2016A&A...594A..94M

Context. The luminous blue variable phase is a crucial transitory phase that is not clearly understood in the massive star evolution.
Aims. We have obtained far-infrared Herschel/PACS imaging and spectroscopic observations of the nebula surrounding the candidate LBV HD 168625. By combining these data with optical spectra of the central star, we want to constrain the abundances in the nebula and in the star and compare them to trace back the evolution of this object.
Methods. We use the CMFGEN atmosphere code to determine the fundamental parameters and the CNO abundances of the central star whilst the abundances of the nebula are derived from the emission lines present in the Herschel/PACS spectrum.
Results. The far-infrared images show a nebula composed of an elliptical ring/torus of ejecta with a ESE-WNW axis and of a second perpendicular bipolar structure composed of empty caps/rings. We detect equatorial shells composed of dust and ionized material with different sizes when observed at different wavelengths, and bipolar caps more of less separated from the central star in Hα and mid-IR images. This complex global structure seems to show two different inclinations: ∼40deg for the equatorial torus and ∼60deg for the bipolar ejections. From the Herschel/PACS spectrum, we determine nebular abundances of N/H=4.1±0.8x10^–4 and C/H∼1.6_–0.35^+1.16x10^–4, as well as a mass of ionized gas of 0.17±0.04M_☉ and a neutral hydrogen mass of about 1.0±0.3M_☉ which dominates. Analysis of the central star reveals T_eff=14000±2000K, logg=1.74±0.05 and log(L/L_☉)=5.58±0.11. We derive stellar CNO abundances of about N/H=5.0±1.5x10^–4, C/H=1.4±0.5x10^–4 and O/H=3.5±1.0x10^–4, not significantly different from nebular abundances. All these measurements taken together are compatible with the evolutionary tracks of a star with an initial mass between 28 and 33M_☉ and with a critical rotational rate between 0.3 and 0.4 that has lost its material during or just after the blue supergiant phase.