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We investigate departures from local thermodynamic equilibrium (LTE) in the line formation of Fe for a number of well‐studied late‐type stars in different evolutionary stages. A new model of the Fe atom was constructed from the most up‐to‐date theoretical and experimental atomic data available so far. Non‐LTE (NLTE) line formation calculations for Fe were performed using 1D hydrostatic marcs and mafags‐os model atmospheres, as well as the spatial and temporal average stratifications from full 3D hydrodynamical simulations of stellar convection computed using the stagger code. It is shown that the Fe i/Fe ii ionization balance can be well established with the 1D and mean 3D models under NLTE including calibrated inelastic collisions with H i calculated from Drawin's formulae. Strong low‐excitation Fe i lines are very sensitive to the atmospheric structure; classical 1D models fail to provide consistent excitation balance, particularly so for cool metal‐poor stars. A better agreement between Fe i lines spanning a range of excitation potentials is obtained with the mean 3D models. Mean NLTE metallicities determined for the standard stars using the 1D and mean 3D models are fully consistent. Moreover, the NLTE spectroscopic effective temperatures and gravities from ionization balance agree with that determined by other methods, e.g. the infrared flux method and parallaxes, if one of the stellar parameters is constrained independently.

The GALAH survey is a large high-resolution spectroscopic survey using the newly commissioned HERMES spectrograph on the Anglo-Australian Telescope. The HER-MES spectrograph provides high-resolution (R ∼28,000) spectra in four passbands for 392 stars simultaneously over a 2 degree field of view. The goal of the survey is to unravel the formation and evolutionary history of the Milky Way, using fossil remnants of ancient star formation events which have been disrupted and are now dispersed throughout the Galaxy. Chemical tagging seeks to identify such dispersed remnants solely from their common and unique chemical signatures; these groups are unidentifiable from their spatial, photometric or kinematic properties. To carry out chemical tagging, the GALAH survey will acquire spectra for a million stars down to V ∼14. The HERMES spectra of FGK stars contain absorption lines from 29 elements including light proton-capture elements, α-elements, odd-Z elements, iron-peak elements and n-capture elements from the light and heavy s-process and the r-process. This paper describes the motivation and planned execution of the GALAH survey, and presents some results on the first-light performance of HERMES.

We investigate departures from local thermodynamic equilibrium (LTE) in the line formation of neutral and singly ionized iron lines and their impact on spectroscopic stellar parameters. The calculations were performed for an extensive grid of 1D marcs models of metal‐rich and metal‐poor late‐type dwarfs and giants. We find that iron abundances derived from Fe i lines are increasingly underestimated in hotter, lower surface gravity and more metal‐poor stars, in a simple and well‐defined pattern, while LTE is usually a realistic approximation for Fe ii lines. For the vast majority of dwarfs and giants, the perturbed ionization balance of Fe i and Fe ii is the main relevant non‐LTE effect to consider in the determination of spectroscopic parameters, while for extremely metal‐poor stars and hot giant stars significant impact is seen also on the excitation balance and on the microturbulence determination from Fe i lines.

We present a re-appraisal of the temperatures of Red Supergiants (RSGs) using their optical and near-infrared spectral energy distributions (SEDs). We have obtained data of a sample of RSGs in the Magellanic Clouds using VLT+XSHOOTER, and we fit MARCS model atmospheres to different regions of the spectra, deriving effective temperatures for each star from (a) the TiO bands, (b) line-free continuum regions of the spectral energy distributions (SEDs), and (c) the integrated fluxes. We show that the temperatures derived from fits to the TiO bands are systematically lower than the other two methods by several hundred Kelvin. The TiO fits also dramatically over-predict the flux in the near-IR, and imply extinctions which are anomalously low compared to neighbouring stars. In contrast, the SED temperatures provide good fits to the fluxes at all wavelengths other than the TiO bands, are in agreement with the temperatures from the flux integration method, and imply extinctions consistent with nearby stars. After considering a number of ways to reconcile this discrepancy, we conclude that 3-D effects (i.e. granulation) are the most likely cause, as they affect the temperature structure in the upper layers where the TiO lines form. The continuum, however, which forms at much deeper layers, is apparently more robust to such effects. We therefore conclude that RSG temperatures are much warmer than previously thought. We discuss the implications of this result for stellar evolution and supernova progenitors, and provide relations to determine the bolometric luminosities of RSGs from single-band photometry.