Young stars are expected to be surrounded by proto-planetary disks, flattened structures of gas and dust from which planets form. In the optical, disks such as that observed with the Hubble Space Telescope in HH30 can only be observed in silhouette against bright nebulae since they are totally opaque to optical photons. The enhanced VLA, however, will not only provide three times better angular resolution than the HST, but will be able to study the inner parts of these disks which are obscured at optical wavelengths. In particular, the expanded VLA is perhaps the only instrument capable of observing the inner few AU of the disk, because the dust present in these objects is optically thick for shorter wavelengths, and thus observations at these wavelengths will ``see'' only the outer parts of the disk. The enhanced VLA may be the best instrument for penetrating all the way down to the region where planet formation occurs.
In the earliest stages of evolution of low mass stars, we expect material to accrete through a disk on size scales of 100 AU, which can be imaged with the VLA at 7mm. Images at shorter wavelengths will reach high optical depths too quickly for viewing the critical inner region. A similar argument holds for proto-planetary disks, though here the total mass might be two orders of magnitude less for a minimum solar mass nebula. As a result, optical depth unity at 7mm occurs at , a scale which the current VLA can just start to resolve. However, one should be be able to image sufficiently bright sources with the expanded VLA on longer baselines.
The enhanced VLA will be able to explore gas kinematics in proto-planetary and proto-stellar disks with spatial resolution currently impossible with other instruments. Consider a constant density sphere filled with gas and dust of standard abundance ratios. Such a sphere will have a optical depth of unity (due largely to dust) at 7mm at a radius AU when it contains 1 solar mass of material. This scale corresponds to for objects in the Ophiuchus cloud. Thus, images in the 45 GHz band with sub-arc-second resolution of such an object should reveal processes occurring on solar system scales. At 3mm, an optical depth of unity would occur for the same object at a radius of several hundred AU and the critical inner region would be obscured from view!
Present estimates suggest that within 200 pc of the Sun there are about 100 proto-planetary disks with total flux densities of order order of 1 mJy at 7mm. The disks are expected to have diameters of about 100 AU ( at 200 pc). In the A configuration an angular resolution of can be achieved in the 45 GHz band, equivalent to a dimension of 10 AU. Within a beam of , a flux density of mJy/beam is expected.
Completion of the 40-50 GHz system will provide a factor of 36 improvement over the present 9 antenna system (a factor of 3 for all 27 antennas, 2 from improved aperture efficiency, 3 from a total bandwidth of 2 GHz, and 2 from better receivers). In observing time, this represents an improvement of 1300! For example, in a 12-hour on-source integration, the VLA will achieve an rms noise of 5 microJy/beam (compared to 0.2 mJy/beam currently achievable), making it possible to image the disks with a signal-to-noise ratio of 20 in this time period. At 40-50 GHz and with the largest VLA configuration, a technique to correct for atmospheric phase noise becomes imperative. Using any nearby masers as a phase reference may require simultaneous observations with two different receiver systems. Alternatives such as fast switching or accurate total power measurements should be explored to provide phase stability.