Presolar grains and solar abundances

Z

As in our Sun, nuclear reactions also take place in other stars. Atomic nuclei react with each other – similarly as atoms in chemical reactions. The results of these reactions are new atomic nuclei. Almost all of the chemical elements we know, and their isotopes - which differ from each other in the number of neutrons in the nucleus, and hence their atomic weight – are products of such nuclear reactions in stars (“nucleosynthesis”). Exceptions are only the lightest elements: hydrogen and helium, which were exclusively and mostly, resp., created during the Big Bang; and the relatively rare lithium, beryllium, and boron nuclei. In the course of nucleosynthesis genenerally lighter elements / isotopes are being “burnt” into heavier ones.

 

The reactions products are available for the formation of new generations of stars, which then are more “metal-rich” than the older ones (see Fig.).

4.57 billion years ago the collapse of a cloud formed from the interstellar medium (“solar nebula”) led to the formation of our solar system. The isotopic composition of the matter in the solar system was largely homogenized in this process. For example, the ratio of the abundances of the two carbon isotopes mit atomic weights 12 and 13 (isotopic ratio ¹²C/¹³C) is ca. 90 almost everywhere in the solar system. Nevertheless information is contained in the meteorites, which dates further back and which includes the processes of element synthesis in stars. Presolar grains (see below) play a special role in this respect. 

Presolar grains

 
During the past three decades mineral phases have been found in meteorites with isotopic compositions which deviate from the average. These “isotope abundance anomalies” provide direct insight into single processes of nucleosynthesis. Especially large are the effects in grains that are older than the solar system itself (“presolar grains”), and which formed directly from material ejected by stars (“stardust”). The isotope ratio ¹²C/¹³C, e.g., in such grains has been found to range from ca. 2 to ca. 7000, i.e. between only 1/100 and 100x the “normal” value of ca. 90. Currently our interest is focused mainly on the isotopic signatures of the presolar nano-diamonds. Further investigations aim at determining an age for the presolar silicon carbide grains.

Further Information:

- Table of the most abundant presolar grains found so far
- Results of some recent (own) work