We present new abundances derived from Cu i , Cu ii , Zn i , and Zn ii lines in six warm ( 5766 \leq T _ { eff } \leq 6427 K ) , metal-poor ( - 2.50 \leq [ Fe/H ] \leq - 0.95 ) dwarf and subgiant ( 3.64 \leq log g \leq 4.44 ) stars .
These abundances are derived from archival high-resolution ultraviolet spectra from the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope and ground-based optical spectra from several observatories .
Ionized Cu and Zn are the majority species , and abundances derived from Cu ii and Zn ii lines should be largely insensitive to departures from local thermodynamic equilibrium ( LTE ) .
We find good agreement between the [ Zn/H ] ratios derived separately from Zn i and Zn ii lines , suggesting that departures from LTE are , at most , minimal ( \lesssim 0.1 dex ) .
We find that the [ Cu/H ] ratios derived from Cu ii lines are 0.36 \pm 0.06 dex larger than those derived from Cu i lines in the most metal-poor stars ( [ Fe/H ] < - 1.8 ) , suggesting that LTE underestimates the Cu abundance derived from Cu i lines .
The deviations decrease in more metal-rich stars .
Our results validate previous theoretical non-LTE calculations for both Cu and Zn , supporting earlier conclusions that the enhancement of [ Zn/Fe ] in metal-poor stars is legitimate , and the deficiency of [ Cu/Fe ] in metal-poor stars may not be as large as previously thought .