Investigating non-LTE abundances of Neodymium (Nd) in metal-poor FGK stars

TL;DR

This paper addresses the need for accurate Nd abundances in metal-poor FGK stars to constrain $r$-process sites and kilonova models, arguing that LTE is insufficient for these low-opacity atmospheres. It develops a comprehensive Nd I / Nd II NLTE model atom, calibrates hydrogen collisions with the Sun and meteoritic Nd to fix $S_H=0.1$, and demonstrates that NLTE effects significantly affect Nd II line formation, especially in optical lines of giants. A large NLTE correction grid is computed for 122 Nd II lines across a wide parameter space ($T_ ext{eff}$, $\\log g$, [Fe/H], $\xi_t$, and Nd abundance) using MULTI2.3 and a GCOG fitting approach, with an accuracy verification showing mean interpolation errors of ~0.001 dex. The results show NLTE corrections ranging from $-0.3$ to $+0.5$ dex, peaking for blue, low-excitation lines in metal-poor giants, and negative corrections for H-band lines, indicating that 1D NLTE analyses are essential for reliable Nd abundances in$r$-process studies and galactic archaeology; future work will extend to 3D NLTE and additional lanthanides.

Abstract

The dominant site(s) of the $r$-process are a subject of current debate. Ejecta from $r$-process enrichment events like kilonovae are difficult to directly measure, so we must instead probe abundances in metal-poor stars to constrain $r$-process models. This requires state-of-the-art Non-Local Thermodynamic Equilibrium (NLTE) modeling, as LTE is a poor approximation for the low-opacity atmospheres of metal-poor giants. Neodymium (Nd) is a prominent $r$-process element detected in both near-infrared kilonovae spectra and spectra of metal-poor stars, so precise Nd stellar abundances are particularly needed to model kilonovae and constrain $r$-process sites. We thus constructed a Nd I / Nd II model atom to compute NLTE abundances in FGK metal-poor stars. We obtain $\mathrm{A(Nd)}_\odot = 1.44\pm0.05$, in agreement with the meteoritic value, when calibrating the model atom with a Drawin hydrogen collision factor of $S_H=0.1$. For a sample of metal-poor $r$-process enhanced stars with observed optical and near-infrared Nd II lines, we find NLTE Nd corrections in the range $-0.3$ to $0.3$ dex. Optical and UV lines have positive NLTE corrections, whereas H band lines have negative corrections. Additionally, we compute a large grid of NLTE corrections for 122 Nd II spectral lines ranging from the UV to the H band, for stellar parameters of typical metal-poor FGK dwarfs and giants with $-3.00\le\mbox{[Fe/H]}\le-1.00$ and $-2.0\le\mathrm{A(Nd)}\le2.0$. Within this grid, we find NLTE corrections ranging from $-0.3$ to $+0.5$ dex. Deviations from LTE are found to be strongest for blue lines with low excitation potentials in the most metal-poor giants.

Figures (13)

• Figure 1: Grotrian diagrams of Nd i and Nd ii displaying all energy levels and bound-bound transitions as generated by FORMATO3. Spectral line data were assembled from VALD, NIST, linemake, and Hasselquist_2016. Dotted lines indicate the ground state and ionization energy of each species, and 122 transitions of interest that have been previously observed in stellar spectra are highlighted in red.
• Figure 2: Curves of growth for two Nd ii lines generated with MULTI2.3 using the solar MARCS model. Measured EWs from denhartog2003 were converted into $\mathrm{A}(\text{Nd})$ values in both NLTE (dark blue) and LTE (light orange) by interpolating from a cubic best-fit curve.
• Figure 3: Nd ii abundance calculations based on observed lines for the Sun (top) and HD222925 (bottom) for LTE and NLTE models with different $S_\text{H}$ factors, compared to published A(Nd) values (shown as blue shaded regions). Vertical axis labels indicate whether the HM model or a MARCS model atmosphere were used for different $S_\text{H}$ factors. Orange lines show the median of the line-by-line Nd abundances, while green triangles show the mean. The lower and upper boundaries of the boxes represent the first and third quartiles of the line-by-line abundances respectively, with outliers shown as black circles.
• Figure 4: Grotrian diagrams of Nd ii for three model atoms: Atom A (top), Atom B (middle), and Atom C (bottom) (see Section \ref{['sec:completeness']} for details on the atoms). Dotted lines indicate the ground state and ionization energy of Nd II. The 122 transitions of interest shown in Figure \ref{['fig:grotrian']} are also highlighted here in red. Transitions are highlighted in blue if their lower energy level is theoretical.
• Figure 5: Observed and synthetic spectra for wavelength regions around two solar Nd ii lines (top) and two Nd ii lines in HD222925 (bottom). For HD222925, abundances were obtained and synthetic spectra were generated using the stellar parameters listed in roederer2018. Shaded regions indicate $\pm0.2$ dex in the NLTE abundance. Prominent spectral lines and features are labeled. Small panels above each plot are zoomed in on the Nd ii lines under consideration.