Spectroscopic analysis of stars in the outskirts of NGC 1851

Abstract

NGC 1851 is a peculiar globular cluster with hints of a bimodal populations of stars (possibly two different episodes of formation of stars that resulted in a small spread in the metallicity in the centre of the cluster). It is suggested that it might have been part of a progenitor dwarf galaxy. Spectroscopic analysis of stars in the outskirts of this cluster can indicate its origin by looking at the metallicity distribution. The stars closer to the core of the cluster have a metallicity of [Fe/H]=-1.18 ± 0.2 and the stars studied here are cluster members of NGC 1851 which are obtained based on Gaia DR2 parallaxes and proper motions and for which we have spectroscopic data from the FLAMES instrument on the Very Large Telescope covering a bigger area of the cluster, all the way to the Jacobi radius. The calcium infrared triplet lines, which are strong absorption lines in the range of 8400 Å to 9000 Å are investigated by looking at their equivalent width. The equivalent width, which is related to the metallicity of a star, is studied for 89 stars which are located in the outer regions of the cluster. Indication of a small spread in [Fe/H] is found which might suggest that the cluster has not been part of a progenitor dwarf galaxy.