High-resolution radio observations of HM Sge - II. Two decades after outburst

HM Sge is a symbiotic binary star that underwent a nova-like outburst in 19 75. Its radio emission has been monitored using MERLIN and the VLA, and map s from 1992 to 1997 are presented. Thermal emission within 0.4 arcsec of th e optical peak is elongated north-south and hotspots appear to be moving ar ound an anticlockwise ellipse. If this is due to motion in an inclined disc , the period is similar to 90 yr. Lyres et al, predicted that if this is du e to Mach shocks in the post-nova wind, proper motions should be seen in ex pansion. This has not been detected since 1992, so an alternative model is adopted, based on the work of Eyres et al. and Kenny et al., of colliding w inds in a binary system following the nova-like outburst. The emission peak s appear to be corotating with the binary orbit as the ionization front and the hot wind from the white dwarf interact with the Mira wind. The positio ns of the stars are estimated, at a separation of similar to 25 au. A dista nce of similar to 1 kpc is most consistent with the observations reported h ere. On arcsecond scales the emission is extended east-west, consistent with a b iconical outflow arising from the collimation of the nova outburst through interaction with the pre-existing cool wind. The presence of non-thermal em ission at a separation of similar to 700 mas from the stars is confirmed. T his is very unusual at such a distance from low-mass stars, and could arise from synchrotron emission in a mu T magnetic field. A model for this is de veloped, which shows that non-thermal emission is expected to decline withi n decades as the nova wind decelerates. This could explain the non-detectio n of non-thermal emission in other symbiotic stars.