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With moderate-to-high resolution gratings and the capability to produce high signal-to-noise data ($S/N>>100/1$), the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope has as one of its most significant attributes the ability to produce unprecedented data on weak UV absorption lines (e.g. $W_\lambda\leq2$ m\AA). For example, the GHRS has been used to obtain new data on weak semi-forbidden transitions of important cosmically abundant species like C II (Cardelli et al. 1991, ApJ, 377, L57; Cardelli et al. 1993, ApJ, 402, L17) and Si II (Cardelli et al. 1993, ApJ, in press) as well as low cosmic abundance species like B II and Co II (Federman et al. 1993, ApJ, 413, L51). In this paper we present data on the first interstellar detections of the heavy elements Tl (Z = 81) and Pb (Z = 82) toward the star $\zeta$ Ophiuchi. Together with recent GHRS detections of Ga (Z = 31), Ge (Z = 32), Kr (Z = 36), Sn (Z = 50) (Cardelli, Savage, and Ebbets 1991, ApJ, 383, L23; Hobbs et al. 1993, ApJ, 411, 750), As (Z = 33), and Se (Z = 34) (Cardelli et al. 1993, ApJ, 416, L41), this brings to eight the number of heavier-than-Zn (Z = 30) elements thus far observed in the ISM. What makes these heavy elements important is that they arise from nucleosynthetic pathways (s- and r-process) uniquely different from those which produce Zn and the lighter elements. Consequently, these elements offer a unique opportunity to study the effects of nucleosynthetic enrichment, the mixing of the interstellar gas, and the efficiency with which heavy elements chemically interact with interstellar dust.
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