¹Gregory A. Brennecka, ¹Thorsten Kleine
The Astrophysical Journal Letters 837, L9 Link to Article [https://doi.org/10.3847/2041-8213/aa61a2]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany

Precise knowledge of the abundances of short-lived radionuclides at the start of the solar system leads to fundamental information about the stellar environment of solar system formation. Previous investigations of the short-lived ${}^{135}\mathrm{Cs}\,\to {}^{135}\mathrm{Ba}$ system (t 1/2 = 2.3 Ma) have resulted in a range of calculated initial amounts of 135Cs, with most estimates elevated to a level that requires extraneous input of material to the protoplanetary disk. Such an array of proposed 135Cs/133Cs initial solar system values has severely restricted the system’s use as both a possible chronometer and as an informant about supernovae input. However, if 135Cs was as abundant in the early solar system as previously proposed, the resulting deficits in its daughter product 135Ba would be easily detectable in volatile-depleted parent bodies (i.e., having sub-chondritic Cs/Ba) from the very early solar system. In this work, we show that angrites and eucrites, which were volatile-depleted within ~1 million years of the start of the solar system, do not possess deficits in 135Ba compared to other planetary bodies. From this, we calculate an upper limit for the initial 135Cs/133Cs of 2.8 × 10−6, well below previous estimates. This significantly lower initial 135Cs/133Cs ratio now suggests that all of the 135Cs present in the early solar system was inherited simply from galactic chemical evolution and no longer requires an addition from an external stellar source such as an asymptotic giant branch star or SN II, corroborating evidence from several other short-lived radionuclides.

¹Conel M. O’D. Alexander,¹Larry R. Nittler,¹Jemma Davidson,²Fred J. Ciesla
Meteoritcs & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12891]
¹Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC, USA
²Department of Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois, USA
Published by arrangement with John Wiley & Sons

The timing and extent to which the initial interstellar material was thermally processed provide fundamental constraints for models of the formation and early evolution of the solar protoplanetary disk. We argue that the nonsolar (solar Δ17O ≈ −29‰) and near-terrestrial (Δ17O ≈ 0‰) O-isotopic compositions of the Earth and most extraterrestrial materials (Moon, Mars, asteroids, and comet dust) were established very early by heating of regions of the disk that were modestly enriched (dust/gas ≥ 5–10 times solar) in primordial silicates (Δ17O ≈ −29‰) and water-dominated ice (Δ17O ≈ 24‰) relative to the gas. Such modest enrichments could be achieved by grain growth and settling of dust to the midplane in regions where the levels of turbulence were modest. The episodic heating of the disk associated with FU Orionis outbursts were the likely causes of this early thermal processing of dust. We also estimate that at the time of accretion the CI chondrite and interplanetary dust particle parent bodies were composed of ~5–10% of pristine interstellar material. The matrices of all chondrites included roughly similar interstellar fractions. Whether this interstellar material avoided the thermal processing experienced by most dust during FU Orionis outbursts or was accreted by the disk after the outbursts ceased to be important remains to be established.