separated by 12.5m, are precisely targeted (by
small steering mirrors) on the target star. The
two path lengths are accurately controlled to be
precisely the same. A phase shifter / beam
combiner (via Mach-Zender interferometer)
produces an output beam consisting of the nulled
sum of the planets light and the stars light.
When properly oriented, the starlight is nulled
by a factor of 10-4, and the planet light is
undimmed. Accurate stellar spectroscopy is used
to subtract this residual starlight, permitting
the detection of planets much fainter than the
host star, and at distances less than the 0.005
required for a similar imaging interferometer.
Operational Fourier-Kelvin Stellar Interferometer
testbed.
Simulated FKSI planet detection Below are images
of a model zodiacal wake about Epsilon Eridani
with source brightness distribution generated
using Zodipic.pro (Kuchner, M. J., Serabyn, E.
2001) and instrument response simulated using
software developed by Jay Rajagopal.
Simulated image at 5 microns but with the ring
and wake enhanced by 100. The main features are
recovered in spite of significant foreground
local-zodi emission.
Simulated image at 10 microns. The
reconstruction includes thermal noise from
mirrors at 40 K and emission from the local zodi.