Древняя космическая пыль: происхождение и изотопный состав досолнечных зерен.

Very old dust in space

Presolar grains are interstellar solid matter in the form of tiny solid grains that originated at a time before the Sun was formed. Presolar stardust grains formed within outflowing and cooling gases from earlier presolar stars. The stellar nucleosynthesis that took place within each presolar star gives to each granule an isotopic composition unique to that parent star, which differs from the isotopic composition of our solar system's matter as well as from the galactic average. These isotopic signatures often fingerprint very specific astrophysical nuclear processes that took place within the parent star and prove their presolar origin.

Meteoriticists often use the term to represent stardust, grains that originated within a single star and which they extract from meteorites for study. Because most interstellar grains are not stardust from a single star, however, being instead interstellar cloud matter accreted by smaller presolar grains, most presolar grains are also not stardust. Logically, all stardust are presolar grains; but not all presolar grains are stardust. This confusing terminology is heavily entrenched among 21st century meteoriticists who prefer to use the terms interchangeably, however, so both usages should employ or utilize the expression presolar stardust grains for stardust.

In the 1960s, the noble gases neon and xenon were discovered to have unusual isotopic ratios in primitive meteorites; their origin and the type of matter that contained them was a mystery. These discoveries were made by vaporizing a bulk sample of a meteorite within a mass spectrometer, in order to count the relative abundance of the isotopes of the very small amount of noble gases trapped as inclusions. During the 1970s similar experiments discovered more components of trapped xenon isotopes. Competing speculations about the origins of the xenon isotopic components were advanced, all within the existing paradigm that the variations were created by processes within an initially homogeneous solar gas cloud. A new theoretical framework for interpretation was advanced during the 1970s when Donald D. Clayton rejected the popular belief among meteoriticists that the solar system began as a uniform hot gas. Instead he predicted that unusual but predictable isotopic compositions would be found within thermally condensed interstellar grains that had condensed during mass loss from stars of differing types. He argued that such grains exist throughout the interstellar medium. Clayton's first papers using that idea in 1975 pictured an interstellar medium populated with supernova grains that are rich in the radiogenic isotopes of Ne and Xe that had defined the extinct radioactivities. Clayton defined several types of stardust presolar grains likely to be discovered: stardust from red giant stars, sunocons (acronym from SUperNOva CONdensates) from supernovae, nebcons from nebular condensation by accretion of cold cloud gaseous atoms and molecules, and novacons from nova condensation. who found using traditional mass spectrometry that the xenon isotopic abundances contained within an acid insoluble carbonaceous residue that remained after the meteorite bulk had been dissolved in acids matched almost exactly the predictions for isotopic xenon in red giant stardust. and silicon carbide grains were found to exist abundantly in those same acid insoluble residues and also to contain large concentrations of noble gases. Significant isotopic anomalies were in turn measured by improvements in secondary ion mass spectrometry (SIMS) within the structural chemical elements of these grains. Improved SIMS experiments showed that the silicon isotopes within each SiC grain did not have solar isotopic ratios but rather those expected in certain red giant stars. The finding of stardust is therefore dated 1987. In January 2020, analysis of the Murchison meteorite found in Australia in 1969 revealed that its stardust formed 5 to 7 billion years ago, older than Earth's 4.6 billion year old sun, making the meteor and its stardust the oldest solid material ever discovered on Earth.

The study of presolar grains provides information about nucleosynthesis and stellar evolution. For example, some presolar grains (supernova grains) have very large excesses of calcium 44, a stable isotope of calcium which normally composes only 2% of the calcium abundance. The calcium in some presolar grains is composed primarily of 44Ca, which is presumably the remains of the extinct radionuclide titanium 44, a titanium isotope which is formed in abundance in Type II supernovae such as SN 1987A after rapid capture of four alpha particles by 28Si, after the process of silicon burning normally begins, and prior to the supernova explosion. However, 44Ti has a half life of only 59 years, and thus it is soon converted entirely to 44Ca. Excesses of the decay products of the longer lived, but extinct, nuclides calcium 41 (half life 99,400 years) and aluminium 26 (730,000 years) have also been detected in such grains. The rapid process isotopic anomalies of these grains include relative excesses of nitrogen 15 and oxygen 18 relative to Solar System abundances, as well as excesses of the neutron rich stable nuclides 42Ca and 49Ti. Other presolar grains (AGB star grains) provide isotopic and physical information on asymptotic giant branch stars, which have manufactured the largest portion 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 the formation of the Solar System. In addition to providing information on nucleosynthesis of the grain's elements, solid grains provide information on the physico chemical conditions under which they condensed, and on events subsequent to their formation.