We present an in-depth study of metal-poor stars, based high resolution spectra combined with newly released astrometric data from Gaia, with special attention to observational uncertainties. The results are compared to those of other studies, including Gaia benchmark stars. Chemical evolution models are discussed, highlighting few puzzles that are still affecting our understanding of stellar nucleosynthesis and of the evolution of our Galaxy.

V4334 Sgr (a.k.a. Sakurai's object) is the central star of an old planetary nebula that underwent a very late thermal pulse a few years before its discovery in 1996. We have been monitoring the evolution of the optical emission line spectrum since 2001. The goal is to improve the evolutionary models by constraining them with the temporal evolution of the central star temperature. In addition the high resolution spectral observations obtained by X-shooter and ALMA show the temporal evolution of the different morphological components.

When a turbulent flow in a porous medium is determined numerically, the crucial question is whether turbulence models should account only for turbulent structures restricted in size to the pore scale or whether the size of turbulent structures could exceed the pore scale. The latter would mean the existence of macroscopic turbulence in porous media, when turbulent eddies exceed the pore size. In order to determine the real size of turbulent structures in a porous medium, we simulated the turbulent flow by direct numerical simulation (DNS) calculations, thus avoiding turbulence modelling of any kind. With this study, which for the first time uses DNS calculations, we provide benchmark data for turbulent flow in porous media. Since perfect DNS calculations require the resolution of scales down to the Kolmogorov scale, often only approximate DNS solutions can be obtained, especially for high Reynolds numbers. This is accounted for by using and comparing two different DNS approaches, a finite volume method (FVM) with grid refinement towards the wall and a lattice Boltzmann method (LBM) with equal grid distribution. The solid matrix was simulated by a large number of rectangular bars arranged periodically. The number of bars in the solution domain with periodic boundary conditions was reduced systematically until a minimum size was found that does not suppress any large-scale turbulent structures. Two-point correlations, integral length scales and energy spectra were determined in order to answer the question of whether or not macroscopic turbulence can be found in porous media.

The common envelope interaction between a giant star and a stellar or substellar companion is at the origin of several compact binary classes, including the progenitors of Type Ia SN. A common envelope is also what will happen when the Sun expands and swallows its planets as far out as Jupiter. The basic idea and physics of the common envelope interaction has been known since the 1970s. However, the outcome of a common envelope interaction - what systems survive and what their parameters are - depends sensitively on the details of the engagement. To advance our knowledge of the common envelope interaction between stars and their stellar and substellar companions, we have carried out a series of simulations with Eulerian, grid-based and Lagrangian, smoothed particle hydrodynamics codes between a 0.88-M⊙, 85-R⊙, red giant branch star and companions in the mass range 0.1-0.9 M⊙. In this contribution, we will discuss the reliability of the techniques, the physics that is not included in the codes but is likely important, the state of the ejected common envelope, and the final binary separation. We also carry out a comparison with the observations. Finally, we discuss the common envelope efficiency parameter, α and the survival of planets.

Tourmalines of unusual (mushroom) habit are common in granitic pegmatites of Momeik, northeast of Mogok, Myanmar. Here, we examine a sample of elbaite of significantly different habit, consisting of a series of diverging crystals, resembling a sheaf of wheat and ranging in colour from light purplish-red at the base to dark purplish-red at the tip with a thin green cap at the termination. The crystal structures of eight crystals are refined to R1-indices of ∼2.5% using graphite-monochromated Mo-Kα X-radiation; the same crystals were analysed by electron microprobe. 11B and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected on four regions of the wheatsheaf crystal, and show ∼0.3 a.p.f.u. [4]B and <0.1 a.p.f.u. [4]Al in the structure. 57Fe Mössbauerspectr oscopy was done on the dark green rim at the termination of the crystal, showing all Fe in this region (∼0.6 a.p.f.u.) to be Fe2+. Detailed electron-microprobe traverses show that the principal compositional variation involves the substitutions [4]B + YAl → Si + YFe*, where transition metals are present, and [4]B2 + YAl → Si2 + YLi, where transition metals are not present, although several other minor substitutions also affect crystal composition. Successive microscopic bifurcation of crystallites causes divergence of growth directions along the c axis, imparting the overall ‘wheatsheaf’ shape to the crystal aggregate. We suggest that such bifurcation is common in pegmatitic elbaite crystals, resulting in their common divergent habit.

