Presolar grains are isotopically-distinct clusters of material found in the fine-grained matrix of primitive meteorites, such as chondrites, whose differences from the surrounding meteorite suggest that they are older than the solar system.[1] Crystallinity in these clusters ranges from that of micrometre-sized silicon carbide crystals, down to that of diamond and unlayered graphene crystals with fewer than 100 atoms. These grains were probably formed in supernovae or the stellar outflows of red giant stars, and then incorporated into the molecular cloud from which the solar nebula separated to form our solar system. Presolar grains identified so far consist of refractory minerals which survived the collapse of the solar nebula, and the subsequent formation of planetesimals.
In the 1960s, neon[2] and xenon[3] components with unusual isotopic ratios were discovered in primitive meteorites. One possible explanation: The existence of intact presolar grains containing anomalous noble gas within these meteorites.[4]
In 1987 diamond[5] and silicon carbide[6] grains were found to be carriers of these noble gases. Significant major-element isotopic anomalies were in turn found within these grains.[7]
Because identified presolar stardust grains formed in the neighborhood of specific stellar nucleosynthesis sources, the isotopic composition of their elements is usually different from the isotopic composition of solar-system matter as well as from the galactic average. These isotopic signatures often fingerprint very specific astrophysical nuclear processes,[8] and thus they go beyond proving interstellar origin to providing independent insight into the way stars work.[9]
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Presolar grains consisting of the following minerals have so far been identified:
Presolar grains are investigated using scanning or transmission electron microscopes (SEM/TEM), and mass spectrometric methods (noble gas mass spectrometry, resonance ionization mass spectrometry (RIMS), secondary ion mass spectrometry (SIMS, NanoSIMS)). Presolar grains that consist of diamonds are only a few nanometers in size, and are therefore also called nanodiamonds. Because of their small size, nanodiamonds are hard to investigate and, although they are among the first presolar grains discovered, relatively little is known about them. The typical sizes of other presolar grains are in the range of micrometres. Presolar grains represent material from outside our solar system.
The study of presolar grains provides information about nucleosynthesis and stellar evolution.[19] Grains bearing the isotopic signature of rapid-process nucleosynthesis are useful in testing models of supernovae explosions. Other grains provide isotopic and physical information on asymptotic giant branch stars, which have manufactured the lion's share of the refractory elements lighter than iron in the galaxy. Because the elements in these particles were made at different times (and places) in the early Milky Way, the set of collected particles further provides insight into galactic evolution prior to formation of our solar system.
Along with providing information on nucleosynthesis, solid grains provide information on the physico-chemical conditions under which they formed, and on events subsequent to their formation. For example consider red giants - which produce much of the carbon in our galaxy. Their atmospheres are cool enough for condensation processes to take place - resulting in the precipitation of solid particles (i.e. multiple atom agglomerations of elements - such as Carbon) - in their atmosphere. This is unlike the atmosphere of our sun, which is too hot to allow build-up of atoms to form more complex molecules. These solid fragments of matter, are then injected into the interstellar medium by radiation pressure. Hence particles bearing the signature of stellar nucleosynthesis are providing us with information on: (i) condensation processes in red giant atmospheres, (ii) radiation and heating processes in the interstellar medium, and (iii) the types of particles that carried the elements of which we are made across the galaxy to our Solar system.