The Lithium Plateau in Super Metal-Rich Stars

Abstract

I present a study of the lithium abundances for a sample of super metal-rich (SMR, [Fe/H] > + 0.20) main sequence dwarfs in the solar neighborhood. The SMR stars were selected to have surface temperatures in the region of the lithium plateau, a narrow region in temperature space where stars are predicted by models to preserve their initial surface Li abundances while on the main sequence. Despite these predictions, observations of lower-metallicity stars indicate that significant depletion occurs during the first few Gyr of stellar evolution. SMR stars, which represent the extreme end of Galactic chemical evolution, present an opportunity to constrain proposed mechanisms to explain this depletion. 100 SMR candidates were selected from existing surveys and followed up with spectroscopic observations using the Hydra spectrograph on the WIYN 3.5m telescope at Kitt Peak National Observatory, as well as photometric observations using the 0.9m WIYN and 40” telescopes at Kitt Peak and Mount Laguna Observatory, respectively. Using the results from ANNA, a new tool that uses a neural network to parameterize stellar spectra, as well as a more traditional equivalent width based analysis, 44 single stars with [Fe/H] > + 0.20 were identified and Li was measured for each star. Consistent with previous studies, the SMR stars can be divided into a sample of stars with measurable Li and a sample with upper limits only. Examining the low-Li stars’ evolutionary states reveals that they are consistent with being evolved Li dip stars and therefore depleted their surface Li while on the main sequence before evolving to the cooler temperatures of the Li plateau. Considering only the high-Li sample, the stars are all consistent with having ages in the range 3 – 4.5 Gyr, indicating that they have already significantly depleted their Li. We find no young (age + 0.20) main sequence dwarfs in the solar neighborhood. The SMR stars were selected to have surface temperatures in the region of the lithium plateau, a narrow region in temperature space where stars are predicted by models to preserve their initial surface Li abundances while on the main sequence. Despite these predictions, observations of lower-metallicity stars indicate that significant depletion occurs during the first few Gyr of stellar evolution. SMR stars, which represent the extreme end of Galactic chemical evolution, present an opportunity to constrain proposed mechanisms to explain this depletion. 100 SMR candidates were selected from existing surveys and followed up with spectroscopic observations using the Hydra spectrograph on the WIYN 3.5m telescope at Kitt Peak National Observatory, as well as photometric observations using the 0.9m WIYN and 40” telescopes at Kitt Peak and Mount Laguna Observatory, respectively. Using the results from ANNA, a new tool that uses a neural network to parameterize stellar spectra, as well as a more traditional equivalent width based analysis, 44 single stars with [Fe/H] > + 0.20 were identified and Li was measured for each star. Consistent with previous studies, the SMR stars can be divided into a sample of stars with measurable Li and a sample with upper limits only. Examining the low-Li stars’ evolutionary states reveals that they are consistent with being evolved Li dip stars and therefore depleted their surface Li while on the main sequence before evolving to the cooler temperatures of the Li plateau. Considering only the high-Li sample, the stars are all consistent with having ages in the range 3 – 4.5 Gyr, indicating that they have already significantly depleted their Li. We find no young (age + 0.20 were identified and Li was measured for each star. Consistent with previous studies, the SMR stars can be divided into a sample of stars with measurable Li and a sample with upper limits only. Examining the low-Li stars’ evolutionary states reveals that they are consistent with being evolved Li dip stars and therefore depleted their surface Li while on the main sequence before evolving to the cooler temperatures of the Li plateau. Considering only the high-Li sample, the stars are all consistent with having ages in the range 3 – 4.5 Gyr, indicating that they have already significantly depleted their Li. We find no young (age < 1 Gyr) SMR stars in the sample, which may explain an observed turnover in the positive correlation between [Fe/H] and initial Li abundance at super-solar metallicities – the only SMR stars in the solar neighborhood are already too old to measure initial Li as they have depleted down to the 2-3 Gyr plateau value. The Li plateau for the SMR sample was measured to be A(Li) = 2.55 dex, which agrees with observations of the similarly-aged super metal-rich cluster NGC 6253 as well as more metal-poor clusters, confirming that the Li plateau abundance in stars older than 2-3 Gyr is apparently insensitive to stellar metallicity. Examining the kinematics and available elemental abundances of the SMR stars, they are shown to be indistinguishable from lower-metallicity thin disk stars aside from their high [Fe/H], consistent with an origin in the inner thin disk.