VLT/UVES spectroscopy of V4332 Sagittarii in 2005: The best view on a decade-old stellar-merger remnant

Context. V4332 Sgr is a red transient (red nova) whose eruption was observed in 1994. The remnant of the eruption shows a unique optical spectrum: strong emission lines of atomes and molecules superimposed on a M-type stellar spectrum. The stellar-like remnant is however not directly obseravable. Presumably it is embedded in a disc-like dusty envelope orientated almost face-on. The observed optical spectrum is supposed to result from interactions of the central-star radiation with dust and gas in the disc and o utflows initiated in 1994. Aims. We aim at studying the optical spectrum of the object in great detail to better understand the origin of the spectrum and the nature of the object. Methods. We have reduced and measured a high-resolution (R≃ 40 000) spectrum of V4332 Sgr obtained with VLT/UVES in April/May 2005. The spectrum comes from the ASO archives and is the best quality spectrum of the object ever obtained. Results. We have identified and measured over 200 emission features be longing to 11 elements and 6 molecules. The continuous, stellar-like component can be classified as ∼M3. The radial velocity of the object, as derived from narrow atomic emission line, is ∼−75 km s −1 . The interstellar reddening was estimated as being 0.35≤ EB−V≤ 0.75. From radial velocities of interstellar absorption features in the Nai D lines we have estimated a lower limit of∼5.5 kpc to the distance of V4332 Sgr. When compared to spectroscopic observations done in 2009, the spectrum of V4332 Sgr considerably evolved between 2005 and 2009. The object significantl y faded in the optical (by∼2 mag. in the V band), which resulted from the main remnant cooled by 300‐350 K corresponding to its spectral type changed from M3 to M5-6. The object however increased in luminosity by∼ 50%, implying a significant expansion of its dimensions. Most of the emission features seen in 2005 significantly fade d or even disappeared from the spectrum of V4332 Sgr in 2009. These resulted from fading of the optical central-star radiation , as well as decreasing of the optical thickness of the cirums tellar matter, presumably due to its expansion. V4332 Sgr bears several resemblances to V1309 Sco, which erupted in 2008. This can evidence a similar nature of the eruptions of both objects. In case of V1309 Sco the outburst resulted from merger of a contact binary.