An international team of scientists has analyzed a diverse set of presolar grains with the goal of realizing their true stellar origins.
Palmerini et al. analyzed the isotopic composition of heavy neutron capture elements measured in SiC stardust grains extracted from pristine meteorites, interpreting them as the outcome of s-process nucleosynthesis occurring in the AGB stages of stars with masses between two and three solar masses and solar metallicities. Image credit: Nan Liu / Andrew Davis / NASA.
Using a state-of-the-art mass spectrometer called NanoSIMS, Dr. Nan Liu of Washington University in St. Louis and colleagues measured isotopes of a suite of elements including nitrogen (N), magnesium (Mg) and aluminum (Al) in presolar silicon carbide (SiC) grains.
By refining their analytical protocols and also utilizing a new-generation plasma ion source, they were able to visualize their samples with better spatial resolution than could be accomplished by previous studies.
“Presolar grains have been embedded in meteorites for 4.6 billion years and are sometimes coated with solar materials on the surface,” Dr. Liu said.
“Thanks to the improved spatial resolution, our team was able to see Al contamination attached on the surface of a grain and to obtain true stellar signatures by including signals only from the core of the grain during the data reduction.”
The researchers sputtered the SiC grains using an ion beam for extended periods of time to expose clean, interior grain surfaces for their isotopic analyses.
They found that the N isotope ratios of the same grain greatly increased after the grain was exposed to extended ion sputtering.
Isotope ratios can be rarely measured for stars, but C and N isotopes are two exceptions.
The new C and N isotope data for the presolar grains directly link the grains to different types of carbon stars based on these stars’ observed isotopic ratios.
“The new isotopic data obtained in this study are exciting for stellar physicists and nuclear astrophysicists like me,” said Dr. Maurizio Busso, a researcher at the University of Perugia.
“Indeed, the ‘strange’ N isotopic ratios of presolar SiC grains have been in the last two decades a remarkable source of concern.”
“The new data explain the difference between what was originally present in the presolar stardust grains and what was attached later, thus solving a long-standing puzzle in the community.”
The authors measured the radioactive isotope 26Al, an important heat source during the evolution of young planetary bodies in the early Solar System as well as extrasolar systems.
They inferred the initial presence of large amounts of 26Al in all measured grains, as predicted by current models.
They determined how much 26Al was produced by the parent stars of the grains they measured.
They concluded that stellar model predictions for 26Al are too high by at least a factor of two, compared to the grain data.
“The data-model offsets likely point to uncertainties in relevant nuclear reaction rates and will motivate nuclear physicists to pursue better measurements of these reaction rates in the future,” Dr. Liu said.
The new results link some of the presolar grains in this collection to poorly known carbon stars with peculiar chemical compositions.
The grains’ isotopic data point to H-burning processes occurring in such carbon stars at higher-than-expected temperatures.
This information will help astrophysicists to construct stellar models to better understand the evolution of these stellar objects.
“As we learn more about the sources for dust, we can gain additional knowledge about the history of the Universe and how various stellar objects within it evolve,” Dr. Liu said.
The findings will be published in the Astrophysical Journal.
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Sara Palmerini et al. 2021. Presolar grain isotopic ratios as constraints to nuclear and stellar parameters of AGB nucleosynthesis. ApJ, in press; arXiv: 2107.12037
