A low abundance of "³⁵cs in the early solar system from barium isotopic signatures of volatile-depleted meteorites

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 ${}^{135}Cs\to {}^{135}Ba$ 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.