The GALAH survey: chemical tagging of star clusters and new members in the Pleiades

Janez Kos*, Joss Bland-Hawthorn, Ken Freeman, Sven Buder, Gregor Traven, Gayandhi M. De Silva, Sanjib Sharma, Martin Asplund, Ly Duong, Jane Lin, Karin Lind, Sarah Martell, Jeffrey D. Simpson, Dennis Stello, Daniel B. Zucker, Tomaž Zwitter, Borja Anguiano, Gary Da Costa, Valentina D'Orazi, Jonathan HornerPrajwal R. Kafle, Geraint Lewis, Ulisse Munari, David M. Nataf, Melissa Ness, Warren Reid, Katie Schlesinger, Yuan Sen Ting, Rosemary Wyse

*Corresponding author for this work

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Abstract

The technique of chemical tagging uses the elemental abundances of stellar atmospheres to 'reconstruct' chemically homogeneous star clusters that have long since dispersed. The GALAH spectroscopic survey - which aims to observe one million stars using the Anglo- Australian Telescope - allows us to measure up to 30 elements or dimensions in the stellar chemical abundance space, many of which are not independent. How to find clustering reliably in a noisy high-dimensional space is a difficult problem that remains largely unsolved. Here, we explore t-distributed stochastic neighbour embedding (t-SNE) - which identifies an optimal mapping of a high-dimensional space into fewer dimensions - whilst conserving the original clustering information. Typically, the projection is made to a 2D space to aid recognition of clusters by eye. We show that this method is a reliable tool for chemical tagging because it can: (i) resolve clustering in chemical space alone, (ii) recover known open and globular clusters with high efficiency and low contamination, and (iii) relate field stars to known clusters. t-SNE also provides a useful visualization of a high-dimensional space. We demonstrate the method on a data set of 13 abundances measured in the spectra of 187 000 stars by the GALAH survey. We recover seven of the nine observed clusters (six globular and three open clusters) in chemical space with minimal contamination from field stars and low numbers of outliers. With chemical tagging, we also identify two Pleiades supercluster members (which we confirm kinematically), one as far as 6° - one tidal radius away from the cluster centre.

Original languageEnglish
Pages (from-to)4612-4633
Number of pages22
JournalMonthly Notices of the Royal Astronomical Society
Volume473
Issue number4
DOIs
Publication statusPublished - 1 Feb 2018

Bibliographical note

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 473, Issue 4, 1 February 2018, Pages 4612–4633, https://doi.org/10.1093/mnras/stx2637. Copyright 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

Keywords

  • Methods: data analysis
  • Open clusters and associations: general
  • Open clusters and associations: individual: Pleiades
  • Stars: abundances

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