A phylogenetic approach to chemical tagging

Reassembling open cluster stars

¹ Observatoire de Genève, Université de Genève, 1290 Versoix, Switzerland
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
e-mail: sblancocuaresma@cfa.harvard.edu
³ Univ. Grenoble Alpes, CNRS, IPAG, Grenoble, France
e-mail: didier.fraix-burnet@univ-grenoble-alpes.fr

Received: 12 February 2018
Accepted: 10 July 2018

Abstract

Context. The chemical tagging technique is a promising approach to reconstructing the history of the Galaxy by only using stellar chemical abundances. Multiple studies have undertaken this analysis and they have raised several challenges.

Aims. Using a sample of open cluster stars, we wish to address two issues: minimize chemical abundance differences whose origin is linked to the evolutionary stage of the stars and not their original composition and evaluate a phylogenetic approach to group stars based on their chemical composition.

Methods. We derived differential chemical abundances for 207 stars, belonging to 34 open clusters, using the Sun as reference star (classical approach) and a dwarf plus a giant star from the open cluster M 67 as reference (new approach). These abundances were then used to perform two phylogenetic analyses: cladistics (maximum parsimony) and neighbor joining, together with a partitioning unsupervised classification analysis with k-means. The resulting groupings were finally confronted to the true open cluster memberships of the stars.

Results. We successfully reconstruct most of the original open clusters when carefully selecting a subset of the abundances derived differentially with respect to M 67. We find a set of eight chemical elements that yield the best result and discuss the possible reasons for these elements to be good tracers of the history of the Galaxy.

Conclusions. Our study shows that unraveling the history of the Galaxy by only using stellar chemical abundances is greatly improved provided that i) we perform a differential spectroscopic analysis with respect to an open cluster instead of the Sun, ii) select the chemical elements that are good tracers of the history of the Galaxy, and iii) use tools that are adapted to detect evolutionary tracks such as phylogenetic approaches.