Isotopic Compositions of Presolar Dust Grains
SiC grains of different classes originated from different types of stars. Mainstream grains and grains of type Y and Z are believed to have condensed in the atmospheres of Asymptotic Giant Branch (AGB) stars of different metallicity. AGB stars are low-to-intermediate mass stars in the late stages of their evolution when they lose large amounts of material in stellar winds from which grains can condense. A minor class of grains, SiC grains of type X come from supernovae, massive stars that explode after the exhaustion of the nuclear fuel in their interior . The isotopic compositions of a handful of grains indicate that they originated from the ejecta of nova, nuclear explosions of a H-rich layer accreted from a companion onto the surface of a white dwarf star. The isotopic compositions of presolar SiC (and other) grains not only allow us to identify their stellar sources but allow us to set constraints on existing models of nucleosynthesis and stellar evolution and, thus, provide new information on the workings of the nuclear furnaces in the interiors of stars.
Presolar grains of SiC, graphite, aluminum oxide, and silicon nitride can be up to several µm in size, but most are less than a micron. The ion microprobe makes it possible to measure isotopic ratios in individual dust grains down to less than 1 µm in size. Such measurements have revealed a tremendous range in isotopic compositions. An example is shown in the graph on the left, which displays the C and N isotopic ratios measured by SIMS in individual SiC grains. On the basis of the C, N, and Si isotopic ratios we can distinguish between six different classes of presolar SiC grains. The abundances of these classes vary greatly and are indicated in the graph.
Presolar Silicate Grains in Meteorites
Although astronomical observations indicate that silicate grains are produced in abundance in the atmospheres of young and evolved stars, early searches for presolar silicates in meteorites were unsuccessful. This situation changed with the advent of the NanoSIMS and its ability to analyze the O isotopic compositions of tens of thousands of sub-¬µm grains by raster imaging. By applying this technique to size-separated matrix grains and to polished sections presolar silicates have been discovered in several primitive carbonaceous chondrites. Abundance estimates show that silicates are the most abundant presolar grain type (with the possible exception of nanodiamonds), with matrix-normalized abundances of ~150–200 ppm. The figure on the left shows O isotopic ratio images of grain size separates from Acfer 094. The grain the arrow is pointing to has a large excess in 17O and deficit in 18O.
During raster ion imaging in the NanoSIMS we also detect secondary electrons (SE) along with secondary ions. We can use the SE image to relocate presolar grains in the SEM and analyze them for their elemental compositions by Auger spectroscopy.
These analyses have shown that most presolar silicates have ferromagnesian compositions, with unexpectedly high Fe contents. These high Fe abundances can be due to secondary processing which results in an influx of Fe into the grains or the result of formation of the grains under non-equilibrium conditions, during which more Fe can be incorporated into the grains than during equilibrium condensation, which predicts Mg-rich compositions. In most cases the latter scenario appears to be more likely, as most of the meteorites in which high abundances of presolar silicates are found are very primitive and show few signs of secondary processing. However, measurements of the Fe isotopes of the grains are required to provide a definitive answer.
In addition to O, Si and Mg isotopes can also be measured in these presolar silicates.. One grain has been found to be enriched in 26Mg from the in situ decay of 26Al. The high inferred initial 27Al/26Al of 0.12 gives more detailed information on the nature of deep mixing processes that occurred in the parent star. In addition to silicate grains, some rare presolar grain types have been identified, including SiO2 grains and Fe,Mg oxide grains. Identification of new presolar grain types introduces new opportunities for studying various stellar environments as well as solar system environments.
Presolar Grains from Supernovae
Low-density graphite grains, SiC grains of type X, and silicon nitride originated in the ejecta of supernovae. Proof for such an origin comes from the initial presence of 44Ti in the grains at the time of their formation. This nuclide, which is produced only in supernovae, is radioactive and decays with a half life of 60 years. Its prior presence in the grains is inferred from huge excesses of its daughter isotope 44Ca. Supernova grains also have large excesses of 28Si and 18O and large inferred 26Al/27Al ratios (determined from excesses in 26Mg, the decay product of the short-lived radioisotope 26Al).
Just before its explosion as a supernova, a massive star has an onion-type structure like the one shown schematically in the figure on the left. The star consists of different layers (indicated in the figure by the most abundant elements) that contain the products of nuclear burning at increasing temperatures from the surface to the core. Both 44Ti and 28Si are produced in an interior zone that consists mostly of 28Si and 32S. In contrast, 18O and 26Al are found in two of the more exterior zones, where He burning produces 18O from 14N and H burning makes 26Al from Mg. The presence in the same grain of isotopes that are produced in very different supernova layers is evidence for turbulent mixing during supernova explosions.
Coordinated TEM and NanoSIMS Studies of Presolar Grains
Transmission electron microscopy (TEM) can reveal the internal structures and chemical compositions of presolar grains. To do this, the grains must be sliced into ultra-thin sections only several hundred atoms thick. However, thinned TEM samples do not have enough atoms for accurate isotopic analysis with most SIMS instruments. This limited earlier presolar grain studies, as it was not possible to obtain both isotopic and microstructural information from the same samples. Now, using the higher sensitivity of the NanoSIMS, we are pursuing coordinated TEM and isotopic studies on presolar grains already mounted on TEM grids. In this way, we can identify the likely stellar sources of individual presolar grains from their isotopic anomalies, and correlate this information directly with the mineralogical results from TEM studies.
