Astronomical Interferometry at Submillimetre Wavelengths
This thesis describes my work on the project to connect the James Clerk Maxwell Telescope (JCMT) and the Caltech Submillimeter Observatory (CSO) to form the world's first astronomical interferometer operating at submillimetre wavelengths. Included are my contributions to the technical design and construction of the instrument and to the subsequent debugging and calibration, and my analysis of the data that was obtained from a number of astronomical objects.
I was responsible for much of the system that brings the signals from the receivers at each antenna to the digital correlator at the JCMT. A key feature is that the signals are carried extensively in single-mode optical fibre. The design of the delay line system is presented; this helps to equalize the travel times from the source to the correlator by inserting different lengths of optical fibre into the two signal paths. Also described is the use of a microcomputer to control a pair of oscillators, where accurate timing is necessary to ensure that a source is tracked correctly as the Earth rotates.
Careful calibration is needed to remove the effects of the instrument from the data. For an interferometer this requires a precise knowledge of many aspects of the system, including, the lengths of the baseline components, thermal drift in the fibre and other components, the deformation of the dishes under gravity, the contribution of the Earth's atmosphere and the complex passband. The techniques used to make these measurements on the JCMT -CSO Interferometer are described. There is also an explanation of how software that I have written processes the raw data from the correlator to give spectra and fluxes.
After four rather brief observing sessions, the interferometer is working well and has been used successfully to observe several astronomical sources. Brief descriptions are given of observations and their interpretation for three sources: the recombination-line maser source MWC 349, the black-hole candidate Sagittarius A* at the centre of our galaxy, and the nearby radio-galaxy Centaurus A. This is followed by a more detailed analysis of data for the evolved star VY CMa and observations of discs around young stars.
Spectra of the 321 GHz H20 line from VY CMa confirm that maser amplification is responsible for the emission. Continuum emission, spatially offset from the masers, was also detected with the interferometer, as was the CO (3-2) line. These data, particularly the offset continuum, are shown to be incompatible with single-star models for VY CMa. Instead, a binary model is proposed. This also helps to account for many previous observational results. Many protostars are suspected of having accretion discs. The JCMT-CSO Interferometer has resolved the dust continuum emission on scales of < 100 AU for the sources HL Tau and L1551-IRS 5. An analysis of the visibility curves is presented, showing that the emission is extended perpendicular to the outflow direction in each case; radii to half-maximum brightness are determined to be 60 AU and 80 AU respectively. A lower limit to the disc masses of ~0.02 M. is also derived.