Stars are born in cold interstellar cloud cores that are so optically thick they are undetectable even in the mid-infrared. After 105 years, a young star emerges, ejecting material along powerful jets and still surrounded by a circumstellar disk. The subsequent evolution is increasingly well studied, but the birth of the star has occurred hidden from view. How does the cloud core collapse? How does subfragmentation occur to produce binary stars? What are the conditions within protoplanetary disks? When, where, and how frequently do these disks form planets? Imaging with the resolution provided by SAFIR <100 AU at 40um for the nearest star forming regions) can probe the density, dynamics, and temperature structure of these ~1000 AU collapsing cores on critical physical scales. In addition, 100 AU resolution will reveal the steps toward binary formation. Far-infrared polarimetry is a powerful probe of magnetic field geometries, both for studying core collapse and mapping the fields that must play an important role in accelerating and collimating jets. Spectroscopy in molecular lines such as H2O and the J>6 high excitation rotational lines of CO, as well as in far-infrared atomic lines, can probe the physical and dynamical conditions in the collapse. Models of collapsing cloud cores (Ceccarelli, Hollenback, & Tielens 1996) show that the [OI] lines have narrow components from the infalling envelope and broad ones from outflow shocks. They are the main coolant of the gas in the intermediate regions of the cloud. Bright H2O lines between 25 and 180um are the dominant coolant in the inner cloud, where a broad component is expected from the accretion shock and a narrow one from the disk. The CO lines from 170 to 520um are the main coolant for the outer cloud; warmer CO from within the cloud can also be studied because of velocity shifts due to the collapse. This suite of lines therefore would allow us to probe thoroughly the kinematics and energetics process of star birth. The combination of angular resolution and sensitivity made possible with SAFIR is essential to these studies.
What were the conditions in the early solar nebula, as the protoplanetary disk formed and planets and small bodies accreted out of it? All the bodies in the inner solar system have been so heavily processed that they no longer reflect the conditions at their formation. The discovery of many small bodies in the Kuiper Belt outside the orbit of Neptune gives access to objects where accretion proceeded slowly, leaving products that are primitive and still reflect conditions in the early solar nebula. There is a large population of Kuiper Belt Objects (KBOs), including objects of size rivaling the largest asteroids. They have a broad variety of surface characteristics. To interpret the clues they provide for evolution of the solar system requires that we understand how this variety of surface chemistry has come about. Two very important parameters are: 1) the albedoes of the surfaces (important to help identify the substances that cover them); and 2) surface temperatures (to understand what chemical reactions can occur and determine the escape rates for different molecules). Both of these parameters can be determined in the far-infrared through measurements of the thermal emission. It is for this reason that the 1998 National Academy of Sciences study on “Exploring the Trans-Neptunian Solar System” placed a very high priority both on large, far-infrared telescopes and on development of high performance far-infrared detector arrays, as are planned for SAFIR. The Kuiper Belt is thought to be the source of short period comets and hence has a central role in the comet impacts that brought water to the earth and made life possible here. Most traces of this process, however, have been erased by time. How can we understand the conditions that regulated the early formation and evolution of the KB and its release of comets toward the inner solar system? The Infrared Astronomy Satellite (IRAS) discovered debris disks around Vega, Pic, and other stars, with evidence for inner voids that might have resulted from planet formation. Many more will be discovered by SIRTF. The Kuiper Belt is similar in many ways to these systems and is interpreted as the debris disk of our solar system. Taking an example, Pic is thought to be only about 20 million years old. Transient and variable absorptions by the [CaII] H and K lines in its spectrum have been interpreted as the infall of small bodies from the debris system( Beust & Morbidelli 2000). This system contains small grains that heat sufficiently to be detected in the mid-infrared and scatter enough light to be seen at shorter wavelengths. Because it should be drawn into the star quickly, this fine dust must have been produced in recent collisions between planetesimals. Thus, this system and others like it demonstrate the potential of examining the early, violent evolution of debris disks and the infall of comets. Debris disks are bright in the far-infrared, where they can be imaged to identify bright zones due to recent planetesimal collisions, as well as voids. The radial zones that are sampled will vary with wavelength, from a few AU near 20um to hundreds of AU in the submillimeter. Figure 2 illustrates the potential advances with SAFIR. Spatially resolved spectroscopy can probe the mineralogy of the debris disks in the 20 - 35um region where ISO has found a number of features diagnostic of crystalline and amorphous silicates, and can locate ice through its 63um emission feature. Giant planets similar to Jupiter and Saturn could be detected to compare their placement with the debris disk structure. The latest community consensus for solar system astronomy from the National Research Council(NRC Solar System Exploration Survey 2002) identifies studies of the outer solar system and the Kuiper Belt debris disk as a top priority for mission development. In addition to analyzing the mission targets directly, SAFIR will complement this effort superbly by enabling comparative studies of debris disks around other stars.