M. D. Gray

Testing the `clump' model of SiO maser emission

Building on the detection of the J = 7 6 SiO maser emission in both the v = 1 and v = 2 vibrational states towards the symbiotic Mira, R Aquarii, we have used the James Clerk Maxwell Telescope to study the changes in the SiO maser features from R Aqr over a stellar pulsational period. The observations, complemented by contemporaneous data taken at 86GHz, represent a test of the popular thermal-instability clump models of SiO masers. The ‘clump’ model of SiO maser emission considers the SiO masers to be discrete emitting regions which differ from their surroundings in the values of one or more physical variables (SiO abundance for example). We find that our observational data are consistent with a clump model in which the appearance of maser emission in the J = 7 6 transitions coincides with an outward-moving shock impinging on the inner edge of the maser zone.

A Model for the Variability of SiO Masers in Mira Variables

Mass loss from M-Miras makes a major contribution to the injection of heavy elements into the interstellar medium and is a key stage in stellar evolution in which a late-type star embarks on its route towards the formation of a white dwarf. Recent VLBI images of SiO maser emission find that SiO masers lie in an incomplete ring around the host M-Mira (Diamond et al. 1994), stellar radius from the photosphere in the case of TX Cam, i.e. in the zone where mass loss is initiated. SiO masers show complicated and time-varying structure. With the new VLBI data and new hydrodynamic pulsation models and maser models, it is now possible to use SiO maser emission to examine critically our understanding of the mass loss mechanism in late-type stars.

A model for polarized OH masers

A semi-classical theory of the propagation of polarized maser radiation through a medium containing an ambient magnetic field is described, including processes involving the overlap of maser lines. The treatment of the partially coherent maser radiation and the problems associated with a computational implementation of the theory are considered.

OH masers in star-forming regions: Interpretation of observations

OH masers are a potentially powerful probe of the physical conditions in massive star-forming regions, giving detailed information on the kinetic and dust temperatures, number densities of H2 and OH, bulk flows and magnetic fields in, the vicinity of massive young stellar objects. Using current theories and data, we describe how bright maser spots, as seen in VLBI at 18cm, 6 cm and 5 cm , may arise and we outline the physical conditions which may be associated with masers at different frequencies.

OH Masers in Outflow Regions

We combine a sophisticated model of maser propagation with a simple model of an accelerating molecular outflow and show that the observation of different OH maser frequencies is consistent with emission from different parts of the outflow.

Modelling OH in regions of star formation

There is a large, diverse and rapidly growing body of OH observations from regions associated with star formation, the physical conditions of which are of great interest. To interpret these observations we need to calculate the populations of the OH energy levels using as accurate a model as is feasible. We have developed a large velocity gradient (LVG) model using the lowest 48 hyperfine states of OH and including the phenomenon of far-infrared line overlap using a new theory which treats all thermal and velocity overlaps. If we use a theory of maser propagation (Field and Gray, 1988) we are then able to see how competing maser lines develop.The generic behaviour which we find is in good agreement with the extensive maser observations of Gaume and Mutel 1987 and our calculations allow us to associate general physical conditions with the four ground state maser frequencies. In addition we are able to present preliminary results of the interpretation of the OH absorption observations of Guilloteauet al. (1984) and Walmsleyet al. (1986) for the compactHII region DR21.