Investigating the Isotopic Abundance of Solar Neighbourhood FGK Stars with the HERCULES Spectrograph

Abstract

This thesis was undertaken with the goal of determining whether the HERCULES instrument on the 1-Metre McLellan Telescope at Ōtehīwai Mt John Observatory is capable of determining Magnesium isotopic abundance from stellar spectra. Observations of 19 stars were taken using HERCULES, a pipeline was developed, and the isotopic abundances were measured. The astrophysical parameters were found for 19 Gaia Benchmark Stars (GBS) verifying the pipeline processes. These were found to be in agreement with the literature to within 2 sigma. Giant stars had less certain parameters due to molecular regions. Using the GBS parameters for spectrum synthesis the isotopic ratio for Mg of 19 stars was determined. It was found that the Sun has a Mg isotopic ratio 24Mg:25Mg:26Mg of 69.60 ± 3.38 : 11.53 ± 0.09 : 18.76 ± 0.92. The 26Mg isotope was overestimated. For αCenA (HD 128620) the 25Mg isotope was overestimated. This overestimation of 25Mg or 26Mg and subsequent underestimation of 24Mg was also seen with HD 128621 and HD 10700. Mg isotopes were measured successfully for most stars but limits were found in the isotope detection. A Teff cut off of 5300K was suggested for isotopic determination. The recommended regions for determining Mg isotope abundances from MgH features for giants are region 3, 4, 5 and 8, and for dwarfs are region 1, 4, 5 and 9. Further refinement could be done to retrieve more regions. A comparison of the MgH line abundance,and lbl [Mg/H] found that an agreement was limited indicating further refinement of isotopic measurements is required. The r−process element Europium did not show the predicted correlation with 24Mg. The s−process element Barium did not show the predicted correlations with 25Mg and 26Mg. This is further evidence that the isotope pipeline is not yet finding the right balance of isotopes