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We use more than 872 000 mid- to high-resolution (R ∼ 20 000) spectra of stars from the GALactic Archaeology with HERMES (GALAH) survey to discern the spectra of diffuse interstellar bands (DIBs). We use four windows with the wavelength ranges 4718–4903, 5649–5873, 6481–6739, and 7590–7890 Å, giving a total coverage of 967 Å. We produce ∼400 000 spectra of interstellar medium (ISM) absorption features and correct them for radial velocities of the DIB clouds. Ultimately, we combine the 33 115 best ISM spectra into six reddening bins with a range of 0.1 < E (B − V) < 0.7 mag. A total of 183 absorption features in these spectra qualify as DIBs, and their fitted model parameters are summarized in a detailed catalogue. From these, 64 are not reported in the literature, of which 17 are certain, 14 are probable and 33 are possible. We find that the broad DIBs can be fitted with a multitude of narrower DIBs. Finally, we create a synthetic DIB spectrum at unit reddening. This should allow us to narrow down the possible carriers of DIBs, to explore the composition of the ISM, and ultimately to model dust and star formation better as well as to correct Galactic and extragalactic observations. The majority of certain DIBs show a significant excess of equivalent width when compared with reddening. We explain this with observed lines of sight penetrating more uniform DIB clouds compared with clumpy dust clouds. [ABSTRACT FROM AUTHOR]

We present isochrone ages and initial bulk metallicities (⁠|$\rm [Fe/H]_{bulk}$|⁠ , by accounting for diffusion) of 163 722 stars from the GALAH Data Release 2, mainly composed of main-sequence turn-off stars and subgiants (⁠|$7000\, \mathrm{ K}> T_{\mathrm{ eff}}> 4000\, \mathrm{ K}$| and |$\log g>3$|  dex). The local age–metallicity relationship (AMR) is nearly flat but with significant scatter at all ages; the scatter is even higher when considering the observed surface abundances. After correcting for selection effects, the AMR appears to have intrinsic structures indicative of two star formation events, which we speculate are connected to the thin and thick discs in the solar neighbourhood. We also present abundance ratio trends for 16 elements as a function of age, across different |$\rm [Fe/H]_{bulk}$| bins. In general, we find the trends in terms of [X/Fe] versus age from our far larger sample to be compatible with studies based on small (∼100 stars) samples of solar twins, but we now extend them to both sub- and supersolar metallicities. The α-elements show differing behaviour: the hydrostatic α-elements O and Mg show a steady decline with time for all metallicities, while the explosive α-elements Si, Ca, and Ti are nearly constant during the thin-disc epoch (ages |$\lesssim \! 12$|  Gyr). The s-process elements Y and Ba show increasing [X/Fe] with time while the r-process element Eu has the opposite trend, thus favouring a primary production from sources with a short time delay such as core-collapse supernovae over long-delay events such as neutron star mergers. [ABSTRACT FROM AUTHOR]

Context. The Orion complex is arguably the most studied star-forming region in the Galaxy. While stars are still being born in the Orion nebula, the oldest part was believed to be no more than 13 Myr old. Aims. In order to study the full hierarchy of star formation across the Orion complex, we perform a clustering analysis of the Ori OB1a region using new stellar surveys and derive robust ages for each identified stellar aggregate. Methods. We use Gaia DR2 parameters supplemented with radial velocities from the GALAH and APOGEE surveys to perform clustering of the Ori OB1a association. Five overdensities are resolved in a six-dimensional parameter space (positions, distance, proper motions, and radial velocity). Most correspond to previously known structures (ASCC 16, 25 Orionis, ASCC 20, ASCC 21). We use Gaia DR2, Pan-STARRS1 and 2MASS photometry to fit isochrones to the colour-magnitude diagrams of the identified clusters. The ages of the clusters can thus be measured with ∼10% precision. Results. While four of the clusters have ages between 11 and 13 Myr, the ASCC 20 cluster stands out at an age of 21 ± 3 Myr. This is significantly greater than the age of any previously known component of the Orion complex. To some degree, all clusters overlap in at least one of the six phase-space dimensions. Conclusions. We argue that the formation history of the Orion complex, and its relation to the Gould belt, must be reconsidered. A significant challenge in reconstructing the history of the Ori OB1a association is to understand the impact of the newly discovered 21 Myr old population on the younger parts of the complex, including their formation. [ABSTRACT FROM AUTHOR]

We present a study using the second data release of the GALAH survey of stellar parameters and elemental abundances of 15 pairs of stars identified by Oh et al. They identified these pairs as potentially co-moving pairs using proper motions and parallaxes from Gaia DR1. We find that 11 very wide (>1 pc) pairs of stars do in fact have similar Galactic orbits, while a further four claimed co-moving pairs are not truly co-orbiting. Eight of the 11 co-orbiting pairs have reliable stellar parameters and abundances, and we find that three of those are quite similar in their abundance patterns, while five have significant [Fe/H] differences. For the latter, this indicates that they could be co-orbiting because of the general dynamical coldness of the thin disc, or perhaps resonances induced by the Galaxy, rather than a shared formation site. Stars such as these, wide binaries, debris of past star formation episodes, and coincidental co-orbiters, are crucial for exploring the limits of chemical tagging in the Milky Way. [ABSTRACT FROM AUTHOR]