How do we identify presolar grains?
We recognize presolar grains on the basis of their unusual isotopic ratios. Isotopes are different atoms of the same chemical element that have slightly different mass. For example, the element carbon has two stable (not radioactive) isotopes: carbon-12 (12C) and carbon-13 (13C). Most of the isotopes of most of the elements are made by nuclear fusion inside stars, a process known as nucleosynthesis. Although different stars produce different relative proportions of the isotopes, the material from these stars gets mixed up in space. Therefore, when our solar system formed, the contributions from the different stellar sources were almost entirely homogenized and the isotopic ratios of the elements are almost identical throughout the solar system. (There are small variations due to chemical and physical processes which do not concern us here.) Thus, carbon on the Sun, Earth, Moon, Mars, and the other planets has about 89 12C atoms for every 13C atom. Presolar grains, on the other hand, contain the original atoms from their parent stars. These atoms did not get mixed up with the atoms from other stars, so the isotopic ratios in presolar grains are remarkably different from solar system stuff, and can span a huge range. The following plot illustrates this diverse range. In contrast to the uniform 12C/ 13C ratio throughout the solar system, presolar silicon carbide (SIC) and graphite grains have C isotopic ratios that range from about 3 to 10,000!
By comparing the isotopic ratios measured in presolar grains to those measured in stars by astronomers and those predicted by theoretical models, we can identify what types of stars the grains formed in, and (hopefully) learn more about how the stars work.
Noble gases: the discovery of presolar grains.
Until the 1960s, most scientists believed that the early solar system got so hot that no presolar grains could have survived. However, in the mid-1960s, researchers started finding evidence that some presolar material might have survived. This evidence was unusual isotopic ratios of the noble gases neon and xenon in certain types of meteorites. Because these gases are very volatile, they are extremely rare in meteorites (any heating of the meteorites tends to lead to the gases escaping). The fact that the anomalous component survived suggested that the gases were trapped in very refractory mineral grains. Throughout the 1970s and 80s, scientists, especially Ed Anders and his co-workers at the University of Chicago, attempted to isolate the carrier grains of the anomlous gases. They did this mostly by trial-and-error, dissolving the meteorites in a particular acid and seeing if the remaining solid residue still had the gases. If it did, the residue would then be attacked further. Finally, in 1987, in collaboration with Ernst Zinner and his colleagues at Washington University in St. Louis, the Chicago group succeeded in identifying diamond and silicon carbide as presolar carriers of the noble gases. In addition to the isotopically unusual noble gases, these grains were found to have unusual isotopic ratios in other elements as well. Since then, several more types of presolar grains have been identified in the same acid residues of meteorites (see Presolar grain types). The Chicago method of isolating presolar grains is still used today. However, it has often been described as "burning down the haystack to find the needle." In fact, we are probably lucky that among the types of dust grains that stars produce are very strong, acid-resistant phases like diamond and SiC. Most likely, there are more fragile presolar grains in meteorites that we dissolve away with our current methods ("hay" grains) and we just have not yet figured out a good way to identify them.