Abstract. The eruption of V4332 Sgr discovered in February 1994 shows striking similarities to that of V838 Mon started in January 2002. The nature of these eruptions is, however, enigmatic and unclear. We present new photometric and spectroscopic data on V4332 Sgr obtained in April-May 2003 at the SAAO. The obtained spectrum shows an unusual emission-line component superimposed on an early M-type stellar spectrum. The emission-line spectrum is of very low excitation and is dominated by lines from neutral elemets (NaI, FeI, CaI) and molecular bands (TiO, ScO, AlO). We also analyse all the observational data, mainly photometric measurements, available for V4332 Sgr. This allows us to follow the evolution of the effective temperature, radius and luminosity of the object since February 1994 till 2003. We show that the observed decline of V4332 Sgr can be accounted for by a gravitational contraction of an inflated stellar envelope. The combined optical and infrared photometry in 2003 shows that apart from the M-type stellar component there is a strong infrared excess in the KLM bands. This excess was absent in the 2MASS measurements done in 1998 but was probably starting to appear in K in 1999 when the object was observed in the DENIS survey. We interpret the results in terms of a stellar merger scenario proposed by Soker & Tylenda. The infrared excess is likely to be due to a disc-like structure which is either of protostellar nature or has been produced during the 1994 eruption and stores angular momentum from the merger event.

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.

V838 Mon erupted at the beginning of 2002 becoming an extremely luminous star with L ≃ 10 6 L ⊙ . Among various scenarios proposed to explain the nature of the outburst the most promising is a stellar merger event. In this paper we investigate the observational properties of the star and its surroundings in the post outburst phase. We have obtained a high resolution optical spectrum of V838 Mon in October 2005 using the Keck I telescope. We have identified numerous atomic features and molecular bands present in the spectrum and provided an atlas of those features. In order to improve the spectrum interpretation we have performed simple modeling of the molecular bands. Our analysis indicates that the spectrum is dominated by molecular absorption features arising in photospheric regions with temperatures of ∼2400 K and in colder outer layers, where the temperature decreases to ∼500 K. A number of resonance lines of neutral alkali metals are observed to show P-Cyg profiles. Particularly interesting are numerous prominent emission lines of [Fe II]. All of them show practically the same profile, which can be well described by a Lorentzian profile. In the blue part of the spectrum photospheric signatures of the B-type companion are easily seen. We have fitted the observed spectrum with a synthetic one and the obtained parameters are consistent with the B3V type. We have also estimated radial and rotational velocities of the companion.

During their asymptotic giant branch evolution, low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the highresolution HIFI spectrometer onboard Herschel. We report on the first detection of H 16 2 O and the rarer isotopologues H 17 2 O and H 18 2 O in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 × 10 −5 , and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H 2 O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of 12 CO, 13 CO, 28 SiO, 29 SiO, 30 SiO, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated.

We report the detection of absorption lines by the reactive ions OH + , H 2 O + and H 3 O + along the line of sight to the submillimeter continuum source G10.6−0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH + at 971 GHz, H 2 O + at 1115 and 607 GHz, and H 3 O + at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6−0.4 as well as in the molecular gas directly associated with that source. The OH + spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H 2 O + lines remains lower than 1 in the same velocity range, and the H 3 O + line shows only weak absorption. For LSR velocities between 7 and 50 km s −1 we estimate total column densities of N(OH + ) ≥ 2.5 × 10 14 cm −2 , N(H 2 O + ) ∼6 × 10 13 cm −2 and N(H 3 O + ) ∼4.0 × 10 13 cm −2 . These detections confirm the role of O + and OH + in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH + by the H 2 O + column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.