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

Abstract

HM Sge is a symbiotic binary star that underwent a nova-like outburst in 1975. Its radio emission has been monitored using MERLIN and the VLA, and maps from 1992 to 1997 are presented. Thermal emission within 0.4 arcsec of the optical peak is elongated north--south and hotspots appear to be moving around an anticlockwise ellipse. If this is due to motion in an inclined disc, the period is ∼ 90 yr. Eyres et al. predicted that if this is due to Mach shocks in the post-nova wind, proper motions should be seen in expansion. 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 winds in a binary system following the nova-like outburst. The emission peaks 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 positions of the stars are estimated, at a separation of ∼ 25 au. A distance of ∼ 1 kpc is most consistent with the observations reported here. On arcsecond scales the emission is extended east--west, consistent with a biconical outflow arising from the collimation of the nova outburst through interaction with the pre-existing cool wind. The presence of non-thermal emission at a separation of ∼700 mas from the stars is confirmed. This is very unusual at such a distance from low-mass stars, and could arise from synchrotron emission in a μT magnetic field. A model for this is developed, which shows that non-thermal emission is expected to decline within decades as the nova wind decelerates. This could explain the non-detection of non-thermal emission in other symbiotic stars.