A variety of mushroom tourmaline from the Kat Chay mine, Momeik, near Mogok, Shan state, Myanmar, consists of a black-to-grey single-crystal core from which a single prismatic crystal reaches to the edge of the mushroom, forming a slight protuberance. The rest of the mushroom (∼50% by volume) consists of extremely acicular sub-parallel crystals that diverge toward the edge of the mushroom. The acicular crystals are dominantly colourless to white, with a continuous black zone (2 mm across) near the edge, and pale pink outside the black zone. The composition varies from ∼Na0.75Ca0.05(Li0.80Al0.70Fe1.10.Mn0.30Ti0.10)Al6Si6(B03)3O18(OH)3(OH,F) at the base of the mushroom to ∼Na0.60Ca0.06(Li1.00Al1.98Fe0.02)Al6(Si5.35B0.65)(BO3)3O18(OH)3(OH,F) close to the edge at the top of the mushroom. The principal substitutions are: (1) YLi + YAl -> yFe* + yFe* and (2) TB + yAl -> Si + yFe*, but there are five other minor substitutions that are also operative. There are six significant compositional discontinuities at textural boundaries in the mushroom, suggesting that the changes in habit are driven in part by changes in external variables such as T and P, plus possible involvement of new fluid phases.

Tourmaline from the Kat Chay mine, Momeik, near Mogok, Shan state, Myanmar, shows a variety of habits that resemble mushrooms, and it is commonly referred to as ‘mushroom tourmaline'. The structure of nine single crystals of elbaite, ranging in colour from pink to white to black and purple, extracted from two samples of mushroom tourmaline from Mogok, have been refined (SREF) to R indices of ~2.5% using graphite-monochromated Mo-Kα X-radiation. 11B and 27Al Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy shows the presence of [4]B and the absence of [4]Al in samples with transition-metal content low enough to prevent paramagnetic quenching of the signal. Site populations were assigned from refined site-scattering values and unit formulae derived from electron-microprobe analyses of the crystals used for X-ray data collection. 57Fe Mössbauer spectroscopy shows that both Fe2+ and Fe3+ are present, and the site populations derived by structure refinement show that there is no Fe at the Z site; hence all Fe2+ and Fe3+ occurs at the Y site. The 57Fe Mössbauer spectra also show peaks due to intervalence charge-transfer involving Fe2+ and Fe3+ at adjacent Y sites. Calculation of the probability of the total amount of Fe occurring as Fe2+–Fe3+ pairs for a random short-range distribution is in close accord with the observed amount of Fe involved in Fe2+–Fe3+, indicating that there is no short-range order involving Fe2+ and Fe3+ in these tourmalines.

We present results of a quantitative spectral analysis of a sample comprising eleven hydrogen deficient post-AGB stars of the spectral type PG 1159 with state-of-the-art NLTE model atmospheres. For all objects high resolution spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) are available. The FUV spectra of PG 1159 stars are dominated by lines of helium, carbon and oxygen, and for the first time we also identified lines of several trace elements as sulfur and silicon which allow to determine the abundances of light metals in the atmospheres of these stars. As these stars show nuclear processed former intershell matter on their surface, the determined abundances also allow us to constrain the predictions of evolutionary models and element abundances in AGB stars.

We report on the progress of our on-going campaign to monitor the evolution of the VLTP objects V4334 Sgr and V605 Aql, as well as the suspected (V)LTP object CK Vul. V4334 Sgr does not show signs of increased ionization compared to our previous observations in 2004. We obtained the first radio detection of V605 Aql, indicating a strong increase in radio flux since 1987. We also present the first radio detection of CK Vul and discuss the expansion of the material ejected during the 1670 event.

We present a study of the s–process nucleosynthesis (weak s-process) occurring during convective core He–burning and convective shell Carbon–burning in a massive star of 25 M$_{\odot}$. We use an updated nuclear network for the various neutron sources and for all neutron captures and β-decay rates involved. Large uncertainties affect the final yields due to the present unsatisfactory knowledge of all neutron capture cross sections involved.