The Inner Nebula and Central Binary of the Symbiotic Star HM Sagittae

We present contemporaneous HST WFPC2 and VLA observations of the symbioticnova HM Sge. We identify a number of discreet features at spatial scalessmaller than approximately 0.1 arcsec embedded in the extended nebula, withradio and optical emission well correlated in the inner 1 arcsec. For the firsttime we measure the positions of the binary components of a symbiotic stardirectly. We estimate the projected angular binary separation to be 40+/-9milli-arcsec, with the binary axis at position angle 130+/-10 degrees. Thelatter is consistent with previous estimates made by indirect methods. Thebinary separation is consistent with a previous estimate of 50 au if thedistance is 1250+/-280 pc. Temperature and density diagnostics show twodistinct regions in the surrounding nebula, with a cool wedge to thesouth-west. An extinction map indicates the true interstellar extinction to beno more than E(B-V) = 0.35. This is consistent with a minimum distance ofapproximatelly 700 pc, but this would be reduced if there is a circumstellarcontribution to the minimum in the extinction map. The extinction map alsosuggests a patchy dust distribution. We suggest that a southern concentrationof 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.Comment: 13 pages including 4 figures. To be published in ApJ vol. 551. Update to rescale Fig.