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

¹ Department of Astrophysics, Nicolaus Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
e-mail: tylenda@ncac.torun.pl
² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile

Received: 28 December 2014
Accepted: 22 March 2015

Abstract

Context. The source 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 atoms and molecules superimposed on an M-type stellar spectrum. The stellar-like remnant is not directly observable, however. It is presumably embedded in a disc-like dusty envelope seen almost face-on. The observed optical spectrum is assumed to result from interactions of the central-star radiation with dust and gas in the disc and outflows 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 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 ESO archives and is the best quality spectrum of the object ever obtained.

Results. We identified and measured over 200 emission features belonging to 11 elements and 6 molecules. The continuous, stellar-like component can be classified as ~M3. The radial velocity of the object is ~–75 km s^-1 as derived from narrow atomic emission lines. The interstellar reddening was estimated to be 0.35 ≤ E_B−V ≤ 0.75. From radial velocities of interstellar absorption features in the Na i D lines, we estimated a lower limit of ~5.5 kpc to the distance of V4332 Sgr. When compared to spectroscopic observations obtained in 2009, the spectrum of V4332 Sgr considerably evolved between 2005 and 2009. The object significantly faded in the optical (by ~2 mag in the V band), which resulted from cooling of the main remnant by 300–350 K, corresponding to its spectral-type change from M3 to M5-6. The object increased in luminosity by ~50%, however, implying a significant expansion of its dimensions. Most of the emission features seen in 2005 significantly faded or even disappeared from the spectrum of V4332 Sgr in 2009. These resulted from fading of the optical central-star radiation and a decrease of the optical thickness of the cirumstellar matter, presumably due to its expansion. V4332 Sgr bears several resemblances to V1309 Sco, which erupted in 2008. This can indicate a similar nature of the eruptions of the two objects. The outburst resulted from merger of a contact binary in V1309 Sco.