We present a chemical abundance study of the brightest confirmed member star of the ultrafaint dwarf galaxy Boötes II from Keck/HIRES high-resolution spectroscopy at moderate signal-to-noise ratios .
At [ Fe/H ] = -2.93 \pm 0.03 ( stat . ) \pm 0.17 ( sys . )
this star chemically resembles metal-poor halo field stars and the signatures of other faint dwarf spheroidal galaxies at the same metallicities in that it shows enhanced [ \alpha /Fe ] ratios , Solar Fe-peak element abundances , and low upper limits on the neutron-capture element Ba .
Moreover , this star shows no chemical peculiarities in any of the eight elements we were able to measure .
This implies that the chemical outliers found in other systems remain outliers pertaining to the unusual enrichment histories of the respective environments , while Boo II appears to have experienced an enrichment history typical of its very low mass .
We also re-calibrated previous measurements of the galaxy ’ s metallicity from the calcium triplet ( CaT ) and find a much lower value than reported before .
The resulting broad metallicity spread , in excess of one dex , the very metal poor mean , and the chemical abundance patterns of the present star imply that Boötes II is a low-mass , old , metal poor dwarf galaxy and not an overdensity associated with the Sagittarius Stream as has been previously suggested based on its sky position and kinematics .
The low , mean CaT metallicity of - 2.7 dex falls right on the luminosity-metallicity relation delineated over four orders of magnitude from the more luminous to the faintest galaxies .
Thus Boötes II ’ s chemical enrichment appears representative of the galaxy ’ s original mass , while tidal stripping and other mass loss mechanisms were probably not significant as for other low-mass satellites .