Stellar 'fossils' in meteorites point to distant stars

Date:
October 12, 2021
Source:
Washington University in St. Louis
Summary:
A new study analyzes a diverse set of presolar grains with the
goal of realizing their true stellar origins.



FULL STORY ==========================================================================
Some pristine meteorites contain a record of the original building
blocks of the solar system, including grains that formed in ancient
stars that died before the sun formed. One of the biggest challenges
in studying these presolar grains is to determine the type of star each
grain came from.


==========================================================================
Nan Liu, research assistant professor of physics in Arts & Sciences at Washington University in St. Louis, is first author of a new study in Astrophysical Journal Letters that analyzes a diverse set of presolar
grains with the goal of realizing their true stellar origins.

Liu and her team used a state-of-the-art mass spectrometer called
NanoSIMS to measure isotopes of a suite of elements including the N and
Mg-Al isotopes in presolar silicon carbide (SiC) grains. By refining
their analytical protocols and also utilizing a new-generation plasma
ion source, the scientists 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,"
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 scientists sputtered the grains
using an ion beam for extended periods of time to expose clean, interior
grain surfaces for their isotopic analyses. The researchers 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 reported in this study 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 Maurizio Busso,
a co- author of the study who is based at the University of Perugia,
in Italy.

"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 study also includes a significant exploration of radioactive isotope aluminum-26 (26Al), an
important heat source during the evolution of young planetary bodies in
the early solar system and also other extra-solar systems.

The scientists inferred the initial presence of large amounts of 26Al
in all measured grains, as predicted by current models. The study
determined how much 26Al was produced by the "parent stars" of the
grains they measured. Liu and her collaborators 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, Liu noted, and will motivate nuclear physicists to pursue better measurements of these reaction rates in the future.

The team's 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," Liu said.

========================================================================== Story Source: Materials provided by
Washington_University_in_St._Louis. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Nan Liu, Barosch Jens, Larry R. Nittler, Conel M. O'D. Alexander,
Jianhua
Wang, Sergio Cristallo, Maurizio Busso, and Sara Palmerini. New
multielement isotopic compositions of presolar SiC grains:
implications for their stellar origins. The Astrophysical Journal
Letters, 2021 DOI: 10.3847/2041-8213/ac260b ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211012102706.htm

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