- Title: Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, galactic chemical evolution, and exoplanets
- Authors: I. Ramirez, J. R. Fish, D. L. Lambert, C. Allende Prieto
- First author’s institution: McDonald Observatory and Department of Astronomy, University of Texas at Austin
The Big Picture
In this paper, Ramirez et al. determine the metallicity, temperature, surface gravity, mass, and lithium abundance of 671 stars to address three questions about galactic and stellar evolution:
- How is lithium depleted inside stars and how does the level of depletion depend on stellar characteristics?
- How has the abundance of lithium changed over the history of the Galaxy?
- How do planets influence the lithium abundance of their host stars?
One question you might have when reading this list of research questions is why Ramirez et al. are so concerned about lithium. Why didn’t they focus the paper on oxygen abundances or carbon abundances? Both of those elements are far more important when considering the formation of habitable planets, but lithium is a useful tool for constraining the ages and evolutionary history of stars because as Ian explained in this astrobite, most lithium is primordial. That means that astronomers know how much lithium a star had when it formed and they can then measure the current abundance of lithium in order to figure out how much lithium has been lost or destroyed. Astronomers can typically only measure the abundance of lithium in the outer layers of the star, so they are really measuring surface depletion: the amount of lithium that has been destroyed near the surface of the star.
There are two primary reasons why the surface abundance of lithium might be lower than expected. The first explanation is that the lithium could have been fused into heavier elements, but the temperatures required to fuse lithium are typically higher than those reached by the outer convective layer of Sun-like stars. For that reason, astronomers have proposed a second explanation that the lithium might have been mixed into the deeper layers of the star. The exact mechanisms governing surface lithium depletion are unclear, but astronomers have discovered that many stars (including the Sun) have lower lithium abundances than would be expected from Big Bang Nucleosynthesis. In this paper, Ramirez et al. measure the lithium abundances of a large sample of stars in order to find correlations between lithium abundance and other properties. Such correlations will help astronomers determine exactly how lithium is destroyed in stars and how the galactic abundance of lithium has changed over the lifetime of the Milky Way.
Ramirez et al. acquired spectra of 671 nearby Sun-like stars (spectral types F, G, and K) using the 2.7 meter Harlan J. Smith Telescope at McDonald Observatory and added their sample to results from the literature to produce a catalog of high-resolution spectra for 1381 Sun-like stars. They determined the lithium abundance and metallicity by examining the spectral lines present in their spectra. The stars were also observed photometrically at optical and near-infrared wavelengths, so Ramirez et al. determined the temperatures of the stars by comparing the observed colors of the stars to known color-temperature relations. All of the stars had parallaxes measured by Hipparcos, which allowed Ramirez et al. to derive the surface gravity and mass of each star by placing the stars on theoretical isochrones.
After creating a fantastic catalog of stellar properties and lithium abundances, Ramirez et al. used the kinematics (3-D velocities) of each of the stars to assign probabilities that each star belonged to the thin disk, thick disk, or halo of the Milky Way. Ramirez et al. found that 898 of their stars belong to the thin disk, 144 are part of the thick disk, and 43 are halo stars. Stars in the thin disk are generally younger and more metal-rich than stars in the thick disk, so astronomers typically expect that thin disk and thick disk stars would have different lithium abundance patterns because they formed at different times under different galactic conditions.
As shown in the left panels of the figure below, the lithium abundances of the thick and thin disks do appear different at first glance. This figure plots lithium abundance versus metallicity for thin disk stars (top panels) and thick disk stars (bottom panels). The solid and dashed lines indicate the upper envelope of the lithium abundance for the thin and thick disk stars, respectively. Although the stars in both populations have a range of lithium abundances, the upper envelope of the lithium abundances of the thin disk stars is significantly higher than the maximum lithium abundances of the thick disk stars.
However, Ramirez et al. point out that the difference in lithium abundances disappears when a correction is applied for the mass difference between the two populations. The thick disk is older than the thin disk, so the mass of a typical thick disk star is lower. When Ramirez et al. correct for the mass difference by plotting only stars with masses below 1.1 solar masses, they find that the difference between the lithium abundance of the thin and thick disk stars shrinks. As shown in the right panels of the figure above, the difference disappears completely when Ramirez et al. restrict the sample to older (> 8 Gyr), low-mass (< 1.1 solar masses) stars. They therefore find that the difference in lithium abundance between the thin and thick disk stars is dominated by age and mass effects rather than environmental effects. More massive stars are hotter, so their convective envelopes are more likely to reach down to the depths where the temperature is hot enough to fuse lithium. Similarly, older stars will have had more time to either fuse lithium into heavier elements and/or mix surface lithium into their interiors. Ramirez et al. also divided up their sample based on planet occurrence. Their catalog contains 165 stars hosting known planets and 360 stars that have been targeted by planet-hunters but do not yet have any known planets. The remaining 856 stars have not yet been searched for planets, so their planet host status is unknown. Although several studies have claimed that planet hosts tend to be more depleted in lithium than non-planet hosts, Ramirez et al. find that the lithium abundance pattern of stars with planets is similar to the abundance pattern for stars without known planets. They note that the correlations observed in previous studies can be explained by biases in the mass, age and metallicity distributions of the planet hosts compared to non-planet hosts. There is a slight hint that planets might affect the lithium abundance of stars within a narrow range of temperatures (5820-6190K), but more research is required before astronomers can provide further comments on the relationship between planets and lithium abundance in that narrow temperature window.