^1,2Thomas Stephan,^1,2,6Reto Trappitsch,³Peter Hoppe,^1,2,4Andrew M. Davis,^1,2,4,5Michael J. Pellin,^1,2Olivia S. Pardo
The Astrophysical Journal 877,101 Link to Article [https://doi.org/10.3847/1538-4357/ab1c60]
¹Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637, USA
²Chicago Center for Cosmochemistry, Chicago, IL, USA
³Max Planck Institute for Chemistry, 55128 Mainz, Germany
⁴The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
⁵Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
⁶Present address: Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.

We have analyzed molybdenum isotopes, together with strontium and barium isotopes, in 18 presolar silicon carbide grains using the Chicago Instrument for Laser Ionization (CHILI), a resonance ionization mass spectrometer. All observed isotope ratios can be explained by mixtures of pure s-process matter with isotopically solar material. Grain residues were subsequently analyzed for carbon, nitrogen, silicon, and sulfur isotopes, as well as a subset for ²⁶Al–²⁶Mg systematics using the NanoSIMS. These analyses showed that all but one grain are mainstream grains, most probably coming from low-mass asymptotic giant branch (AGB) stars. One grain is of the AB type, for which the origin is still a matter of debate. The high precision of molybdenum isotope measurements with CHILI provides the best estimate to date for s-process molybdenum made in low-mass AGB stars. The average molybdenum isotopic abundances produced by the s-process found in the analyzed mainstream SiC grains are 0% ⁹²Mo, 0.73% ⁹⁴Mo, 13.30% ⁹⁵Mo, 36.34% ⁹⁶Mo, 9.78% ⁹⁷Mo, 39.42% ⁹⁸Mo, and 0.43% ¹⁰⁰Mo. Solar molybdenum can be explained as a combination of 45.9% s-process, 30.6% r-process, and 23.5% p-process contributions. Furthermore, the observed variability in the individual grain data provides insights into the variability of conditions (neutron density, temperature, and timescale) during s-process nucleosynthesis in the grains’ parent stars, as they have subtle effects on specific molybdenum isotope ratios. Finally, the results suggest that the ratio between p– and r-process molybdenum in presolar SiC from many different types of parent stars is Mo _p /Mo _r  = 0.767, the value inferred for the solar system and consistent with what has been found in bulk samples and leachates of primitive meteorites.

¹Sota Arakawaand,¹Taishi Nakamoto
The Astrophysical Journal 877, 84 Link to Article [https://doi.org/10.3847/1538-4357/ab1b3e]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

Shock-wave heating within the solar nebula is one of the leading candidates for the source of chondrule-forming events. Here we examine the possibility of compound chondrule formation via optically thin shock waves. Several features of compound chondrules indicate that they are formed via the collisions of supercooled precursors. We evaluate whether compound chondrules can be formed via the collision of supercooled chondrule precursors in the framework of the shock-wave heating model by using semi-analytical methods and discuss whether most of the crystallized chondrules can avoid destruction upon collision in the post-shock region. We find that chondrule precursors immediately turn into supercooled droplets when the shock waves are optically thin, and they can maintain supercooling until the condensation of evaporated fine dust grains. Owing to the large viscosity of supercooled melts, supercooled chondrule precursors can survive high-speed collisions on the order of 1 km s⁻¹ when the temperature is below ~1400 K. From the perspective of the survivability of crystallized chondrules, shock waves with a spatial scale of ~10⁴ km may be potent candidates for the chondrule formation mechanism. Based on our results from one-dimensional calculations, a fraction of compound chondrules can be reproduced when the chondrule-to-gas mass ratio in the pre-shock region is ~2 × 10⁻³, which is approximately half of the solar metallicity.