We utilize elemental-abundance information for Galactic red giant stars in five open clusters ( NGC 7789 , NGC 6819 , M67 , NGC 188 , and NGC 6791 ) from the Apache Point Observatory Galactic Evolution Experiment ( APOGEE ) DR13 dataset to age-date the chemical evolution of the high- and low- \alpha element sequences of the Milky Way .
Key to this time-stamping is the cluster NGC 6791 , whose stellar members have mean abundances that place it in the high- \alpha , high- [ Fe/H ] region of the [ \alpha /Fe ] - [ Fe/H ] plane .
Based on the cluster ’ s age ( \sim 8 Gyr ) , Galactocentric radius , and height above the Galactic plane , as well as comparable chemistry reported for APOGEE stars in Baade ’ s Window , we suggest that the two most likely origins for NGC 6791 are as an original part of the thick-disk , or as a former member of the Galactic bulge .
Moreover , because NGC 6791 lies at the high metallicity end ( [ Fe/H ] \sim 0.4 ) of the high- \alpha sequence , the age of NGC 6791 places a limit on the youngest age of stars in the high-metallicity , high- \alpha sequence for the cluster ’ s parent population ( i.e. , either the bulge or the disk ) .
In a similar way , we can also use the age and chemistry of NGC 188 to set a limit of \sim 7 Gyr on the oldest age of the low- \alpha sequence of the Milky Way .
Therefore , NGC 6791 and NGC 188 are potentially a pair of star clusters that bracket both the timing and the duration of an important transition point in the chemical history of the Milky Way .