Supernova graphites capture and preserve other high temperature grains during their growth, and observations of the composite objects allow us to infer phase condensation sequences, and temperature and pressure conditions in the SN outflows. However, relating these observations back to nucleosynthesis and grain condensation in SN outflows requires improved modeling of these complex environments. It was thought that the presence of non-equilibrium processes in SNe, such as intense radiation and grain-gas collisions, would make certain types of simplifications (such as equilibrium condensation modeling) invalid. However, our recent observations of SN graphites match very well with the prior predictions based on equilibrium thermodynamical models, suggesting that these existing thermodynamical and nucleosynthetic models can be further refined and tested through comparison with laboratory measurements.
Al-26 in Presolar Grains and Deep Mixing in AGB Stars
Many presolar grains show excesses in 26Mg due to the decay of the radioisotope 26Al (half life = 730,000 years). The figure shows inferred 26Al/27Al ratios in presolar oxide and SiC grains. The highest ratios are found in SiC grains of type X, which originated in supernova ejecta. Mainstream, Y, and Z SiC grains as well as almost all oxide grains are believed to come from Asymptotic Giant Branch (AGB) stars, whereas the stellar sources of A+B grains are still not well established. Oxide grains have, on average, much higher 26Al/27Al ratios than SiC grains from AGB stars. The 18O/16O ratios of these oxide grains are less than ~10-3, lower than those predicted by standard stellar evolution models. They have been explained by invoking extra deep mixing (also called cool bottom processing) of material from the star’s envelope to hot zones close to the H-burning shell. Deep mixing is also required to explain 26Al/27Al ratios larger than ~3×10-3. Thus, the parent stars of many oxide grains must have experienced deep mixing. In contrast, the 26Al/27Al ratios of most SiC grains with an AGB origin are within the limits expected from shell H burning in standard stellar evolution models. This is surprising because continued dredge-up of 12C produced in the He shell during thermal pulses is expected to turn O-rich AGB stars, the parents of oxide grains, into C-rich stars (“carbon stars”), from which SiC grains can condense. Thus it seems that most parent stars of SiC grains do not undergo deep mixing. An explanation for this observation is that only AGB stars that do not experience deep mixing become carbon stars or, conversely, that deep mixing prevents AGB stars from becoming carbon stars. The mechanism for deep mixing is still not well understood. The study of presolar grains not only provides information about nucleosynthetic processes taking place in the interiors of stars but also about mixing processes that bring the nucleosynthetic products to the stars’ surfaces where they can be observed astronomically and where they are incorporated into grains.
Surface Properties of Pristine Circumstellar SiC Grains
We are studying the morphologies and surface properties of circumstellar SiC grains that have been gently isolated from their parent meteorites. The objective of the work is to understand the history of the grains from initial production to incorporation into the meteorite parent bodies. Circumstellar SiC grains were first found in acid resistant residues of carbonaceous meteorites, and because of this their properties prior to chemical etching were unknown.
This issue was partially resolved when we developed a x-ray mapping technique that located the SiC grains in situ in polished sections, allowing us to use x-ray mapping to isolatecircumstellar grains that have been neither etched nor polished. High-resolution scanning electron images of these SiC grains show several distinct morphologies. While some grains are highly angular, most are rounded indicating various degrees of erosion. The nature of this rounding and where and when it took place – perhaps by oxygen chemical attack in the solar nebula or possibly by processing by supernovae shock waves in interstellar space– are questions that we are attempting to answer. If the rounding has resulted from oxidation, a thin layer of SiO2 may be present on some grains. It is also possible that the least modified grains were protected by ice or organic mantles acquired in the interstellar medium. These issues are being pursued by applying a variety of analytic techniques to study the surface properties of the grains in more detail.
Presolar Refractory Metal Nuggets
Refractory metal nuggets (RMNs) are small grains found within some meteorites that are highly enriched in refractory elements such as Os, W, and Ru. The chemical compositions of many RMNs are consistent with condensation directly from the gas and their isotopic compositions match our solar system, suggesting they may be the first solid objects formed in the early solar system.
Recently, we have discovered RMNs embedded inside of presolar graphites (with large isotopic anomalies indicating formation in ancient stars), of a type that are only consistent with condensation directly from the gas. The chemical compositions of these pRMNs match well with meteoritic RMNs discovered earlier, and with their predicted compositions for condensation from the gas. These findings strengthen the argument that the mRMNs are among the earliest solar system condensates. Application of the existing models also reveals more about the ancient stars in which the presolar RMN and its host graphite formed. The composition of each presolar RMN reflects the temperature of its last equilibration with the gas, which in turn reveals the condensation temperature of the graphite in which it was captured. This yields detail information about circumstellar grain formation in stars and also reveals a trend showing that high-density graphites typically condense at a higher temperature than low-density ones.