A small fraction of the halo field is made up of stars that share the light element ( Z \leq 13 ) anomalies characteristic of second generation globular cluster ( GC ) stars .
The ejected stars shed light on the formation of the Galactic halo by tracing the dynamical history of the clusters , which are believed to have once been more massive .
Some of these ejected stars are expected to show strong Al enhancement at the expense of shortage of Mg , but until now no such star has been found .
We search for outliers in the Mg and Al abundances of the few hundreds of halo field stars observed in the first eighteen months of the Gaia-ESO public spectroscopic survey .
One halo star at the base of the red giant branch , here referred to as 22593757-4648029 is found to have [ Mg / Fe ] = -0.36 \pm 0.04 and [ Al / Fe ] = 0.99 \pm 0.08 , which is compatible with the most extreme ratios detected in GCs so far .
We compare the orbit of 22593757-4648029 to GCs of similar metallicity and find it unlikely that this star has been tidally stripped with low ejection velocity from any of the clusters .
However , both chemical and kinematic arguments render it plausible that the star has been ejected at high velocity from the anomalous GC \omega Centauri within the last few billion years .
We can not rule out other progenitor GCs , because some may have disrupted fully , and the abundance and orbital data are inadequate for many of those that are still intact .