TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 3
3.1.1 Darwin/TPF-I Properties
We assume a baseline TPF-I architecture consisting of four free-flying telescopes plus a beam combiner.
The apertures are approximately D = 4 meters, the maximum baselines are approximately B = 500 m,
and the operating wavelength range is between 5 and 15 μm with a spectral resolution of at least R = 50.
Although the current prime-mission (planet finding) requires a nulling interferometer configuration, we
will assume that imaging interferometry without nulling will also be possible. Additionally, we will
consider upgrades in spectral resolution, wavelength coverage, baseline length, and multi-beam
interferometry of two or more objects distributed over the field-of-view of each telescope.
These parameters translate to a primary-beam (diffraction spot size of each telescope) θ P = λ / D = 0.25”
to 0.75”, and a synthesized beam (interferometric angular resolution) of θ I = λ / B = 0.002 to 0.006” (2 to
6 milli-arcseconds or mas) over the assumed operating wavelength range of 5 to 15 μm and B = 500 m.
The sensitivity is about 20th magnitude, and may be considerably better if the coherence time can be
improved.
Extension of the operating wavelength range down to 2 μm (the wavelength beyond which thermal
emission from the atmosphere makes ground-based observations difficult for all but the brightest
sources), or extension of maximum baselines to 1,000 m would enable an angular resolution of 1 mas to
be reached or exceeded. Note however, that excess thermal noise introduced by the increasing visibility
of the thermal shields of the telescopes on long baselines will likely result in loss of sensitivity and
contrast. Studies are therefore needed to assess the maximum baselines that can be used. The nulling
mode will be useful in the study of the environments of bright object such as quasars, stars, and luminous
pre- and post-main sequence objects, such as the Becklin-Neugebauer object in Orion or the massive post-
LBV, eta-Carinae. Multi-object interferometry will enable the precise determination of relative positions,
parallax, and proper motions.
Ground-based interferometers suffer from the randomly phased fluctuations introduced by the
atmosphere. Even extreme adaptive optics (AO) systems will exhibit residual phase noise that limits
sensitivity. In comparison, space-based interferometry has the enormous advantage of exquisite phasestability
limited only by metrology and path-length-difference compensation errors. On-the-fly recording
of fringes will enable excellent sampling of the u-v plane required for high-fidelity imaging of complex
sources.
3.1.2 Diagnostics in the TPF-I/Darwin Bands
The wavelength region between 5 and 15 μm is rich in diagnostics for probing physical conditions in
astrophysical environments. Emission from warm dust in the range 100 to 1,000 K, the characteristic
temperature of grains located in and near the habitable zones of stars (about 0.3 to 3 AU for Solarluminosity
stars) peaks in this spectral domain. TPF-I/Darwin will be the most powerful probe of dust in
star forming-cloud cores heated by young stars and clusters. This instrument is ideally suited for imaging
of the region from 0.1 to 10 AU where planets form around Solar-mass stars located within a few hundred
pc of the Sun, warm dust located from 10 to 1000 AU around massive stars located anywhere in the
Galaxy, and the dust surrounding the Active Galactic Nucleus (AGN) and extragalactic star-forming
regions in our locale in the Universe.
40