The abundance of interstellar krypton

We present high signal-to-noise ratio HST Goddard High Resolution Spectrograph (GHRS) observations of the weak interstellar Kr I lambda 1236 absorption toward the stars tau CMa, kappa Ori, epsilon Ori, lambda Ori, delta Sco, omega(1) Sco, epsilon Per, and zeta Per. In combination with previous GHRS measurements of Kr I in two other sight lines (zeta Oph and 1 Sco), these new observations yield a mean interstellar gas-phase krypton abundance (per 10(9) H atoms) of 10(9) Kr/ H = 0.96 +/- 0.05. There is no statistically significant variation from sight line to sight line in the measured Kr I abundance and, in particular, no evidence for any correlation with the fraction of hydrogen in molecular form. Since Kr, as a noble gas, is not expected to deplete into dust grains, its gas-phase abundance should reflect the total interstellar abundance. Consequently, the GHRS observations imply that the interstellar Kr abundance in the vicinity of the Sun is about 60% of the solar system value of 10(9) Kr/H = 1.70 +/- 0.30. This interstellar abundance deficit is similar to that recently found for oxygen with GHRS.