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We present a study of interstellar Zn and Cr absorption toward 20 stars observed with the Hubble Space Telescope\/ Goddard High Resolution Spectrograph based on echelle spectra obtained through the small science aperture prior to 1993 September and extracted from the HST\/ archive. The goal of this work is to provide an accurate measure of the Zn/Cr column density ratio in the Galactic ISM and quantify the range in values determined from disk and halo lines of sight. Zinc is widely regarded as an accurate metallicity indicator in neutral interstellar gas. In contrast, chromium is readily depleted onto grains with typically only a few percent of the total Cr abundance existing in the gas phase.
The ultraviolet lines of Cr II and Zn II have been used to measure the metallicity and depletion characteristics of high-redshift ($1\leq z \leq 3$) damped Lyman-$\alpha$ QSO absorption systems. These lines are well suited for this purpose since for $z\geq0.7$ they are redshifted into the optical where they can be observed by ground-based telescopes. In addition, the lines are typically weak ($W_\lambda\leq 0.2$ \AA) allowing for the derivation of accurate column densities and abundance information. Typical values for the Cr/Zn column density ratio in these systems range between $2 - 4$, indicating that while $\approx 65-80$\% of the Cr has been removed from the gas phase (Cr$_\odot$/Zn$_\odot=11$), there has still been significantly less processing of heavy elements into dust at high redshift than in local disk material.
We have derived column densities from multi-component Voigt profiles fitted to the archive spectra, and we find the Cr/Zn column density ratio varies by a factor of 25 ($\sim 0.1 - 2.5$) in our sample of Galactic stars. The standard values for $\zeta$ Oph and $\xi$ Per against which the high-$z$ results are usually compared are in the low end of this range, while in the highest ratios only $\approx$ $75-80$\% of the Cr has been removed onto dust grains which overlaps the highest depletion values seen toward QSOs. We will be using these results to further interpret existing high-redshift damped Lyman-$\alpha$ depletion measurements.
