The GALAH survey: Data Release 2

The Galactic Archaeology with HERMES (GALAH) survey is a large-scale stellar spectroscopic survey of the Milky Way and is specially designed to deliver chemical information complementary to a large number of stars covered by the Gaia mission. In April 2018, the GALAH survey announced its second public data release (GALAH DR2) of 345678 stars. For these stars, the GALAH collaboration is providing stellar parameters and abundances for up to 23 elements to the community. We provide the necessary information on target selection, observation, data reduction, as well as a detailed explanation of how the spectra were analysed to estimate stellar parameters and element abundances. For the stellar analysis, we have used a multi-step approach by combining the physics-driven spectrum synthesis by Spectroscopy Made Easy (SME) to derive stellar labels (Teff, logg, [Fe/H], [X/Fe], vmic, vsini, Ak) for a representative training set of stars and propagate this information with the data-driven Cannon on to the whole survey by estimating quadratic models to link the stellar flux for each wavelength pixel and the stellar labels. For the spectrum synthesis part, special care has been exercised to only consider spectral lines that have reliable atomic input data and are little affected by blending lines. Departures from local thermodynamic equilibrium (LTE) are considered for several key elements, including Li, O, Na, Mg, Al, Si and Fe, within the framework of 1D MARCS stellar atmosphere models.Various validation tests such as repeat observations, Gaia benchmark stars, open and globular clusters, and K2 asteroseismic targets lend confidence in our methods and results as do comparisons with other spectroscopic and photometric surveys. We provide a description of the contents of the publicly released data and give information on where and how the catalogue can be queried and downloaded. GALAH DR2 is a valuable source of stellar parameters and abundances for an unprecedented number of stars.

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