Speaker
Description
Presolar grains help us to study the complex system of dying stars through their isotopic signatures. One important concept presolar grains help us explore is the neutron density and temperature of ejected material from AGB stars through measurements of isotopes impacted by s-process branching [1]. However, getting these grains is no simple task. Orgueil, a CI chondrite, contains ~10 ppm presolar graphite [2], and Murchison, a CM chondrite, has a concentration of ~1 ppm for grains larger than 1 μm in size [3]. These abundances indicate that at least one grain >1 μm should be exposed at the surface of a 1 $mm^{2}$ polished section. Previous presolar grain research on Murchison and Orgueil often relied on a chemical separation method to extract presolar graphite grains from their matrix [3, 4]. Although this method is helpful for concentrating presolar grains, it destroys the material surrounding the grains and thus removes their petrographic context. Furthermore, CI and CM chondrites are some of the least abundant meteorite classes but have the highest concentrations of presolar grains [2, 3]. Thus, using nondestructive techniques for presolar grain identification, like Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS), allows us to capitalize on the large abundance of presolar grains in CI and CM material without their destruction. These techniques can also be used for asteroid samples from Ryugu and Bennu, which have similar presolar graphite abundances as CI meteorites [5, 6]. More than 100 $mm^{2}$ each of polished sections of the Orgueil and Murchison meteorites have been imaged and chemically analyzed using SEM and EDS. From these images and chemical maps, hundreds of carbon-rich objects have been identified and documented. Following identification, closer SEM imaging revealed that many objects appear to be epoxy-filled holes or organic matter. A few of the identified carbon-rich objects show similarities to SEM images taken of separated presolar graphite grains. Isotope measurements of the elements Mo, Zr, Sr, and Ti using the Chicago Instrument for Laser Ionization (CHILI) [7] are planned to help us better constrain s-process branching points.
Silicates and oxides are also important types of presolar grains. One formation environment of such grains is early in the AGB thermal pulsing sequence when the envelope still has a C/O < 1 [8]. Isotope analysis of elements heavier than iron has yet to be measured in presolar silicates and oxides. Thus, nucleosynthetic signatures of these grains are not fully constrained, leaving many open questions concerning the early stages of AGB stars or other oxygen-rich stellar environments where these grains may have formed. To achieve the goal of measuring these elements (Mo, Zr, Sr, and Ti) in presolar silicates and oxides, previous measurement techniques must first be modified, as these phases are transparent to the nanosecond UV laser we have used for presolar SiC and graphite [7]. We are optimizing CHILI’s femtosecond ablation laser wavelength, pulse length, power, and timing to allow accurate isotopic measurements of optically transparent presolar grains.
[1] Busso M. et al. (1999) Annu. Rev. Astron. Astrophys. 37, 239–309. [2] Huss G. R. and Lewis R. S. (1995) Geochim. Cosmochim. Acta 59, 115–160. [3] Amari S. et al. (2014) Geochim. Cosmochim. Acta 133, 479–522. [4] Jadhav M. et al. (2013) Geochim. Cosmochim. Acta 113, 192–224. [5] Barnes J. J. et al. (2025) Nat. Astron. 9, 1785–1802. [6] Nguyen A. N. et al. (2023) Sci. Adv. 9, 28. [7] Stephan T. et al. (2016) Int. J. Mass Spectrom. 407, 1–15. [8] Nittler L. R. et al. (2008) Astrophys. J. 682, 1450–1478.
| Career stage | Graduate student |
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