Barium abundances of A--F--G type stars in the Hyades cluster

TL;DR

This study quantifies barium abundances in Hyades A–F–G-type stars by performing non-LTE spectrum synthesis that includes hyperfine-structure of Ba II lines at 6141 and 6496 Å. A grid of model atmospheres and NLTE departure coefficients is used to derive accurate Ba abundances for 89 Hyades members and 80 field A-type stars, with results indicating homogeneous, mildly supersolar Ba among Hyades G stars and a Teff- and vsini-dependent Ba excess in A-type stars. Hyades F-type stars exhibit a notable Ba-dip around $T_{ m eff} \sim 6500$ K, coinciding with the Li-dip, while A-type stars show a wide dispersion in Ba enhancements that increase with $T_{ m eff}$ and decrease with $v_{ m e}\sin i$. The findings imply that NLTE effects partially alleviate previously reported subsolar Ba trends from LTE analyses and highlight the roles of $T_{ m eff}$ and rotation in the Am phenomenon, with implications for cluster chemical tagging and diffusion-related abundance patterns.

Abstract

With an aim of clarifying the extent and parameter-dependence of compositional anomaly of barium in A-type stars, Ba abundances were spectroscopically determined based on BaII 6141/6496 lines for 89 (23 A-type and 66 F--G-type) main-sequence stars belonging to the members of Hyades cluster by taking into account the non-LTE effect and the hyper-fine-structure effect. While the non-LTE effect tends to strengthen lines in G stars, it acts in the direction of line weakening in the regime of A stars due to increasing imortance of overionization. The Ba abundances of G stars turned out almost constant (<A>= 2.33), indicating that the primordial composition of Ba in Hyades is mildly supersolar by ~+0.2dex. In contrast, A-type stars show Ba overabundances of considerably large dispersion (0~<[Ba/H]~<2). Since this Ba excess tends to increase with an increase/decrease in Teff/vsini, these two parameters may be essential for producing or controling the anomaly. Regarding Hyades F-type stars, their Ba abundances are not uniform but show a broad depression (by <~0.3dex) around Teff~6500K, interestingly coinciding with the location of Li-dip.

Figures (9)

• Figure 1: Number population fraction ($f$) of (a) neutral, (b) once-ionized, and (c) twice-ionized barium species relative to the total Ba atoms [e.g., $f$(Ba ii) $\equiv N$(Ba ii)/$N_{\rm total}^{\rm Ba}$], plotted against the continuum optical depth at 5000 Å. Calculations were done for four $\log g = 4.0$ model of different $T_{\rm eff}$ (6000, 7000, 8500, and 10000 K) as indicated in each panel. The runs of $f$ for these three stages at $\tau_{5000} = 0.1$ with $T_{\rm eff}$ are also depicted in panel (d). All these calculations were done in LTE (use of Saha's equation).
• Figure 1: Left panels (a)--(d): atmospheric parameters of 80 field A-type stars ($\log g$, $v_{\rm t}$, $v_{\rm e}\sin i$, and $A$(Fe)) are plotted against $T_{\rm eff}$, as done in Fig. 2a--2d for Hyades stars. Right panels (e)--(h): Ba ii 6141 line-related quantities ($W_{6141}$, $A_{6141}^{\rm L}$, $\Delta_{6141}$, and $A_{6141}^{\rm L}$) derived for 80 field A stars are plotted against $T_{\rm eff}$, as done in Fig. 7a--7d for Hyades stars.
• Figure 2: The atmospheric parameters of the program stars adopted in this study are plotted against $T_{\rm eff}$. (a) $\log g$ (surface gravity), (b) $v_{\rm t}$ (microturbulence), (c) $v_{\rm e}\sin i$ (projected rotational velocity), and (d) $A$(Fe) (logarithmic number abundance of Fe in the usual normalization of H=12). The data of Hyades A stars and F--G stars are plotted in blue symbols and green symbols, respectively.
• Figure 3: Synthetic spectrum fitting around the Ba ii 6141 line region for Ba abundance determination. The best-fit theoretical spectra are depicted by black solid lines, while the observed data are plotted by colored symbols (inappropriate rejected data in evaluating the goodness of fit are highlighted in light green). The leftmost panel (a) is for 23 Hyades A-type stars arranged in the descending order of $v_{\rm e}\sin i$. Panels (b), (c), and (d) in the right-hand side are for 66 Hyades F- and G-type stars (along with Procyon and Sun) arranged in the descending order of $T_{\rm eff}$ (from top to bottom; from left to right). HD number is indicated in each spectrum. An offset of 0.2 (in unit of the continuum-normalized flux) is applied to each spectrum relative to the adjacent one. The position of the Ba ii 6141 line is indicated by vertical dashed lines. Note that, while the whole wavelength region adopted for fitting (cf. Table 1) is shown for A stars in panel (a), only the restricted region around the Ba ii line is displayed for F--G stars in panels (b)--(d).
• Figure 4: Synthetic spectrum fitting around the Ba ii 6496 line region for Ba abundance determination. The position of the Ba ii 6496 line is indicated by vertical dashed lines. Otherwise, the same as in Fig. 3.