Molecular lines with enhanced blue shifted emission, believed to be indicative of gravitational infall, have recently been detected towards several young low luminosity stars including the well known source B335. However, the infall interpretation of such spectral features is not unique. This is especially true as the circumstellar regions of young stars are probably not the simple, radially collapsing regions which have been assumed when modeling these infall sources. For example, all young stars have powerful stellar winds which have sufficient energy and momentum to alter the structure of the circumstellar regions significantly.
In order to understand the processes at work close to young stars and to investigate whether the observed line asymmetries are indeed due to gravitationally accreting material, detailed studies of the circumstellar material around young stars are necessary. Important aspects of such studies are identifying the precise location of the young stars, sufficiently high spatial resolution to probe the circumstellar regions, high spectral resolution to map the velocity field and the ability to distinguish infall from the effects of outflow. These are all areas in which the enhanced VLA will make new and significant contributions.
An important signature of gravitational infall is that material closest to the central young star will have the highest infall velocities. However, the precise location of young stars is often uncertain. Simultaneous spectral and continuum observations offer the best way to accurately determine the relative location of the central star (traced by its continuum emission) and the circumstellar dense gas (traced by the spectral line emission).
For a 1 solar mass star, the infall velocity only exceeds 0.5 km/s within 1800 AU of the star, which corresponds to in Orion. The VLA can obtain angular resolutions which well sample such small regions. Obtaining resolution images of young stars in Taurus ( = 140 pc) will probe material within 28 AU of the central star.
The 45 GHz band (7mm) contains transitions of many molecules which, because of their excitation, provide probes of the material which is infalling but has not been heated by the central young star. Some molecular species, such as CS and SO, trace material which participates in the outflows from young stars or has been disturbed by the outflows. Simultaneous observations of one of these species and a transition of a molecule which does not trace the outflow but traces the cooler infalling circumstellar material, such as HCO, HCN or CH, will provide the means of identifying and mapping the properties of the cool infalling material and distinguish it from the outflowing material.