Presolar grains

Boeing Delta II rocket carrying the Stardust spacecraft waiting for launch. Stardust had a close encounter with the comet Wild 2 in January 2004 and also collected interstellar dust containing pre-solar interstellar grains.

Presolar grains are interstellar solid matter originating at a time before the sun was born. (presolar: before the sun)

Presolar stardust grains formed within outflowing and cooling gases from earlier (pre-Sol) stars. The stellar nucleosynthesis that took place within the presolar star gave the grains an isotopic composition unique to that 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^[1] that took place within the parent star and prove their extra-Sol origin.^[2][3]

History

In the 1960s, the noble gasses neon^[4] and xenon^[5] were discovered to have unusual isotopic ratios in primitive meteorites. Their origin and the type of matter they contained was a mystery. In the mid-1970s, Donald D. Clayton predicted that unusual isotopic compositions would be found within thermally condensed grains produced during mass loss from stars of differing types, and argued that such grains exist throughout the interstellar medium. ^[6][7] He defined several different types of presolar grains that might be found: stardust from red giant stars, sunocons from supernovae, nebcons from nebular accretion, and novacons from novae. His suggestions lay dormant for a decade until such grains were discovered within meteorites. The first unambiguous demonstration of the existence of stardust within meteorites came from the laboratory of Edward Anders in Chicago,^[8] who found that the xenon isotopic abundances contained within an insoluble carbonaceous residue that remained after the meteorite bulk matched almost exactly the predictions for red-giant stardust.^[7] There followed a decade of intense experimental searching to isolate single grains of those xenon carriers.

In 1987 diamond^[9] and silicon carbide^[10] grains were found to contain noble gases. Significant isotopic anomalies were in turn measured within the structural chemical elements of these grains.^[11]

In meteoritics

Presolar grains are the solid matter that was contained in the interstellar gas before the sun was born. They can be identified in the laboratory by their abnormal isotopic abundances and consist of refractory minerals which survived the collapse of the solar nebula and the subsequent formation of planetesimals.

To meteorite researchers, the term presolar has come to mean presolar grains found in meteorites. Such grains comprise only about 0.1 percent of the total mass of particulate matter found in meteorites. Such grains are isotopically-distinct clusters of material found in the fine-grained matrix of meteorites, such as primitive chondrites. Their isotopic differences from the surrounding encasing meteorite suggest that those clusters predate the solar system.^[12] The crystallinity of those clusters ranges from micrometer-sized silicon carbide crystals, down to that of diamond, and unlayered graphene crystals of fewer than 100 atoms. The refractory grains achieved their mineral structures by condensing thermally within the slowly cooling gases of nebula, of supernovae, and the outflows of red giant stars.

Characterization of presolar material

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, 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 micrometers.

Presolar grains consisting of the following minerals have so far been identified:

• diamond (C) nanometer-sized grains (~2.6 nanometres (1.0×10⁻⁷ in) diameter)^[13] possibly formed by vapor deposition^[14]
• graphite (C) particles and onions,^[15] some with unlayered graphene cores^[16]
• silicon carbide (SiC) submicrometer to micrometer sized grains. Presolar SiC occurs as single-polytype grains or polytype intergrowths. The atomic structures observed contain the two lowest order polytypes: hexagonal 2H and cubic 3C (with varying degrees of stacking fault disorder) as well as 1-dimensionally disordered SiC grains.^[17] In comparison, terrestrial laboratory synthesized SiC is known to form over a hundred different polytypes.
• titanium carbide (TiC) and other carbides within C and SiC grains^[18]
• silicon nitride (Si₃N₄)
• corundum (Al₂O₃)^[19]
• spinel (MgAl₂O₄)^[20]
• hibonite ((Ca,Ce)(Al,Ti,Mg)₁₂O₁₉)^[21]
• titanium oxide (TiO₂)
• silicate minerals (olivine and pyroxene)

Information carried by presolar grains

The study of presolar grains provides information about nucleosynthesis and stellar evolution.^[22] Grains bearing the isotopic signature of rapid neutron capture process and alpha capture process of nucleosynthesis are useful in testing models of supernovae explosions. 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 Ca-44, which is presumably the remains of the extinct radionuclide Ti-44, a titanium isotope which is formed in abundance in Type IIa supernovae after rapid capture of eight alpha particles by Si-28, after the process of silicon burning normally begins, and prior to the supernova explosion. However, Ti-44 has a half-life of 59 years, and thus it is soon converted entirely to calcium-44. Excesses of the decay products of the longer lived, but extinct, nuclides Ca-41 (half-life 350,000 years) and Al-26 (730,000 years) have also been detected in such grains. The rapid-process isotopic anomalies of these grains include relative excesses of N-15 and O-18 relative to solar system abundances, as well as excesses of neutron-rich Ca-42, and Ti-49.^[23]

Other presolar grains (AGB star 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 the formation of our 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. 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 atoms to build-up into 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 provide 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.

See also

• Glossary of meteoritics
• List of meteorite minerals
• circumstellar dust
• cosmic dust
• cosmochemistry
• extraterrestrial materials
• interplanetary dust cloud

References

External links

• Presolar grain research
• Presolar grains in meteorites
• Moving Stars and Shifting Sands of Presolar History
• Presolar Grains in Meteorites: An Overview and Some Implications
• (images of presolar grains, text in German)
• (large database of isotopic data on presolar grains) (authorization required)