Context : Recent spectroscopic surveys have begun to explore the Galactic disk system on the basis of large data samples , with spatial distributions sampling regions well outside the solar neighborhood .
In this way , they provide valuable information for testing spatial and temporal variations of disk structure kinematics and chemical evolution .

Aims : The main purposes of this study are to demonstrate the usefulness of a rigorous mathematical approach to separate substructures of a stellar sample in the abundance-metallicity plane , and provide new evidence with which to characterize the nature of the metal-poor end of the thin disk sequence .

Methods : We used a Gaussian mixture model algorithm to separate in the [ Mg/Fe ] vs. [ Fe/H ] plane a clean disk star subsample ( essentially at { \textmd { R } } _ { \textmd { G } C } < 10 kpc ) from the Gaia-ESO survey ( GES ) internal data release 2 ( iDR2 ) .
We aim at decomposing it into data groups highlighting number density and/or slope variations in the abundance-metallicity plane .
An independent sample of disk red clump stars from the Apache Point Observatory Galactic Evolution Experiment ( APOGEE ) was used to cross-check the identified features .

Results : We find that the sample is separated into five groups associated with major Galactic components ; the metal-rich end of the halo , the thick disk , and three subgroups for the thin disk sequence .
This is confirmed with the sample of red clump stars from APOGEE .
The three thin disk groups served to explore this sequence in more detail .
The two metal-intermediate and metal-rich groups of the thin disk decomposition ( [ Fe/H ] > -0.25 dex ) highlight a change in the slope at solar metallicity .
This holds true at different radial regions of the Milky Way .
The distribution of Galactocentric radial distances of the metal-poor part of the thin disk ( [ Fe/H ] < -0.25 dex ) is shifted to larger distances than those of the more metal-rich parts .
Moreover , the metal-poor part of the thin disk presents indications of a scale height intermediate between those of the thick and the rest of the thin disk , and it displays higher azimuthal velocities than the latter .
These stars might have formed and evolved in parallel and/or dissociated from the inside-out formation taking place in the internal thin disk .
Their enhancement levels might be due to their origin from gas pre-enriched by outflows from the thick disk or the inner halo .
The smooth trends of their properties ( their spatial distribution with respect to the plane , in particular ) with [ Fe/H ] and [ Mg/Fe ] suggested by the data indicates a quiet dynamical evolution , with no relevant merger events .

Conclusions :