Dissecting stellar chemical abundance space with t-SNE
  In the era of industrial Galactic astronomy and multi-object spectroscopic
stellar surveys, the sample sizes and the number of available stellar chemical
abundances have reached dimensions in which it has become difficult to process
all the available information in an effective manner. In this paper we
demonstrate the use of a dimensionality-reduction technique (t-distributed
stochastic neighbour embedding; t-SNE) for analysing the stellar
abundance-space distribution. While the non-parametric non-linear behaviour of
this technique makes it difficult to estimate the significance of found
abundance-space substructure, we show that our results depend little on
parameter choices and are robust to abundance errors. By reanalysing the
high-resolution high-signal-to-noise solar-neighbourhood HARPS-GTO sample with
t-SNE, we find clearer chemical separations of the high- and low-[$\alpha$/Fe]
disc sequences, hints for multiple populations in the high-[$\alpha$/Fe]
population, and indications that the chemical evolution of the
high-[$\alpha$/Fe] metal-rich stars is connected with the super-metal-rich
stars. We also identify a number of chemically peculiar stars, among them a
high-confidence s-process-enhanced abundance-ratio pair (HD91345/HD126681) with
very similar ages and $v_X$ and $v_Y$ velocities, which we suggest to have a
common birth origin, possibly a dwarf galaxy. Our results demonstrate the
potential of abundance-space t-SNE and similar methods for chemical-tagging
studies with large spectroscopic surveys.
