Chemical Tagging

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.

Quantifying chemical tagging: towards robust group finding in the Galaxy

The first generation of large-scale chemical tagging surveys, in particular the High Efficiency and Resolution Multi-Element Spectrograph (HERMES)/Galactic Archaeology with HERMES million star survey, promises to vastly expand our understanding of the chemical and dynamical evolution of the Galaxy. This, however, is contingent on our ability to confidently perform chemical tagging on such a large data set.

The Galactic Archaeology with HERMES Survey

HERMES is a multi-fibre spectrograph being built for the AAT 3.9m telescope, designed to simultaneously obtain high resolution (R ˜ 28000) spectra for ˜ 400 stars over a 2° field of view. The Galactic Archaeology with HERMES (GALAH) Survey is a major Australian-led project to obtain detailed elemental abundances for a million stars, with the goal of using chemical tagging to decipher the formation history of the Milky Way.

The HERMES Project: Reconstructing Galaxy Formation

The primary driver for the HERMES multi-object high resolution spectrometer on the AAT is a Galactic archaeology survey of about a million stars with V < 14. I will give a brief overview of the instrument, Galactic archaeology and chemical tagging, and then describe the goals and plans for the GALAH survey.

Structure and Evolution of the Milky Way

This review discusses the structure and evolution of the Milky Way, in the context of opportunities provided by asteroseismology of red giants. The review is structured according to the main Galactic components: the thin disk, thick disk, stellar halo, and the Galactic bar/bulge. The review concludes with an overview of Galactic archaeology and chemical tagging, and a brief account of the upcoming HERMES survey with the AAT.

Principal component analysis on chemical abundances spaces

In preparation for the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) chemical tagging survey of about a million Galactic FGK stars, we estimate the number of independent dimensions of the space defined by the stellar chemical element abundances [X/Fe]. This leads to a way to study the origin of elements from observed chemical abundances using principal component analysis.

The Chemical Signatures of the First Star Clusters in the Universe

The chemical abundance patterns of the oldest stars in the Galaxy are expected to contain residual signatures of the first stars in the early universe. Numerous studies attempt to explain the intrinsic abundance scatter observed in some metal-poor populations in terms of chemical inhomogeneities dispersed throughout the early Galactic medium due to discrete enrichment events. Just how the complex data and models are to be interpreted with respect to "progenitor yields" remains an open question. Here we show that stochastic chemical evolution models to date have overlooked a crucial fact.

The Long-term Evolution of the Galactic Disk Traced by Dissolving Star Clusters

The Galactic disk retains vast amount of information about how it came to be and how it evolved over cosmic time. However, we know very little about the secular processes associated with disk evolution. One major uncertainty is the extent to which stars migrate radially through the disk, thereby washing out signatures of their past (e.g., birth sites). Recent theoretical work finds that such "blurring" of the disk can be important if spiral arms are transient phenomena. Here we describe an experiment to determine the importance of diffusion from the Solar circle with cosmic time.

Reconstructing Fossil Sub-structures of the Galactic Disk: Clues from Abundance Patterns of Old Open Clusters and Moving Groups

The long term goal of large-scale chemical tagging is to use stellar elemental abundances as a tracer of dispersed substructures of the Galactic disk. The identification of such lost stellar aggregates and the exploration of their chemical properties will be key in understanding the formation and evolution of the disk. Present day stellar structures such as open clusters and moving groups are the ideal testing grounds for the viability of chemical tagging, as they are believed to be the remnants of the original larger star-forming aggregates.

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