The inner nebula and central binary of the symbiotic star HM Sge

Abstract

We present contemporaneous HST WFPC2 and VLA observations of the symbiotic nova HM Sge. We identify a number of discreet features at spatial scales smaller than approximately 0.1 arcsec embedded in the extended nebula, with radio and optical emission well correlated in the inner 1 arcsec. For the first time we measure the positions of the binary components of a symbiotic star directly. We estimate the projected angular binary separation to be 40+/-9 milli-arcsec, with the binary axis at position angle 130+/-10 degrees. The latter is consistent with previous estimates made by indirect methods. The binary separation is consistent with a previous estimate of 50 au if the distance is 1250+/-280 pc. Temperature and density diagnostics show two distinct regions in the surrounding nebula, with a cool wedge to the south-west. An extinction map indicates the true interstellar extinction to be no more than E(B-V) = 0.35. This is consistent with a minimum distance of approximatelly 700 pc, but this would be reduced if there is a circumstellar contribution to the minimum in the extinction map. The extinction map also suggests a patchy dust distribution. We suggest that a southern concentration of dust and the south-west wedge are associated with the cool component wind. Alternatively, the southern dust concentration is the cause of the cool wedge, as it shields part of the nebula from the hot component radiation field.