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

Abstract

We present contemporaneous Hubble Space Telescope Wide Field Planetary Camera 2 and Very Large Array observations of the symbiotic nova HM Sge. We identify a number of discreet features at spatial scales smaller than ~0farcs1 embedded in the extended nebula, with radio and optical emission well correlated in the inner 1''. 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 mas, with the binary axis at a position angle of 130° ± 10°. 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 southwest. 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 ~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 southwest wedge are associated with the cool component wind. Alternatively, the southern dust concentration is the cause of the cool wedge, since it shields part of the nebula from the hot-component radiation field.