Late-type giant and supergiant stars show maser emission in molecular transitions of hydroxyl, water, and silicon monoxide. These transitions are found at frequencies ranging from 1.6 GHz all the way to 43 GHz. The molecules are found in the circumstellar envelope produced by mass loss from the dying star. The masers appear in a somewhat hierarchical distribution from the central star. Hydroxyl masers are found far out in the shell, and the silicon monoxide masers are found quite close in to the photosphere; water masers are usually found somewhere in between. Since these are masers, they are bright and small, thus serving as excellent tracers of the dynamics of the circumstellar shell. These masers are similar in character to interstellar masers, but are usually somewhat weaker, and the maser ``spot'' sizes are somewhat larger than those of interstellar masers. Much work has been done on these sources using both connected-element and VLB interferometers, giving a reasonably well defined picture of the structures of circumstellar shells.
But there is much more to be done, and the enhanced VLA can contribute substantially to understanding of such masers in circumstellar shells. Much of the science would be dependent upon filling the VLA/VLBA ``gap'' (see below), but first we address a few studies which could be done without that the VLA Expansion. One example is the detection of the photosphere in the radio continuum by phase referencing to the masers. High frequencies (the 22.5, 33, 45 GHz bands) maximize the sensitivity to the continuum since the emission is thermal (see figure 2). The HO masers at 22 GHz, and SiO masers at 43 GHz allow tracking of the phase, removing the effects of phase distortion from the Earth's atmosphere and thereby allowing the expected rms noise to be reached. An upgrade of the 22.5 GHz receivers, with a large continuum bandwidth will allow a factor of 10 improvement in the continuum sensitivity. This limit can be reached by phase referencing to the maser emission. This technique has been used successfully for studies of a few stars, but the Ultra-Sensitive Array will allow many more stellar photospheres to be detected, mapped, and placed within the distribution of masers. The same technique could be used in the 45 GHz band, and outfitting all of the antennas, coupled with large continuum bandwidths, will allow the detection and mapping of the same photospheres at this frequency. Using the frequency agility of the VLA will allow relative position measurements of the HO , SiO, and the photospheric emission. Another example of an observation that could be made with the Ultra-Sensitive Array is astrometric observations of the SiO and HO masers. Having the whole VLA operating at 45 GHz will allow much improved astrometry of the SiO masers. For the 22.5 GHz band, upgrading the sensitivity of the receivers will allow more, and closer, calibration sources to be used. A final possibility that takes advantage of the frequency coverage is observing excited states of OH (at 5 GHz and 6 GHz) and sensitive searches for other maser lines (e.g. methanol) in these circumstellar shells.
The expansion to the A+ configuration opens up a large range of projects which would enhance our understanding of the dynamics and structure of the shell. VLBI observations of SiO masers at 43 GHz using the VLBA show clear ``ring'' structures formed from the maps of SiO maser spots. However, the flux density obtained from such high resolution observations is that obtained from single-dish observations. Thus half the flux density must be found on scales larger than those corresponding to the shortest projected baseline from the VLBI observations. A recent lunar occultation experiment found the missing flux for one source-R Leo-on roughly the scale of the ``ring'' itself. Thus the baselines between the VLA A configuration and the VLBA are optimal for probing these structures. Another example of the use of the A+ configuration is the polarization observations of OH in the far reaches of the circumstellar shell. The resolution of the VLA in the A configuration just barely resolves the shell structures but VLBI data resolve most of the larger scale structure. Even with the A configuration observations, the spectral resolution of the current correlator is too coarse (the best that can be done at 18cm is 0.5 km/s, when using full Stokes' polarimetry). We need both the VLA/VLBA extension and higher spectral resolution (at least 0.1 km/s) to resolve the maser emission spectrally and spatially. Polarization observations throughout the circumstellar shell are important because they can give significant information about magnetic fields and/or the maser process itself.