Optical Studies of V4332 Sagittarii: Detection of Unusually Strong K I and Na I Lines in Emission

We present optical observations of the enigmatic nova-like variable V4332 Sgr. The importance of this object should not be understated since it is considered to be the possible prototype of a new class of eruptive variables. These objects have been the subject of considerable studies at present primarily because of the spectacular eruption of V838 Mon—another member of this class—recently in 2002. The cause of the outburst in such objects is not well understood. Our recent work has shown striking changes in the near-IR spectrum of V4332 Sgr since its 1994 outburst. The optical spectrum presented here confirms that V4332 Sgr is indeed an unusual and extremely interesting object. This spectrum, the first to be taken after a hiatus of nearly 10 years after the outburst, shows several lines in emission but is dominated by exceptionally strong emission in the resonance doublet of K I at 7665 and 7699 Å and to a slightly lesser strength in the unresolved Na I doublet at 5890 and 5896 Å. The K I lines are shown to be optically thick and considerably broadened. We investigate the site of origin of the K I and Na I emission. Considering the strength of the alkali metal lines—seen at similar strength only in L- and T-type dwarfs (although in absorption)—we discuss whether the outburst of V4332 Sgr was an explosion on an L- or T-type dwarf. However, BVRI photometry does not support such a scenario but rather shows the central object of V4332 Sgr to be an M-type star with a temperature of 3250 K.