Darwin/TPF-I will have a resolution of 1 AU at a distance of 500 pc, the distance to the Orion massive star forming region. In the nearest regions of low-mass star formation (~125 to 150 pc), 0.25 AU structures will be resolved.

Stars are the fundamental building blocks of the baryonic Universe. Short-lived massive stars and clusters are responsible for the nucleosynthesis of elements heavier than helium, for the UV radiation that re-ionizes the Universe at the end of the cosmic Dark Ages, and for regulating the physical and chemical state of the interstellar media (ISM) of galaxies. Their powerful stellar winds and terminal supernova explosions dominate the generation of random motions in the ISM. Long-lived low-mass stars provide the stable environment needed for the formation of planetary systems and the evolution of life. Star formation is the process that determines the mass-spectrum (Initial Mass Function -IMF; Kroupa 2000) of stars. Star formation determines how galaxies consume their interstellar media, how they convert this material into long-lived low-mass stars, and controls the rate and nature of galactic evolution.

In the standard model of star formation, the inside-out gravitational collapse of a rotating cloud core leads to the formation of a protostellar core on a time-scale of about ten thousand years. The infall of high angular momentum gas forms a spinning, circumstellar disk through which most of the star's final mass spirals onto the protostar on a time-scale of about a hundred thousand years. Entrained and dynamo-generated magnetic fields launch powerful jets and bipolar outflows along the rotation axis of the system for a period of order a hundred thousand to a million years (Reipurth & Bally 2001). Planetary systems form and mature from remnants of the disk in about a million to a hundred million years years; low-mass stars reach the main-sequence (MS) in ten million to over a hundred million years (see the Grenoble stellar evolutionary tracks, Siess, Dofour, & Forestini 2000).

During the last decade, observations have shown that most stars form in highly over-dense, but short-lived clusters that form from the collapsing and fragmenting cores of turbulent molecular clouds (MacLow & Klessen 2004). Furthermore, the birth and early evolution of most stars occurs in the close proximity of luminous, massive stars that irradiate the birth environments with intense UV radiation and which explode as supernovae on time-scales ranging from 3 to 40 Myr, the same time-scale on which planetary systems mature, their central stars reach the main-sequence, and birth clusters and associations disperse (Lada & Lada 2001; Adams et al. 2004). While in the standard model for the formation of low-mass stars, they evolve gradually from accreting (Class 0 and I) protostars into Class II and III T Tauri stars, in the modern clustered formation paradigm, catastrophic events such as dynamical interactions with sibling stars (Reipurth 2000; Tan 2004), evaporation of envelopes and disks by the intense radiation fields (Johnstone, Hollenbach, & Bally 1998), winds, and supernova explosions punctuate early stellar and planetary system evolution (Hollenbach et al. 2000). Contrary to being hazardous, UV radiation in from both the central protostar and nearby massive stars may actually promote the formation of planetesimals by selectively removing light gases and small particles (Throop & Bally 2005).

Massive stars appear to be preferentially formed in ultra-dense proto-cluster environments where cluster -stellar densities higher than a hundred thousand stars per cubic parsec. With a resolution of 2 to 10 mas in the thermal infrared, Darwin/TPF-I will resolve a projected interstellar separation of 10 to 50 AU at a distance of 5 kpc, enabling the cores of embedded proto-clusters to be resolved, and their structure analyzed anywhere in the Galaxy. Does the high multiplicity fraction of massive stars originate in the primordial fragmentation of the parent cloud core, or does it develop later in the evolution of the cluster by means of stellar dynamical processes such as three-body encounters?

Thermal IR interferometry will resolve the acceleration region where stellar winds are produced in pre- main-sequence stars, Wolf-Rayet stars, and giants. Many such systems are in multiple systems where phenomena associated with wind-wind interactions can be directly imaged. Interferometric spectro-imaging of highly ionized species will provide unique diagnostics of these systems that will complement radio wavelength observations.

Darwin/TPF-I will resolve forming super-star clusters and starbursts in our Galaxy and in nearby galaxies. Milli-arcsecond angular resolution will enable TPF to peer inside clusters to determine the volume density of stars and to directly test formation models for the most massive stars in such systems even when the clusters are still highly embedded and their stars are still accreting. Do massive stars sink rapidly to the cluster center due to on-going accretion and dynamical friction? Do massive stars always form by accretion, or do stellar interactions and mergers contribute? The multiplicity fraction of massive stars as a function of location within a cluster, and as a function of cluster age will provide clues.

TPF / Darwin will have the capability to resolve the structure of circumstellar and planet forming disks, trace the shadows cast by the "dust walls" formed at the disk inner edge, spiral waves, gaps created by forming giant planets, and the compositional variations resulting from condensation sequences. The inner radii associated with the evaporation of various ices such as water, ammonia, methanol should be detectable by their characteristic spectral bands. Polarization measurements will enable the measurement of magnetic field geometries by their power to align dust grains. These observations will produce direct tests of planetary system formation models to distinguish between competing paradigms such as core-accretion and formation via gravitational instability.

The ionization and shock fronts produced by ionizing radiation will be resolved and diagnosed by the analysis of spectral-line ratios. The highest resolution observations should resolve the inner regions where jets and disk winds are launched, thereby providing direct tests of outflow generation models. Are jets formed by ordinary stellar winds, the magnetic X-points where stellar magnetospheres interact with the circumstellar disk, or are they launched by magnetic fields entrained or dynamo-amplified in the disk itself? Detection of molecular bands from simple molecules as well as PAH and silicate features will enable the characterization of chemical and physical gradients in disks.

Darwin/TPF-I will be especially sensitive to young planets which tend to be larger, hotter, and therefore brighter than mature objects. Forming gas and ice giants will be easy to detect. Even forming or young, rocky terrestrial planets are expected to be brighter, especially following recent accretion events or impacts.

Observations of more mature planetary systems and debris disks with ages ranging from a few million to several hundred million years will lead to direct tests of planetary system evolution models. The Darwin/TPF-I planet finding capability will enable the direct detection of forming and evolving planets. Large impacts on rocky planets are expected to produce global "lava oceans" that will glow at 10 µm for thousands of years. Thus, direct observations of the conditions following giant impacts and equivalents of the "Late Phase Heavy Bombardment" suspected to have occurred about 800 Myr after the birth of our own Solar System may be observable in other planetary systems. Source selection would rely on the detection of extensive debris disks that may indicate a high rate of collisions.

The Darwin/TPF-I observations of forming and evolving planetary systems will complement the prime mission of planet detection and characterization by providing direct tests of planet formation and evolution models.