Using Lithium and Beryllium to Study Structure and Evolution of Rotating Stars: Spite Plateau of Halo Stars

Abstract

The observed lithum (Li) abundance of Galactic halo stars mainly fall within the range of 2.0--2.4 dex. This nearly constant value, known as the Spite plateau, is approximately a factor of three lower than the value predicted from cosmic microwave background measurements and standard Big Bang Nucleosynthesis (BBN) calculations. This discrepancy -- referred to as the cosmological Li problem -- is considered a potential indication of new physics or astrophysical processes. We employed models incorporating gravitational settling, diffusion, rotation, and magnetic fields to explain the Spite plateau. The rotating models predict that Li abundances in stars with ages of roughly 8--13 Gyr and effective temperatures between 6400 and 5900 K generally fall within 2.0--2.4 dex, forming a well-defined Li plateau, followed by a sharp decline in Li abundance down to about 5200 K. The Li plateau results from the combined effects of variations in convection zone depth, gravitational settling, diffusion, rotation, and magnetic fields. For red giant branch stars with $T_{\mathrm{eff}} \lesssim$ 5200 K, the rotating models predict another Li plateau with an abundance of about 1.0 dex. These results are in good agreement with observations. Moreover, the initial Li abundance of 2.72 dex adopted in the models matches the BBN prediction, implying that the Li problem arises from stellar Li depletion. Furthermore, the rotating models also reproduce the Li and Be distributions of the sample that exhibit the Spite plateau meltdown and Be deviation.

Figures (12)

• Figure 1: (a) Hertzsprung-Russell diagram of rotating models with initial metallicity [Fe/H]$_{0} = -1.0$. (b) Surface lithium abundance as a function of effective temperature ($T_{\mathrm{eff}}$) for different models. The ages of 8, 11, and 13 Gyr are indicated along the tracks by open red triangles, squares, and pentagons, respectively. Filled circles denote observed Li abundances, while inverted triangles represent Li upper limits determined by charb05. The metallicities of the observed stars are indicated by symbol colors. The vertical dashed and dotted lines correspond to $T_{\mathrm{eff}}$ = 6000 K and 5700 K, respectively. The horizontal dotted, dashed, and dash-dotted lines indicate Li abundances of 2.72, 2.4, and 2.1 dex, respectively.
• Figure 2: (a) Comparison of Hertzsprung-Russell diagrams for rotating and nonrotating models. Filled circles and inverted triangles represent observational data from charb05. The magenta cross, triangle, square, pentagon, and star along the track mark the ZAMS, MSTO, terminal-age MS (TAMS), middle of the SGB, and base of the RGB, respectively. The TAMS is defined as the point where the central hydrogen content drops to $10^{-7}$. These models are also listed in Tables \ref{['tab1']} and \ref{['tab2']}. (b) Comparison of surface Li abundances between rotating and nonrotating models. Filled circles denote lithium detections, while inverted triangles indicate Li upper limits charb05. (c, d) Mass of the CZ and temperature at the BCZ of nonrotating models as a function of stellar age. Vertical dotted lines indicate the end of the fully convective phase.
• Figure 3: (a), (b), (c), (d), (e), (f) Lithium profiles as a function of radius for different models. (g), (h), (i), (j), (k), (l) Lithium profiles as a function of temperature for different models. Horizontal dotted lines indicate the initial Li abundance, while the vertical dotted lines denote the BCZ of nonrotating models. For M = 0.82 $\mathrm{M}_\odot$, at ages of 11 and 13 Gyr, the star is located near the MSTO and the middle of the SGB, respectively.
• Figure 4: Same as Figure \ref{['fig1']}, but for models with [Fe/H]$_{0} = -1.5$ dex (a, b) and [Fe/H]$_{0} = -2.0$ dex (c, d).
• Figure 5: (a) Surface lithium abundance as a function of effective temperature for different models. The ages of 8, 11, and 13 Gyr are indicated along the tracks by open red triangles, squares, and pentagons, respectively. (b, c) Mass of the CZ and temperature at the BCZ of NMs as a function of stellar age. The cross and star along the tracks mark the ZAMS and MSTO, respectively.