An image of the open cluster Trumpler 20. Credit: University of Vienna
An international team has detected two new giant lithium-abundant stars in an old open cluster named Trumpler20. The discovery could yield new important information regarding the phenomenon of omnipresence of lithium-rich stars in different environments throughout the universe. The newly found stars were designated MG 340 and MG 591. They were detected during an analysis of a sample of 40 giant stars of the 1.66 billion-year-old Trumpler 20 cluster, for which high-resolution spectra were obtained by the Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) in Chile. The observations were carried out under the Gaia-ESO Public Spectroscopic Survey, which utilizes VLT and ESA’s Gaia spacecraft to study Milky Way’s bulge, thick and thin discs and halo components, and open star clusters. One of the goals was to provide the first overview of the distributions of kinematics and elemental abundances in our galaxy.
Thanks to data by UVES, Smiljanic et al derived atmospheric parameters and lithium abundances of MG 340 and MG 591 and estimated the stars to be 1.5 and 3.6 solar masses. Lithium-rich giants have been observed in different environments: open clusters, globular clusters, metal-rich and metal-poor field stars, the Galactic bulge, as well as in dwarf galaxies. In previous studies, it was found that some of these host planets and thus the surface lithium enrichment could be caused by planet engulfment. This process could activate internal production of lithium and induce its mixing to the surface. According to the research team, the new discovery supports a different theory.
“The properties of many lithium-rich giants discovered within the Gaia-ESO Survey seem to be consistent with those of giants that engulfed close-in giant planets before evolving up the red giant branch. However, a small fraction of cases still require alternative explanations. Here, we report the discovery of two lithium-rich giants that could be examples of such an alternative formation channel in the open cluster Trumpler 20,” the researchers wrote.
One explanation is suppressed all instances of extra-mixing processes, so the surface abundance of lithium in these stars remained at the level predicted by standard stellar evolution models.
“We argue that the fraction of Li-rich giants found in our sample is consistent with these giants being evolved counterparts of magnetic Ap-type dwarfs. In this case, the extra-mixing processes could have been inhibited by the action of magnetic fields,” the paper reads. This scenario would likely apply to many, if not all, lithium-rich giants with similar masses and abundances of this element. ie they would not have fresh lithium production, but would have preserved part of their original abundance. Other hypotheses seem less likely, but cannot be fully excluded.
http://phys.org/news/2016-05-astronomers-giant-lithium-rich-stars-cluster.htmljCp
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