Ionized Nebulae Around Two New Symbiotic Stars: GR Cygni and [D75] 141

Abstract

In the course of selecting targets for a spectroscopic survey of faint planetary-nebula nuclei, we encountered two anomalous cases of late-type variable stars surrounded by nebulosities discovered by amateur astronomers. We obtained follow-up deep narrow-band images of the two nebulae: StDr Objet 14 (diameter ∼ 1.′5 ) and StDr 4 (diameter ∼ 2.′ 5). StDr Objet 14 is found to be a low-excitation nebula, bright only in Hα+[N ii], and showing a unique double-ring “bull’s-eye” morphology. StDr 4 exhibits a bow-shock structure, bright in Hα and [S ii], embedded in an extended emission nebula. Both central stars, respectively GR Cyg and [D75] 141, are found to be pulsating semiregular variable stars. Our spectroscopic observations show GR Cyg to be a cool carbon star of type C9,2, with emission lines of the Balmer series, [O ii] and [O iii], [Ne iii], [S ii], and Fe ii. A synthetic narrow-band image of GR Cyg shows that the source of its strong [O ii] λ3727 emission line is a knot displaced from the star by about 1.″6 , indicative of a recent collimated ejection event. [D75] 141 is a M6 III red giant, with weak Balmer-series emission. Both stars show a blue excess, and GR Cyg was detected in the near-UV by the GALEX spacecraft. These data establish both stars as new symbiotic binaries, containing a late-type red giant and a hot companion accreting from its stellar wind. We speculate that the double rings surrounding GR Cyg were ejected during helium-shell thermal pulses (or conceivably from recurring nova outbursts), while the nebula around [D75] 141 may be the result of a collision of the symbiotic star’s ejecta with an interstellar cloud. Our accidental discoveries of these two binaries because they are surrounded by faint nebulae supports the view that there is a significant population of “missing” symbiotic stars yet to be discovered.