MERLIN observations of bipolar outflow from HM Sagittae

MERLIN maps of HM Sagittae at 6-cm and 18-cm wavelengths are presented. At 6 cm, the inner nebula has been mapped at a resolution of 60 milli-arcsec (mas), revealing two parallel ridge structures to the north and south of a deep central minimum. In addition, these central features are surrounded by a complex, weak structure. The ridges and many of the weak features are correlated with ultraviolet (UV) structure observed with the Hubble Space Telescope (HST). The 18-cm map reveals elongated structure to the east and west of the stars position. There is strong evidence that the eastern and western edges of this extension are non-thermal. The E-W and N-S features are associated with components detected with optical spectroscopy. The radial velocities of these components suggest that the E-W features are a high-velocity bipolar outflow. We argue that the brightness temperature of ~ 45000K for the N-S features results from shock heating by the outflow. The resulting expansion velocity, when compared with the observed proper motion, implies a distance of 3.2 kpc.