Master`s Thesis defense by Terese Hansen – Niels Bohr Institute - University of Copenhagen

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Master`s Thesis defense by Terese Hansen

Formation of the Heavy Elements in the Early Universe
 Extremely metal poor stars can provide us with insight into early nucleosynthesis
and chemical enrichment of the interstellar medium of our galaxy. A small fraction of the poor halo giants exhibit uniform r-process element abundances that vary as a group by factors up to ~ 60 relative to those of the iron peak and below. Despite great efforts in the area, the nature and location of the production site(s) of the r-process elements remain unknown. This work presents the long term radial velocity monitoring of 17 extremely metal poor r-process enhanced stars, with the  object of assessing the frequency and orbital of any properties any binaries. Medium resolution spectra have been obtained with the Nordic Optical Telescope over a period of four years. These have been reduced and a multi order cross correlation have been  performed to determine any variations in  the radial velocity of the stars. Three, possibly four, binary systems are detected with orbital periods and eccentricities normal for giant binaries with lower-mass main-sequence companions. Showing no signs that the secondary have been AGB stars or a supernovae.The other 14 stars in the sample, showing no variation in radial velocity within our typical precision ~ 200 m/s, appear to be single. The conclusion is, the r-process enhancement of these stars is not a local event due to mass transfer from a binary companion, but this enhancement was inherent in the parent clouds of these (single and binary) stars; and the r-process elements must have been ejected in processes separate from those supplying the bulk of the iron-peak and lighter elements, perhaps by SN with jets, and later mixed in  variable proportions. These stars are thus prominent chemical tags of their birth places in the Galaxy or the fragments merging to form the halo.
Supervisor: Johannes Andersen, Niels Bohr Institute