A New Test of Copper and Zinc Abundances in Late-Type Stars Using Ultraviolet Cu II and Zn II Lines

TL;DR

This study uses ultraviolet and optical spectra to accurately measure copper and zinc abundances in metal-poor stars, confirming minimal non-LTE effects for zinc and highlighting LTE underestimations for copper.

Contribution

It provides the first consistent comparison of Cu and Zn abundances from neutral and ionized lines, validating non-LTE models and refining understanding of element enhancement in metal-poor stars.

Findings

Zn abundances from Zn I and Zn II agree within 0.1 dex.

Cu II lines yield higher Cu abundances than Cu I lines in metal-poor stars.

Results support non-LTE calculations and previous theories on element enhancement.

Abstract

We present new abundances derived from Cu I, Cu II, Zn I, and Zn II lines in six warm (5766 < Teff < 6427 K), metal-poor (-2.50 < [Fe/H] < -0.95) dwarf and subgiant (3.64 < log g < 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 (< 0.1 dex). We find that the [Cu/H] ratios derived from Cu II lines are 0.36 +/- 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.