¹Yuji Matsumoto,²Sota Arakawa
The Astrophysical Journal 948, 73 Open Access Link to Article [DOI 10.3847/1538-4357/acc57c]
¹National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, 181-8588 Tokyo, Japan; yuji.matsumoto@nao.ac.jp
²Japan Agency for Marine-Earth Science and Technology, 3173-25, Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan

Shock-wave heating is a leading candidate for the mechanisms of chondrule formation. This mechanism forms chondrules when the shock velocity is in a certain range. If the shock velocity is lower than this range, dust particles smaller than chondrule precursors melt, while chondrule precursors do not. We focus on the low-velocity shock waves as the igneous rim accretion events. Using a semianalytical treatment of the shock-wave heating model, we found that the accretion of molten dust particles occurs when they are supercooling. The accreted igneous rims have two layers, which are the layers of the accreted supercooled droplets and crystallized dust particles. We suggest that chondrules experience multiple rim-forming shock events.

¹Steven J. Desch,^2,3Emilie T. Dunham,¹Ashley K. Herbst,⁴Cayman T. Unterborn,¹Thomas G. Sharp,¹Maitrayee Bose,⁵Prajkta Mane,⁶Curtis D. Williams
The Astrophysical Journal 953, 146 Open Access Link to Article [DOI 10.3847/1538-4357/acdeed]
¹School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404, USA; steve.desch@asu.edu
²Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA 90095, USA
³Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA 95064, USA
⁴Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
⁵Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA
⁶Arctic Slope Regional Corporation Federal, 11091 Sunset Hills Road, Suite 800, Reston, VA 20190, USA

Most meteoritic calcium-rich, aluminum-rich inclusions formed from a reservoir with ²⁶Al/²⁷Al ≈ 5 × 10⁻⁵, but some record lower (26Al/27Al)0, demanding they sampled a reservoir without live ²⁶Al. This has been interpreted as evidence for “late injection” of supernova material into our protoplanetary disk. We instead interpret the heterogeneity as chemical, demonstrating that these inclusions are strongly associated with the refractory phases corundum or hibonite. We name them “low-²⁶Al/²⁷Al corundum/hibonite inclusions” (LAACHIs). We present a detailed astrophysical model for LAACHI formation in which they derive their Al from presolar corundum, spinel, or hibonite grains 0.5–2 μm in size with no live ²⁶Al; live ²⁶Al is carried on smaller (<50 nm) presolar chromium spinel grains from recent nearby Wolf–Rayet stars or supernovae. In hot (≈1350–1425 K) regions of the disk, these grains and perovskite grains would be the only survivors. These negatively charged grains would grow to sizes 1–10³μm, even incorporating positively charged perovskite grains, but not the small, negatively charged ²⁶Al-bearing grains. Chemical and isotopic fractionations due to grain charging was a significant process in hot regions of the disk. Our model explains the sizes, compositions, oxygen isotopic signatures, and the large, correlated ⁴⁸Ca and ⁵⁰Ti anomalies (if carried by presolar perovskite) of LAACHIs, and especially how they incorporated no ²⁶Al in a solar nebula with uniform, canonical ²⁶Al/²⁷Al. A late injection of supernova material is obviated, although formation of the Sun in a high-mass star-forming region is demanded.

^1,2Prajkta Mane,¹Maitrayee Bose,¹Meenakshi Wadhwa,^3,4Céline Defouilloy
The Astrophysical Journal 946, 37 Open Access Link to Article [DOI 10.3847/1538-4357/acb156]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA; pmane@lpi.usra.edu
²Lunar and Planetary Institute, Universities Space Research Association, Houston, TX 77058, USA
³WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
⁴CAMECA, F-92622 Gennevilliers Cedex, France

The calcium–aluminum-rich inclusions (CAIs) from chondritic meteorites are the first solids formed in the solar system. Rim formation around CAIs marks a time period in early solar system history when CAIs existed as free-floating objects and had not yet been incorporated into their chondritic parent bodies. The chronological data on these rims are limited. As seen in the limited number of analyzed inclusions, the rims formed nearly contemporaneously (i.e., <300,000 yr after CAI formation) with the host CAIs. Here we present the relative ages of rims around two type B CAIs from NWA 8323 CV3 (oxidized) carbonaceous chondrite using the 26Al–26Mg chronometer. Our data indicate that these rims formed ∼2–3 Ma after their host CAIs, most likely as a result of thermal processing in the solar nebula at that time. Our results imply that these CAIs remained as free-floating objects in the solar nebula for this duration. The formation of these rims coincides with the time interval during which the majority of chondrules formed, suggesting that some rims may have formed in transient heating events similar to those that produced most chondrules in the solar nebula. The results reported here additionally bolster recent evidence suggesting that chondritic materials accreted to form chondrite parent bodies later than the early-formed planetary embryos, and after the primary heat source, most likely 26Al, had mostly decayed away.