A major astronomical goal of the 21st century will be an understanding of how stars and planets form. The large collecting area and high resolution of the LSA/MMA will make it the best instrument for studying how gas and dust evolves from a collapsing cloud core into a circumstellar disk that can form planets. The array will be able to directly observe astrophysical phenomena that have until now only been conjectured in our theoretical models of the early stages of star formation. At the distance of the nearest young stars, in Taurus and Ophiuchus, the synthesised beam of the LSA/MMA -- 4 AU at 1.3 mm -- will allow us to constrain different models of forming stars. The data will yield new unique information on the gravitational contraction of protostellar cloud cores, with accurate kinematics and mass distributions inside the cores and their envelopes. The results will give new clues to the role of the magnetic field in the cloud cores, the circumstellar disks, and its influence on the accretion disk and the outflow jets that carry away the original angular momentum. A good example of bipolar outflow jets is shown in Fig. 2.3, which is a map of the high-velocity CO gas streaming out in two opposed jets of the source HH 211. The flow emerges from an embedded protostar surrounded by a disk of cold dust that is detected in the 1.3 mm continuum at the center of the flow. At their terminal points, the lobes of the jets produce shocks that emit in the near infrared lines of molecular hydrogen.
For the later stages, when the newly-formed stars are surrounded by
protoplanetary disks, imaging the gas and dust on scales of few tens
of AU will be the only way to study the disk physics and chemistry in
the earliest stages of planet formation. Current mm arrays do not
have small enough beams to study the physics of the inner disks
(inside the 
central AU), and even the rotating molecular
disks recently resolved by mm arrays around single T Tauri stars like
DM Tau and GM Aur are actually very large structures that would be
beyond the Kuiper Belt in our Solar System -- well beyond the orbits
of all the planets. The necessary resolution will be provided by the
LSA/MMA, and maps of cold dust and optically thin molecular lines with
0.1'' to 0.05'' beams will provide crucial data on the chemistry,
the gas/grain coupling, the reservoirs of the biogenic elements, and
the timescales on which planets form. The high sensitivity of the
array will allow us to make unbiased surveys of pre-main sequence
stars to obtain the statistics of disk properties and frequency of
protoplanetary systems in different star-forming regions.
For such studies, the LSA/MMA and the VLT will be complementary in the same sense as the current 4m optical telescopes and the IRAM mm array, examples of which are given by the recent combined imaging with the CHFT and the IRAM array of the low-mass, pre-Main Sequence binary star GG Tau. The near-IR images (Roddier et al., 1996) and mm data (Guilloteau et al., 1998, in prep.) show the circumbinary ring of GG Tau (Fig. 2.4) which is truncated by tidal effects at an inner radius of 1.2''. These results provide a tantalizing preview of what will become possible with the next generation of telescopes, which will have 10 times better resolution in the mm/sub-mm and the near-IR/optical ranges. This will enable the LSA/MMA to resolve, within proto-planetary disks, the gaps that are tidally cleared by Jovian sized planets at distances of a few AU from their young, central stars. With the LSA/MMA and the VLT, multi-wavelength studies of such objects will be powerful tools for analysing the dust and gas properties on the scale of the Solar System.
The LSA/MMA will also allow us to study disks around newly-formed, higher mass stars, which has been impossible up to now, due to lack of resolving power. An example of the strong signals we can expect is shown in the VLA map of the ammonia (4,4) main line emission around the young high-mass star that is exciting the ultracompact H II region G10.47+0.03 (Fig. 2.5). The brightness temperatures of the NH3 line in the 0.4'' beam range from 20 to 180 K. Because similar temperatures are found in optically thick 3 mm lines of molecules like methyl cyanide (CH3CN), we expect from the sensitivities listed in Table 2.1 that the LSA/MMA will be able to image structures as small as 0.1'' without much difficulty.
