Presolar or stardust grains are very small pieces (micrometer-sized: 1 micrometer = 1/1,000,000 meter) of dust grains that formed from gases thrown out by certain types of stars as they are dying. About 4.6 billion years ago, our Solar System was formed from a cloud of molecular gas that contained such stardust grains from millions of stars that had died. Hence, the name "presolar" grains because these dust grains are older than our Solar System. As this early molecular cloud rotated and collapsed in on itself, it heated most of the gas and dust while these materials were getting incorporated into planets and other rocky bodies. Most of the stardust was lost to large planetary bodies; however, some stayed intact in very primitive bodies, such as some asteroidal and cometary bodies that escaped heating. These bodies are called primitive because they preserve early materials that contain records of what the conditions were like when our Solar System formed. Certain primitive asteroids are the parent bodies of special types of meteorites that fall to Earth, called carbonaceous chondrites. Stardust or presolar grains were first discovered (in the 80's) in carbonaceous chondrites by dissolving away 99.9 % of the rock with harsh acids, a process commonly known as "burning the haystack to find the needle". For more than 3 decades, these tiny pieces of stars have provided scientists astonishing insights into their parent stars and the nuclear reactions that took place in them while they were alive, before the Solar System formed. This field has burgeoned and allowed scientists to glean information on a wide range of topics from the Milky Way galaxy to the parent bodies of these grains. These grains have so far been found in primitive meteorites, interplanetary dust particles, and sample collection missions from asteroids and comets.

Presolar grains preserve an isotopic record of the nuclear reactions that took place in their parent stars during their lifetimes. Thus, these grains can be identified as presolar on the basis of large isotopic anomalies (deviations from Solar System values) measured in individual grains. For e.g., the ¹²C/¹³C ratio of any material made in the Solar System ranges from 88 - 90; however, the same ratio in carbonaceous presolar grains (e.g., SiC, graphite) shows a variation of over three orders of magnitude (¹²C/¹³C ~ 2 - 7000). Natural processes in the Solar System cannot give rise to such variation and these anomalies can only be explained by nuclear reactions taking place in nature's nuclear reactors - stars! Isotopic ratios of a number of elements can be measured in individual grains using cutting edge mass spectrometry tools. Nano Secondary Ion Mass Spectrometry (NanoSIMS) and Resonance Ion Mass Spectrometry (RIMS) are used to measure light and heavy element isotopes, respectively, in individual presolar grains. Such measurements are complicated because these techniques are destructive and microscopic presolar grains have very little material available for analyses. Each grain represents one star at a particular time in its life; thus, extracting as much information as possible from a single grain is of highest priority in presolar grain studies.

This study was successful in measuring Mo, Zr, Sr, Rb, Gd, and Ru isotopes on approximately 100 presolar graphite grains. The isotopes of these heavy elements are synthesized in stars by the slow and rapid neutron capture processes. Other processes like the gamma process are responsible for creating proton-rich nuclides. This study found the first laboratory evidence that ⁸⁴Sr, a p-nuclide, is made in supernovae. We were also able to find new stellar sources for ⁹⁶Zr. The stellar origins of these nuclides were previously ambiguous. The p-process result is currently being reviewed by Nature Communications. The remaining results are currently being prepared for publishing in peer-reviewed journals and the graduate student's PhD dissertation. Six to seven high impact factor publications are estimated as this study is wrapped up.

Thus, this highly successful project aimed to find answers to the most fundamental question of the origin of elements in the Universe and to discern the properties of the materials that contributed to the formation of our Solar System. Some of the measurements had not been performed before and the PI and her collaborators developed new measurement protocols. The results from this study will provide tight constraints to theoretical stellar nucleosynthesis models. The PI and her student obtained cutting edge analytical skills by learning the powerful RIMS technique. Furthermore, the PI was successful in establishing a long-term collaboration with the RIMS laboratory at Lawrence Livermore National Laboratory that will be an invaluable asset to the PI's future career in cosmochemistry. The graduate student and PI were successful in obtaining the prestigious NASA FINESST grant based on the preliminary results from this study. This grant will fund the graduate student's education until she obtains her PhD in 2025.

Last Modified: 08/24/2024
Modified by: Manavi M Jadhav