Interstellar Helium Trapped with the COLLISA Experiment on the MiR Space Station-Improved Isotope Analysis by In Vacuo Etching

We have redetermined the helium isotopic composition of the local interstellar cloud (LIC) in the framework of the Swiss-Russian Collection of Interstellar Atoms (COLLISA) collaboration. Based on in vacuo etching analyses of foils that have been exposed to the interstellar neutral particle flux on board the Mir space station, we obtain (³He/⁴He)_LIC=(1.62+/-0.29)×10^-4. This is the most precise determination of the He isotopic composition of the LIC, with errors being lower by more than a factor of 2 compared to an earlier experiment on similar foils and compared to the analysis of interstellar pick-up ions. Comparing our improved result with current models of Galactic chemical evolution reveals that close to 100% of all Galactic low-mass stars must have undergone extra mixing associated with cool bottom processing. The ³He abundance relative to ⁴He in the LIC is within the uncertainty identical to the value of (1.66+/-0.06)×10^-4 inferred for the protosolar cloud (PSC), showing that no significant evolution of the ³He abundance took place since 4.6 Gyr ago at the solar distance (~8 kpc) from the Galactic center. Our value sets new rigorous constraints on Galactic evolution models. Since the Big Bang nucleosynthesis theory predicts a value of <=(1.1+/-0.2)×10^-4, a noticeable increase of the relative abundance of ³He has apparently occurred in the Galaxy before the formation of the solar system, but, contrary to earlier expectations, only a modest or negligible increase is registered after the formation of the solar system.