Discovery of true, likely and possible symbiotic stars in the dwarf spheroidal NGC 205

Abstract

In this paper we discuss the photometric and spectroscopic observations of newly discovered (symbiotic) systems in the dwarf spheroidal galaxy NGC 205. The Gemini Multi-Object Spectrograph on-off band [O III] 5007 A emission imaging highlighted several [O III] line emitters, for which optical spectra were then obtained (Gonçalves et al. 2014). The detailed study of the spectra of three objects allow us to identify them as true, likely and possible symbiotic systems (SySts), the first ones discovered in this galaxy. SySt-1 is unambiguously classified as a symbiotic star, because of the presence of unique emission lines which belong only to symbiotic spectra, the well known O VI Raman scattered lines. SySt-2 is only possibly a SySt because the Ne VII Raman scattered line at 4881 A, recently identified in a well studied Galactic symbiotic as another very conspicuous property of symbiotic, could as well be identified as N III or [Fe III]. Finally, SySt-3 is likely a symbiotic binary because in the red part of the spectrum it shows the continuum of a late giant, and forbidden lines of moderate- to high-ionization, like [Fe v] 4180 A. The main source for skepticism on the symbiotic nature of the latter systems is their location in the PN region in the [O III] 4363/Hγ vs [O III] 5007/Hβ diagnostic diagram (Gutierrez-Moreno et al. 1995). It is worth mentioning that at least another two confirmed symbiotics, one of the Local Group dwarf spheroidal IC 10 and the other of the Galaxy, are also misplaced in this diagram.