ar X iv : a st ro - p h / 01 06 20 5 v 3 2 5 A ug 2 00 1 Experimental Limit to Interstellar 244 Pu Abundance

Short-lived nuclides, now extinct in the solar system, are expected to be present in interstellar matter (ISM). Grains of ISM origin were recently discovered in the inner solar system and at Earth orbit and may accrete onto Earth after ablation in the atmosphere. A favorable matrix for detection of such extrater-restrial material is presented by deep-sea sediments with very low sedimentation rates (0.8–3 mm/kyr). We report here on the measurement of Pu isotopic abundances in a 1-kg deep-sea dry sediment collected in 1992 in the North Pacific. Our estimate of (3 ± 3) × 10 5 244 Pu atoms in the Pu-separated fraction of the sample shows no excess over the expected stratospheric nuclear fallout content and under reasonable assumptions sets a limit of 0.2 244 Pu atoms/cm 2 yr for extra-terrestrial deposition. Using available data on ISM steady-state flux on Earth, we derive a limit of 2 × 10 −11 g-244 Pu/g-ISM for the abundance of 244 Pu in ISM.