Sachiko Amari Washington University St. Louis MO, USA Presolar diamond in meteorites: A few outstanding problems (and how we plan to investigate them) Sachiko Amari Washington University St. Louis MO, USA

Presolar Grains Dust produced in the stellar outflow or stellar ejecta Incorporated into meteorites 4.6 billion years ago (=older than the solar system) STARDUST IN METEORITES (not to be confused with the dust recovered from the STADUST mission) Abundance: ≤ 0.1 weight percent of a bulk meteorite Allende meteorite

Presolar diamond First mineral type that was isolated from meteorites (1987) One of the least understood presolar grain types Abundance: ~1000 ppm in primitive meteorites Small grain size: 3nm Only bulk (=aggregate) analysis is possible Contains an isotopically anomalous Xe called Xe-HL Enriched in L -- p-process only isotopes 124 and 126 H -- r-process only isotopes 134 and 136 Xe-L Xe-H

Puzzle 1 The excess in the p-only isotopes (124 & 126) and that in the r-process only isotopes (134 & 136) are always correlated. (= Xe-H and Xe-L are ALWAYS correlated.) This correlation has been observed in all diamond separates from any type of primitive meteorites. Close association of the two different nucleosynthetic processes. Possible explanation Diamond grains are very small (average size: 3nm) and grains with Xe-L and those with Xe-H are homogeneously mixed.

Puzzle 2 r- and p-Process patterns inferred from Xe-HL are NOT what is expected from the Solar System abundance and models of stars. Xe-HL = R-process in diamond + P-process in diamond + solar Xe (Pure r- and p-process Xe does not contain 130) R-process in diamond = measured Xe-H - solar Xe P-process in diamond = measured Xe-L - solar Xe (134/136)Diamond-R = 0.699, (124/126)Diamond-P = 2.205 R- and p-process Xe in the Solar System (134/136)SS-R = 1.207* * Solar Xe - (s-process Xe in SiC) (Ott, 1996) (124/126)SS-P = 1.157 Theoretical models (Rayet et al., 1995) (124/126)SN20Msun = 1.188

Effort to explain the r-process pattern in diamond 1. Rapid separation model (Ott, 1996) Precursors of 134Xe and 136Xe (Te and I) have different half-lives. 136Te (T1/2 = 17.5 s) 136I (T1/2 = 1.39 m) 134Te (T1/2 = 42 m) 134I (T1/2 = 53 m) Difficulties Separation of Xe and the unstable precursors (Te and I) need to take place in a timescale of hours. Needs a VERY fine tuning of the timing of the separation. 124Xe and 126Xe can be explained in the same manner with Ba Cs precursors (Ott, 1996).

2. Mix with n-burst material Effort to explain the r-process pattern in diamond (continued) 2. Mix with n-burst material Neutron burst (Meyer et al., 2000) Occurs in shocked He-rich matter T=109K ~1017 neutrons/cm3 Mix of neutron burst and solar material Solar = 0.79 N-burst = 0.21 Xe-L: left unexplained

Other elements in diamond Isotopic analyses of heavy elements that were produced in the same processes as diamond may shed light on the origin of Xe-HL. Isotopic anomalies in other elements are very small compared with those in Xe (Richter et al., 1998; Mass et al., 2001) Tellurium [Te] (128 & 130 r-only, 124 s-only) 128Te/124Te & 130Te/124Te < 1.009 times solar Pd (104 s-only, 110 r-only) 110Pd/104Pd: 1.01 times solar Possible explanation Presence of contaminants (Ir-nuggets, SiC etc.)

Isotopic analysis of heavy elements in diamond Low concentrations Any contaminants compromise the analysis Our Plans First Step Produce diamond separates of ultra-high purity. Complete removal of SiC, Ir nuggets Will be done at Washington University Second Step Perform isotopic analyses of heavy elements SIMS -- high mass resolution power (M/M > 10,000 required) RIMS (Resonant Ionization Mass Spectrometry) (Lewis et al., 1991)

RIMS analysis at Argonne National Laboratory (Michael Pellin and Michael Savina and their coworkers) Atoms are desorbed from the sample with a laser, resonantly ionized with several tuned lasers, and analyzed with a time-of-flight mass spectrometer. Advantage of the instrument An element of interest is selectively ionized and detected. CHARISMA (The Chicago Argonne Resonance Ionization Spectrometer for Microbeam Analysis) SARISA (Surface Analysis by Resonant Ionization of Sputtered Atoms) Useful yields (ions detected/atoms removed) CHARISMA - 1 to 2% SARISA - up to 25% (Nicolussi et al., 1998)

Summary Diamond One of the least understood presolar grains Puzzles 1) Excesses in the p-only isotopes (124 & 126) and in the r-process only isotopes are always correlated. 2) Their patterns are different from what is derived from the solar system abundance and from models. Future plans 1) Produce ultra-high purity diamond 2) Analyze isotopic ratios of heavy elements using RIMS