12mm is ALMA's Band 0 - the missing piece from the jigsaw that ALMA is going to fill in for the millimetre-wave Universe. It is also a band in transition, from molecular cloud astrophysics to the physics of the ionized medium. Ammonia is the great molecule in this band, the best probe of the dense molecular medium, unlike CO which traces the diffuse regions of molecular clouds. Measurement of the various rotational lines in the 12mm band also provides an immediate measure of the density and temperature of the emitting gas, parameters which are much harder to determine using CO lines. Ammonia emission is, in general, extended, which greatly complicates the interpretation of emission maps made by an interferometer like ATCA. Parkes will also have better than twice the resolution of Mopra at 12mm, and six times the collecting area. Parkes is thus well suited for sharper, sensitive imaging of ammonia clouds mapped by Mopra through the HOPS survey. Parkes will be able to zoom into the Mopra ammonia maps in a similar manner to the way Mopra is zooming into the NANTEN CO maps of the galactic plane.

Ammonia mapping at Parkes may also be used to investigate the most energetic phenomena in astrophysics, gamma-rays induced by ultra-high energy cosmic rays, which use dense molecular cores as their targets to roduce the particle collisions. It is actually unclear whether cosmic ray diffusion will limit the penetration of medium-energy cosmic rays into cloud cores, to be targeted by the GLAST mission. By contributing to the determination of the density structure of molecular clouds, in particular the relative proportions of low density (i.e. CO) and high density (i.e. NH3) gas, which is partially responsible for the observed gamma ray spectrum, Parkes might contributing to the understanding of the physics behind cosmic ray acceleration to the highest energies.

The 12mm band also hosts a plethora of molecular lines, particularly from long carbon-chain molecules with large dipole moments. Sensitivity is needed to probe the amazing astrochemistry underway, as the lines are weak and the molecules rare. Limited work has been done in this regime as a result, using giant telescopes like the Nobeyama 45m and the Greenbank 100m. Parkes can contribute much to this field, in particular because the most interesting galactic sources for such studies all lie in the South.

Mapping investigations at 12mm would be facilitated by the use of a multibeam receiver array, as they also would at 3mm with Mopra. A Mopra-style wide-band, high-spectral resolution correlator would allow the simultaneous measurement of the ammonia rotational lines, and so to mapping the density and temperature structure of cloud cores. It would also greatly enhance the study of carbon-chain chemistry, as it is essential to measure many lines to provide a handle to understand what species are present, and what chemistry is actually taking place.