Title: Lithium Pollution of White Dwarfs and Other Secrets of MORDOR
Running diagonally down the middle of the Hertzsprung-Russell Diagram (H-R Diagram) between the white dwarf cooling track and the lower main sequence lies a less frequently studied region of the H-R Diagram populated by dim objects. We have termed this area the Medial Overlooked Region’s Dim Object Range (MORDOR), and we present the results of an optical spectroscopic survey of 83 objects in MORDOR using the Goodman Spectrograph mounted on the SOAR telescope. We discovered a variety of new and interesting objects including white dwarf-M dwarf binaries that could reveal the composition of M dwarf wind. We performed a thorough analysis of another MORDOR class: the white dwarfs with metal lines (DZs).
These DZs have accreted planetesimals, which can reveal the elemental abundances of rocky bodies in exoplanetary systems. Those abundances provide information on the composition of the nebula from which the systems formed and the geological processes that the rocky material experienced. A DZ we found in MORDOR and a second DZ outside MORDOR exhibited the first known lithium and potassium in white dwarfs. We find their lithium abundances to be elevated relative to the baseline for the Solar System, which we propose was a consequence of the accreted planetesimals originating from the earliest epochs of the Galaxy, when the ratio of lithium to heavier elements was high due to Big Bang nucleosynthesis.
Another observer discovered three additional white dwarfs that are polluted by lithium from extrasolar planetesimals and proposed the lithium excess was caused by the accretion of continental crust. We analyze the abundances of all five Li-bearing white dwarfs in the context of the two competing hypotheses: planetary differentiation and the nucleosynthetic evolution of the Galaxy. We find that none of the five white dwarfs are consistent with accreting unaltered continental crust, but three are consistent with accreting primitive planetesimals with a Li excess originating from the nucleosynthetic evolution of the Galaxy. In the future with further discoveries and the means to constrain the protostellar metallicities of the systems, Li-polluted white dwarfs will allow for the first proper test of the predicted Big Bang nucleosynthetic Li abundance.
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