What types of presolar grains are known?
Currently, seven different types of presolar grains have been discovered in primitive meteorites:
Diamonds are the smallest presolar grains that have been identified; they are typically about 2 nanometers (2 billionths of a meter) in diameter and only contain on order of one thousand atoms. They are believed to be presolar because they contain isotopically unusual xenon and nitrogen. The xenon composition suggests that the grains originated in supernova explosions. However, only about one diamond in a million contains any xenon atoms at all, so it is possible that most of the diamonds formed somewhere else. On the right is a micrograph (obtained by transmission electron microscopy) of some presolar diamonds. (Photo courtesy T. Daulton)
Silicon Carbide (SiC):
Presolar SiC is probably the best-studied of any of the known presolar grain types. SiC grains range in size from about 0.1 micron up to as large as 20 microns ( 1 micron= 1 millionth of a meter). Shown on the right is a scanning electron micrograph of a SiC grain from the Murchison meteorite; the grain is about 2 microns in diameter. Just about every element ever measured in presolar SiC grains has been found to be isotopically unusual, including silicon, carbon, nitrogen, magnesium, calcium, titanium and many others. Most presolar SiC is believed to have formed in "AGB" stars, a certain type of very old red giant star that is rich in carbon, but some of the grains have isotopic signatures indicating other stellar sources, including supernovae and novae.
Presolar graphite grains, like SiC, can be relatively large, up to 20 microns in diameter. They also have unusual isotopic compositions in a large number of elements. Many graphite grains, like the one shown on the right have an "onion"-like structure, consisting of concentric sheets of graphitic material. Although some graphite is believed to have formed in AGB stars, the majority of the grains probably formed in very massive stars (more than ten times the mass of the Sun) either during a very strong stellar wind or in a supernova explosion. Graphite grains have also been found to contain tiny sub-grains of other minerals, including titanium carbide, iron carbide and iron-nickel metal. (Photo courtesy S. Amari)
Silicon Nitride (Si3N4):
Only a few Si3N4 grains have been found in meteorites so far. Although similar in size and appearance to presolar SiC grains, presolar silicon nitride is much less common. The isotopic compositions of these grains indicate they probably formed in supernova explosions, and are similar to certain types of SiC and graphite grains.
Aluminum Oxide (Al2O3), Spinel (MgAl2O4), Titanium Oxide (TiO2):
Only about 150 presolar O-rich grains have now been identified, in contrast to the thousands of SiC and graphite grains that have been measured. Most of these (like the photo on the right) are aluminum oxide, but a few grains of spinel have also been found. Only one oxide grain that is not rich in aluminum has been found, a grain of TiO2. The isotopic compositions of most of the oxide grains suggest they formed around red giant stars and oxygen-rich AGB stars. However, one or two of the grains probably formed in supernovae, and a few grains are not yet explained.