Tracing back the evolution of the candidate LBV HD168625
  Aims. We obtained Herschel/PACS imaging and spectroscopic observations of the nebula surrounding the candidate LBV HD168625. By combining these data with optical spectra of the star, we want to constrain the abundances in the nebula and in the star and compare them to trace back its evolution.   Methods. We use CMFGEN 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.   Result. The FIR 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 H$\alpha$ and mid-IR images. This complex global structure seems to show 2 different inclinations: 40{\deg} for the equatorial torus and 60{\deg} for the bipolar ejections. From the Herschel/PACS spectrum, we determine nebular abundances of $\mathrm{N/H} = 4.1 \pm 0.8 \times~10^{-4}$ and $\mathrm{C/H} \sim 1.6_{-0.35}^{+1.16} \times~10^{-4}$, as well as a mass of ionized gas of $0.17\pm 0.04$M$_{\odot}$ and a neutral hydrogen mass of about $1.0\pm 0.3$M$_{\odot}$ which dominates. Analysis of the central star reveals Teff = 14000K, log g = 1.74 and log (L/L$_{\odot}$) = 5.58. We derive stellar abundances of about $\mathrm{N/H} = 5.0 \pm 1.5 \times~10^{-4}$, $\mathrm{C/H} = 1.4 \pm 0.5 \times~10^{-4}$ and $\mathrm{O/H} = 3.5 \pm 1.0\times~10^{-4}$. All these measurements taken together are compatible with the evolutionary tracks of a star with an initial mass between $28$ and $33$M$_{\odot}$ and with a crit. rotational rate between 0.3 and 0.4 that has lost its material during or just after the BSG phase. 
L. Mahy, D. Hutsem\'ekers, P. Royer, C. Waelkens
astro-ph.SR