The VLA Development Plan addresses the demands of a wide variety of scientific programs for greatly increased sensitivity, much broader frequency coverage, enhanced spectral line capabilities, and better angular resolution. It does so largely by returning the VLA to the state-of-the-art in receiver technology, in the transmission and processing of broadband signals, and in correlator design. The scientific requirements also pose new technological challenges. How can optimum performance (polarization and sensitivity) be maintained across the large bandwidths now proposed? Can broadband, high-performance, low-frequency feeds be designed? What is the optimum way to transmit broadband signals from antennas hundreds of kilometers from the VLA for real-time ultra-high-resolution interferometry?
The impact on astrophysics of returning the VLA to the state of the art near the millenium will be profound. Many hard limitations now constraining VLA observations will be removed or greatly relaxed. The continuum sensitivity will increase by ten-fold in several bands. New frequency bands and increased bandwidth ratios will increase frequency coverage almost three-fold. The bandwidth which can be processed by the spectrometer, and its spectral resolution, will simultaneously increase by about 10-fold. The minimum beam area will improve fifty-fold. Finally, the new instrument, when cross-linked with the VLBA, will result in a VLBI instrument with greatly increased dynamic range, field of view and frequency-scalability relative to the present VLBA.
The VLA Development Plan thus offers far more than an incremental improvement to existing scientific capabilities, though almost all current areas of research done with the VLA will benefit greatly from it. It provides fundamentally new science in many arenas, much of which depends on the cumulative effect of many improvements in the VLA Development Plan, rather than critically on any one of them. For example, high-resolution imaging of stellar thermal emission requires the sensitivity improvements and the A+ configuration; imaging proto-planetary disks requires the 40-50 GHz upgrade and enhanced sensitivity; deep H I surveys require extending the 1.4 GHz band to lower frequencies and the E configuration.
Chapters 2 to 4 below turn to a detailed description of the scientific program which motivates enhancing the VLA. If past experience is any guide, this initial account of the possible scientific program, while exciting, will prove to be far from complete. Some scientific arenas that will benefit from the enhancement may simply have been overlooked, and innovative uses of the improved instrument will not have been anticipated. Nevertheless, we believe that the science described here clearly demonstrates the need for enhancing the VLA's major capabilities and illustrates how all the key features of the enhancement project will be used in practice.
The science discussions also point to areas where technological challenges exist, and to others where tradeoffs - driven by technological and/or budgetary considerations - are inevitable. These areas are specifically addressed in Chapter 5, which also provides illustrative, but as yet by no means definitive, cost estimates. This Chapter is intended to serve as as background for the detailed studies that will lead to a detailed technical design and better cost estimates.
We strongly urge everyone in the VLA user community to add their thoughts on the astrophysical goals and technical challenges of the enhanced VLA to these discussions. This document is intended to evolve into one that makes the scientific case to the NSF for supporting these enhancements. All comments on it will be welcomed. E-mail comments can be sent to firstname.lastname@example.org.