V4332 Sagittarii: a circumstellar disc obscuring the main object

Abstract

Context. V4332 Sgr experienced an outburst in 1994 whose observational characteristics in many respects resemble those of the eruption of V838 Mon in 2002. It has been proposed that these objects erupted because of a stellar-merger event. Aims. Our aim is to derive, from observational data, information on the present (10-15 yrs after the outburst) nature and structure of the object. Methods. We present and analyse a high-resolution (R ≈ 21 000) spectrum of V4332 Sgr obtained with the Subaru Telescope in June 2009. Various components (stellar-like continuum, atomic emission lines, molecular bands in emission) in the spectrum are analysed and discussed. We also investigate a global spectral energy distribution (SED) of the object mostly derived from broadband optical and infrared photometry. Results. The observed continuum resembles that of an ∼M 6 giant. The emission features (atomic and molecular) are most probably produced by radiative pumping. The observed strengths of the emission features strongly suggest that we only observe a small part of the radiation of the main object responsible for pumping the emission features. An infrared component seen in the observed SED, which can be roughly approximated by two blackbodies of ∼950 and ∼200 K, is ∼50 times brighter than the M 6 stellar component seen in the optical. This further supports the idea that the main object is mostly obscured for us. Conclusions. The main object in V4332 Sgr, an ∼M 6 (super)giant, is surrounded by a circumstellar disc, which is seen almost edgeon so the central star is obscured. The observed M 6 spectrum probably results from scattering the central star spectrum on dust grains at the outer edge of the disc.

Optical spectropolarimetry of V4332 Sagittarii

“…The observed strong polarization of the stellar-like continuum, and depolarization in regions dominated by emission features is consistent with the interpretation of the observed spectrum of V4332 Sgr proposed in Kamiński et al (2010) and . According to the hypothesis outlined in these references, the main object, resembling an M-type giant, is surrounded by a dusty disc or disc-like envelope, most probably formed during the 1994 eruption of V4332 Sgr.…”

“…The result is displayed in Fig. 2, as is a standard M5 III spectrum reddened as V4332 Sgr, i.e., with E B−V = 0.32 (Kamiński et al 2010). As can be seen from the upper part of We also present results of our spectropolarimetric observations in terms of QU plots in Fig.…”

“…Their radiation is not seen directly, because it is mostly hidden by the disc and reaches the observer only owing to scattering in the disc and in the polar outflow. The spectrum of V4332 Sgr is the only instance where emission bands of CrO have been observed (Kamiński et al 2010). This suggests some abundance peculiarities in the molecular envelope of this exotic object.…”

“…Subsequent polarimetric measurements done by in the V and R photometric bands have shown that the optical light from V4332 Sgr is indeed strongly polarized (26% and 11%, respectively). As proposed in Kamiński et al (2010), the emissionline spectrum results from radiative excitation of atoms and molecules by strong radiation of the (hidden for us) central object. Thus, unlike the observed continuum, the emission lines should be unpolarized.…”

Pure rotational spectra of TiO and TiO2in VY Canis Majoris

“…Although emission bands of TiO are rarely observed, they are present in VY CMa, and very rich TiO emission is seen in the spectra of U Equ and V4332 Sgr (Barnbaum et al 1996;Kamiński et al 2010 (Zickgraf et al 1989) and a few protostars (Hillenbrand et al 2012), including some that are known to be explosive objects. Recently, an emission feature of TiO was discovered in the IR (around 10 μm) in four S-type Mira variables that are large-amplitude pulsators (Smolders et al 2012).…”