TPF-I SWG Report - Exoplanet Exploration Program - NASA

G ENERAL A STROPHYSICS
A variety of molecular bands and solid-state features (arising from grains and ices) will provide powerful
diagnostics of composition and molecular structure (amorphous vs. crystalline). This wavelength region
contains bands produced by PAHs polycyclic aromatic hydrocarbons (PAHs), the bands of amorphous
and crystalline silicate dust around 10 μm, and a variety of ice features due to water, carbon monoxide,
carbon dioxide, and methanol, molecular vibrational bands of many common organic and inorganic
substances, fine structure lines of many elements and ions, and the spectral lines of atomic and molecular
hydrogen.
TPF- I/Darwin will be highly complementary to giant ground-based facilities being deployed during the
next decades. The ALMA (Atacama Large Millimeter Array) will probe molecules and cold dust in the
outer portions of protostellar environments and disks beyond 10 AU, but with only 0.05” to 0.5”
resolution. Future ground-based ELTs (Extremely Large Telescopes with apertures of 30 m or more that
are equipped with extreme AO) may probe the hot gas, dust, and plasma that shine below a wavelength 2
μm with a resolution approaching 0.01”. While ELTs will probe stars and plasmas at a narrow band
centered at 10 μm, TPF-I/Darwin will be uniquely suited to investigate warm dust, ices, molecules, and a
variety of atomic and ionic species with at least an order of magnitude better angular resolution over the
much wider spectral range of 5–15 μm. TPF-I/Darwin is especially well suited for probing in the
planetary region between 0.1 and 10 AU around forming, maturing, and dying stars with more than an
order of magnitude better angular resolution than any other conceived facility.
The TPF-I/Darwin spectral domain contains the lines of may ions and atoms (H, He, Ne, Ar), including
several ionization stages of hard-to-deplete noble gases, the rotational and vibrational transitions of a
variety of molecules including H 2 , forbidden fine-structure lines, continua from dust, and a variety of
solid state features from ices and PAH molecules (at wavelengths of 6.2, 7.7, 8.6, 11.3, 12.7, 14.2, and
16.2 μm). Combined, these tracers can be used to map temperature, density, metallically and kinematics
of gas at intermediate to cold temperatures (10 to over 10,000 K) and moderate densities (10 to over 10 6
cm -3 ).
Extension of the wavelength coverage from about 2 μm to as long as 30 μm should be considered.
Extension to the shorter wavelengths would permit overlap with ground-based AO-assisted
interferometry, improved resolution, and access to the vibrational transitions of H 2 , CO, and other
molecules and ices. Extension to longer wavelengths would permit observations of cooler, more
embedded targets; would provide access to the 24-μm iron complex, the ground-rotational transitions of
H 2 , and the 20 micron silicate feature, and would extend the use of PAH, [Ne II], Brackett α-based
distance, and metallicity determinations to very high redshifts. Darwin/TPF-I capabilities will
revolutionize our ability to observe the formation and maturation of stars, planetary systems, and star
clusters ranging from loose associations to super-star clusters that evolve into globular systems.
The spectral energy distributions of normal nearby galaxies peak at a rest-frame wavelength of a few
microns. For galaxies at high redshifts, this peak will be shifted to observed wavelengths of 5−10 µm.
High resolution sensitive measurements at 5−10 µm are crucial for tracing the formation and evolution of
high redshift galaxies. At the highest redshifts, rest-frame visual wavelength emission will fall into the
Darwin/TPF-I windows. Therefore, this mission will diagnose the very first stars and galaxies to emerge
from the “Dark Ages” of the Universe with an angular resolution sufficient to resolve the ionized bubbles
they create (star-forming regions of ioninzed hydrogen, HII) and other global properties. A central
feature of TPF-I/Darwin is its ability to resolve a length scale of order 100 pc – the size of giant molecular
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