The broad lessons to be drawn from the history of radioastronomy are that large, general-purpose instruments, such as the LT, have dominated the discoveries made by the discipline. By the nature of research, many observing programmes involve working close to current performance limits. Hence raw sensitivity, resulting from a combination of collecting area and the best-available receiving equipment, is a sure-fire route to success. The science case concentrates on the new astrophysical potential which can be unlocked by extending the highest frequency at which the LT's large collecting area can be brought into play. However, it is important to stress that, by securing the long-term viability of the LT, the upgrade will also ensure that the present range of world-leading work can be continued. Some outstanding aspects of this current work are touched on at the start of the main science case.
A common thread, much of it new to LT studies, runs through this future work - the life-cycles of stars in our own Galaxy and throughout the Universe. The upgraded-LT will provide new insights into the many processes involved: from starbirth in collapsing dark clouds of molecular gas, to the collimated jets from single young stars, to the copious chemically-enriched winds from old red-giant stars, to the powerful interactions of stars in binary systems, and finally to the violent explosions seen as novae and supernovae which mark the last phases of the lives of many stars. The embers left behind by supernovae are pulsars, neutron stars spinning many times per second. Supernovae and their remnants, can be studied in distant galaxies and by this means a picture can be built up of the overall history of star-formation over the lifetime of the Universe.
A search for pulsars near the centre of the Galaxy
Dispersion and multi-path propagation effects arise from the increased density of ionized gas near the centre of the Galaxy and make searches for pulsars close to and beyond the galactic centre impossible at the wavelengths currently accessible to the LT. Working at shorter wavelengths will allow the LT to be used for searches in the region where the density of stars is also highest. The pulsars in or behind this region have a significant probability of being gravitationally micro-lensed and precise pulse timing can yield strong constraints on the amount of stellar matter in these regions which are obscured to visible light.
A survey for faint extragalactic radio sources and new radio stars
Working at 5 GHz with the 25-m Mk2 telescope on the Jodrell Bank site, the upgraded LT can make a definitive (ten times deeper than is presently available) large-area survey for faint extragalactic radio sources. New surveys are the way to discover new things, as was exemplified by the discovery of the first gravitational lens as a direct result of survey work carried out with the LT-Mk2 combination. A second definitive survey can be made to even fainter flux levels in restricted areas, in order to establish the types of galactic star which show radio emission at a given luminosity.
Studies of stars at different phases in their evolution
An unusual amount of radio emission is a signature of a star currently undergoing changes. From the collapse of clouds in starbirth, to the interaction of binary companions, to the copious chemically-enriched winds from red giants and finally to a planetary nebula or supernova, MERLIN and the EVN are ideal instruments for investigating these milestones in stellar evolution. Radio observations are often the only means of probing the interaction of stars with their surroundings, which are usually cool or obscured at other wavelengths by dust clouds. Stellar radio emission is usually quite weak and hence the scientific return of current arrays can be greatly increased simply by increasing their sensitivity. The proposed enhancement of the LT does exactly that at wavelengths ideal for stellar radioastronomy. The full science case gives details of the many types of stellar systems which will be accessible to the LT-enhanced imaging arrays.
Starburst Galaxies and the rate of star-formation in the Universe
Almost all galaxies go through a relatively brief period when they form large numbers of stars rapidly - the "starburst" phase. The most massive stars live for a very short time and then explode in supernova events whose remnants are ideally-suited for study with high-resolution imaging arrays like MERLIN and the EVN. In the visible waveband, starburst galaxies are often obscured by dust and hence are hard to observe. Each starburst galaxy is a laboratory in which to investigate the formation of massive stars. By measuring the sizes and expansion rates of supernova remnants, one can place strong constraints on the current formation rate of massive stars in nearby galaxies. This calibration of the local universe is important for our understanding of the rate of star-formation in the early universe where many galaxies are seen to be undergoing a massive burst of star-formation. The required high-resolution radio observations are sensitivity-limited both locally and at high redshift. The addition of the upgraded LT into MERLIN and the EVN will therefore have a great impact in this exciting area which is also a target for the Next Generation Space Telescope and the international mm-wave-array.
Gravitational lenses provide a unique way of probing the distribution of matter in the Universe at cosmologically-important distances. Detailed studies of individual lens sytems enable the mass distributions of the galaxies acting as the lenses to be determined. With the aid of this knowledge, measurements of the time delay between intensity variations in different lensed images can be used to determine Hubble's Constant and hence the overall scale of the Universe. For these detailed, high-resolution studies of gravitational lens systems, imaging sensitivity is at a great premium. The extra sensitivity provided by the upgraded LT at the international-standard frequency of 5 GHz will allow MERLIN and the EVN to make definitive measurements on many of the known lenses.