Science

The GALAH Survey and Gaia DR2: (Non)existence of four sparse high-latitude open clusters

Sparse open clusters can be found at high galactic latitudes where loosely populated clusters are more easily detected against the lower stellar background. As bursty star formation takes place in the thin disk, the population of clusters far from the Galactic plane is hard to explain. We combined spectral parameters from the GALAH survey with the Gaia DR2 catalogue to study dynamics and chemistry of 5 old sparse high-latitude clusters in more detail.

Catalog of carbon enhanced stars and CEMP candidates

The Galactic Archaeology with HERMES (GALAH) survey is a large scale, magnitude limited, southern stellar spectroscopic survey providing spectra, stellar parameters and chemical abundances for stars located in different components of the Galaxy. Random selection, based only on object's brightness and its celestial coordinates, implies that a representative set of peculiar stellar types are observed.

Here we focus on carbon enhanced stars which can be identified by peculiar features present in their GALAH spectra.

VELOCITY FLUCTUATIONS IN THE MILKY WAY USING RED CLUMP GIANTS

Context: If the Galaxy is axisymmetric and in dynamical equilibrium, we expect negligible fluctuations in the residual line-of-sight velocity field. However, non-axisymmetric structures like a bar, spiral arms and merger events can generate velocity fluctuations. Recent results using the APOGEE survey claim significant fluctuations in velocity for stars in the mid plane (|z|<0.25 kpc) out to 5 kpc and suggest that the dynamical influence of the Milky Way's bar extends out to the Solar neighborhood.

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The GALAH Survey: Stellar streams and how stellar velocity distributions vary with Galactic longitude, hemisphere and metallicity

Using GALAH survey data of nearby stars, we look at how structure in the planar (u,v) velocity distribution depends on metallicity and on viewing direction within the Galaxy. In nearby stars, with distance d < 1 kpc, the Hercules stream is most strongly seen in higher metallicity stars [Fe/H] > 0.2. The Hercules stream peak v value depends on viewed galactic longitude, which we interpret as due to the gap between the stellar stream and more circular orbits being associated with a specific angular momentum value of about 1640 km/s kpc.

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We look at distributions of stars from regions that are near the Sun but seen in different directions on the sky. We find that there are differences in the distributions of stellar velocities.

Non-LTE abundance patterns in M67

One of the main goals of the Galah survey is to find stellar siblings in the Galactic disk and associate them to a common parent cluster by means of chemistry and dynamics. The success of such chemical tagging hinges critically on our ability to determine the abundances of late-type dwarf and giant stars with high precision, but also to assess whether their present-day abundance patterns truly reflect their original compositions.

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Confirmation of co-moving star pairs identified in Gaia DR1

Recently, Oh et al published "Co-moving stars in Gaia DR1" (http://arxiv.org/abs/1612.02440) which identified over 13000 co-moving star pairs in Gaia DR1 based upon their proper motions and parallaxes. Some of the pairs form larger groups (e.g., they recovered the Pleiades).

About 350 of these pairs of stars are also in the Cannon 1.3 table. Oh et al found a similar sized overlap with RAVE. We would be able to use the GALAH results to investigate the 3D space motions and chemistry of these stars to identify those stars which true binaries and clusters.

t-SNE as a tool for studying clustering in the elemental abundance space

One of the main motivations for the GALAH survey is to measure abundances of many elements in sufficiently large number of stars that some of them can be identified as stars that were born in the same cluster but all indications of this fact have been lost, except for the chemical fingerprint. Chemical tagging can reveal the connection between such stars, but state of the art observations and analytical methods will be needed to actually perform this task.

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The GALAH Survey: Separating the thin and thick disks

Almost all spiral galaxies have a second disk component, the thick disk, in addition to the thin disk which defines their disk structure. Thick disks are believed to be ancient structures that predates the formation of the thin disks, but how they fit in to the overall picture of galaxy formation remains unknown. Although our Galaxy has a thick disk, the properties of this ancient component are not yet well determined.

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