Infrared interferometry in space with a constellation of telescopes and an image combiner flying in formation will be a gateway to milli-arcsecond (mas) angular resolution astronomical imaging and spectroscopy of the future. The TPF-I from NASA and Darwin from ESA are the first formation-flying interferometer concepts to be seriously investigated for both technical feasibility and scientific potential. These missions were conceived for a very specific goal - the detection and characterization of terrestrial planets in terrestrial orbits around about 150 spectral-type G stars within 30 pc of the Sun. However, the stringent performance requirements imposed on these missions by planet finding and characterization makes TPF-I/Darwin a powerful tool for many other astronomical applications. Space enables phase-stability (unachievable by an Earth-based system) that will be limited only by the metrology used in establishing the flux-collector-combiner separations. TPF/Darwin will be a technological pathfinder for future micro-and nano-arcsecond resolution instruments at infrared and other wavelengths. Here, we explore the general astrophysics enabled by milli-arcsecond angular resolution and micro-Jansky sensitivity in the 5 to 15 µm wavelength regime with possible extension to wavelengths from 2 to 30 µm.

Darwin / TPF Properties

We assume a baseline TPF interferometer architecture consisting of four free-flying telescopes plus a beam combiner. The apertures are around D = 4 meters, the maximum baselines are around B = 500 meters, 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) of 0.25" to 0.75", and a synthesized beam (interferometric angular resolution) of 2 to 6 milli-arcseconds 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 meters 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 random phased fluctuations introduced by the atmosphere. Even extreme AO systems will exhibit residual phase noise that limits sensitivity. In comparison, space-based interferometry has the enormous advantage of exquisite phase-stability 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.

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 Solar luminosity 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 AGN and extragalactic star forming regions in our locale in the Universe.

A variety of molecular bands and solid-state features 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), 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 an 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 meters or more that are equipped with extreme adaptive optics) may probe the hot gas, dust, and plasma that shines 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 H₂, 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 1,000,000 per cubic cm).

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 H₂, CO, and other molecules, and ices. Extension to longer wavelengths would permit observations of cooler, more embedded targets, provide access to the 24 µm iron complex, the ground-rotational transitions of H₂, the 20 micron silicate feature, and extend the use of PAH, [Ne II], and Brackett α based distance, and metallicity determinations to very high redshifts. Darwin/TPF capabilities will revolutionize our ability to observe the formation and maturation of stars, and 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 microns. High resolution sensitive measurements at 5 - 10 microns 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 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 (HII regions) 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 clouds or an OB association - anywhere in the Universe, enabling the detailed investigation of the cosmic evolution of galactic structure.

Darwin/TPF will open a gateway to future space-based interferometry to deliver ever increasing angular resolution throughout the electromagnetic spectrum. The baseline design will provide order-of-magnitude improvements in angular resolution over any other instrument. Combined with sensitivity to objects as faint as magnitude 20, Darwin/TPF capabilities have the potential of revolutionary advances in all areas of astrophysics and planetary science. This mission will transform our understanding of galaxy formation and evolution, stellar and planetary system origins, the cycles oaf matter and energy in the cosmos, and enable the detailed mapping of surfaces and weather patterns in Solar system objects. The utility of this instrument for general astrophysics and planetary science will only be limited by the lack of available observing time.