The 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 broad-band 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 broad-band, high-performance, low-frequency feeds be designed? What is the optimum way to transmit broad-band 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 current state of the art 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 ten-fold. The minimum beam area will improve a hundred-fold. Finally, cross-linking the upgraded VLA with the VLBA will produce a VLBI instrument with greatly increased dynamic range, field of view and frequency scalability relative to the present VLBA.
The thus offers far more than an incremental improvement to existing scientific capabilities. Almost all areas of research now done with the VLA will benefit greatly from it. It also provides fundamentally new science in many arenas. Much of the new scientific capability depends on the cumulative effects of many improvements in the , 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 protoplanetary disks requires the 40-50 GHz upgrade and enhanced sensitivity; deep surveys require extending the 1.4 GHz band to lower frequencies and the new correlator.
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 will almost surely have been overlooked, and innovative uses of the improved instrument will not have been anticipated.
We 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 newvla@nrao.edu