Interstellar Lead

Abstract

The spectrum of 1 Sco has been recorded in the region of the 1434 A line of Pb II at S/N = 220, using the ECH-A mode of the HST/GHRS. Absorption by interstellar Pb II is seen with an equivalent width W lambda = 0.3 +/- 0.2 (2 sigma ) mA, at a radial velocity which agrees satisfactorily with that of the strongest component of the interstellar Na I D1 line seen in high-resolution optical spectra. If a theoretical oscillator strength f = 0.865 is adopted, the resulting logarithmic depletion is D = -0.97 (+0.22, -0.48) with respect to the meteoritic abundance of Pb. This depletion, which is consistent with Cardelli's result toward zeta Oph, is stronger than that expected from the low condensation temperature, Tc = 496 K, of Pb, in light of the general correlation between the depletions and condensation temperatures of 29 other elements. Some previous ideas about the formation and the evolution of interstellar grains are briefly considered.