Research Byte

Published in the RSAA Lunations
Vol1 Issue3 1–30 April 2020

Hydrogen, helium and lithium were created in the Big Bang. Predictions of how much hydrogen and helium were formed through Big Bang nucleosynthesis are remarkably consistent with observations of distant neutral hydrogen gas clouds, and He emission from nearby metal-poor galaxies. Measurements of metal-poor stars however indicate lithium abundances 3 times smaller than predicted from Big Bang nucleosynthesis. To further complicate this discrepancy, lithium has two stable isotopes: Li-6 and Li-7. The Big Bang is predicted to have created mostly Li-7, and negligible amounts of Li-6. Yet, Li-6 appears to have been detected in several stars, with ratios Li-6/Li-7 at a level of 5%, more than a thousand times higher than predicted. Not only do we find too little Li-7, but also too much Li-6! 

 The isotopic splitting in the Li line is small, and can be confused with signatures of atmospheric convection and rotation. Proper measurement of Li-6 requires very high quality spectra and accurate modelling taking into account effects of convection. We analyse HD84937, a Spite plateau star where Li-6 was previously detected by several groups. We use exquisite observations from ESPRESSO at the VLT, with SNR > 1000, and R ≈ 140000, and a Monte Carlo Markov Chain fitting technique, breaking the degeneracies with rotational velocity and radial velocity through measurements of Fe lines. We measure Li-6/Li-7 = 0.74 + 0.67/ - 0.70 %, as shown in the figure, implying a 3 sigma upper limit on the ratio at a level of 2%. This shows that the previous apparent detections were caused by an unfortunate coincidence of isotopic splitting and convection.  

 This analysis will be presented in a forthcoming paper. The error bars are actually plotted in the top figure, but you can’t see them because they’re smaller than the line width.

Dr Thomas Nordlander & Ella Wang (PHD Student)