V417 Centauri: a yellow symbiotic system in a resolved nebula

We report the discovery of a faint nebula surrounding the yellow symbiotic star V417 Cen, the detection of light variation in the system from 238 photographic plates taken between 1919 and 1934, yielding a period of 245.68 days and an amplitude of 0.5mag, and the observation of large spectral changes based on recent investigations. The spectral energy distribution shows the presence of hot dust, unique in yellow symbiotics; the broad IR excess indicates stratification of the dust temperature. In the (J-K), (H-K) two-colour diagram, V417 Cen lies at the position of D-type symbiotic stars. This is confirmed by high resolution spectroscopy: a narrow Hα line and the presence of [NII] lines are typical for D-type symbiotics. Low resolution spectra show that the cool component of V417 Cen is a G2 Ib-II star. The presence of such a high luminosity object in a symbiotic system is unusual. The emission line spectrum is strongly variable. In 1988 only [OIII] and Hei lines were seen in emission, in 1993 the Balmer lines and [NII] were also pronounced. A deep Hα image was taken with the coronographic facility of EFOSC-1 attached to the ESO 3.6m telescope. V417 Cen is surrounded by a faint ring-like nebula with dimensions 30" x 15", as well as a very faint asymmetric emission to the west, reminiscent of a highly developed "Southern Crab" (He 2-104), which suggests bipolar outflow from the central object. We interpret the ring-like nebula as an evolved planetary nebula and the extensions from the shell as remnants of the heavy mass loss during the AGB phase of the progenitor. The symbiotic nebula remains unresolved. Estimates on mass, size, mass loss and mass accretion rate of the system give a consistent picture of a 246-day period binary composed of a supergiant and a white dwarf.