Interstellar Graphite Onion - Core/Rim Ratios

Contributors: Kay Brewer, David Dawkins, and Minh Truong.

Since the early 1960's, noble gas (i.e. xenon, argon, neon) anomalies in primitive carbonaceous chondrite meteorites have pointed toward the existence of materials, unhomogenized with the rest of the meteorite, containing atoms nucleosynthesized in the interior of different types of stars. In the past decade, such noble gas isotopic anomalies have been tracked to nanometer-sized diamonds, and to micron-sized silicon carbide crystals and graphite particles, in these meteorites. The graphite particles so identified appear with two morphologies, reminescent respectively of the vegetables cauliflower and onions. We focus here on the onions.

A Possible Source of Core-Rim Ratio Variability

  • Radiation pressure theory and classical gravitation, along with knowledge of red-giant luminosities, suggest that core-radius r and total radius R might obey: R - r ~ Sqrt[2r] * 0.1Ám, where 0.1 is an empirical constant from laboratory data, and R and r are in microns.

    For more on this, cf. Dave Dawkin's webnote.

    One Strategy for Accessing Core/Rim Data from Ultra-Microtomed Graphite Spheres

    Adjacent slices may remain connected between knife slices (and hence found together in a section, if the slice is folded down & pulled under the knife (as sometimes happens when slicing salami) by the flexible but high lateral-strength skin (provided in our onions by a graphite rim, in a salami by the enclosing skin).

    For more on this, cf. a slide I put together for Minh's presentation in Spring 1999.

    Related Links: Invited overview presented at the 1998 AAPT meeting in New Orleans, and an Image of the Month Note on high-resolution studies of these spheres.