Highlights

Focused Ion Beam Welding of Stardust Grains

Scientific Achievement

Presolar stardust grains are samples of stellar matter ejected from stars before the solar system formed.  Isolated from primitive meteorites, they constitute one component of the material from which the Sun and planets formed.  The vast majority of stardust grains initially present were melted in the early solar system, thus creating new materials representing an average over all the stars that contributed to it.  This is what we see in the Sun and planets today.  However, some grains survived and are found today in primitive meteorites.  Each surviving stardust grain carries a record of its parent star and, and presents a unique opportunity to study stars directly. 

Several stardust minerals have been identified, including, silicon carbide, graphite, and others. Some grains are large enough (at least 0.5 micron in diameter) to be analyzed individually.  Silicon carbide grains are the most extensively studied, and much has been learned from them.  Graphite stardust grains are also large and abundant.  They derive mostly from the same stars as the SiC grains, but from different stages of stellar evolution, and thus carry different information.

Graphite grains present unique analytical challenges.  While SiC is an extremely hard mineral, graphite is extremely soft.  SiC grains are immobilized for analysis by being pressed into a gold foil.  This technique fails for graphite, producing only a smear where the grain used to be.  The  Electron Microscopy Center (EMC) has developed a focused ion beam (FIB) technique that solves the problem by "welding" the grains to the gold surface with amorphous carbon.  Initial experiments proved highly successful in locating, welding and marking the grains on the foil for subsequent laser analysis.  Further, secondary electron microscopy performed in the EMC allows us to catalog each grain’s shape and size prior to analysis, and to later quantify how much material was removed during analysis in order to determine the absolute abundance as well as the isotopic composition of the elements studied.

Significance

This work has enabled us to make some of the first heavy metal analyses in graphite stardust grains.  Prior to developing this technique it was impossible to do the analysis because the grains would hop off the foil during analysis and be lost.  We have determined that these graphite grains came from asymptotic giant branch stars 1.5 to 3 times more massive than the Sun, and show the signature of certain neutron capture processes occurring in the parent stars.

Performers

M. Savina, J. Hiller (ANL-MSD); E. Zinner, M. Jadhav (Washington U. St. Louis)