A Near-Infrared and Optical Study of NGC 5822: An Open Cluster Hosting Barium-stars and Lithium-Enriched Giant Stars

TL;DR

The study probes the chemical composition of thirteen red giant members of NGC 5822, including two Ba-stars and three Li-enriched giants, by combining high-resolution optical FEROS and near-infrared IGRINS spectra to measure 23 elements and several isotopic ratios. Membership is established with Gaia DR3 and cluster parameters are constrained as age $\log t = 9.018$, distance $d \approx 0.8$ kpc, and turn-off mass $M_{TO} \approx 2.05\,M_\odot$, enabling robust interpretation of evolutionary stages. The cluster shows a fluorine gradient with slope $-0.05\pm0.03$ dex kpc$^{-1}$ in the inner disk, and Ba-star abundances are best matched by low-mass polluters around $M_p \sim 3.0$ and $3.75\,M_\odot$ depending on the model. The Li-enriched giants are consistent with red-clump status and do not reveal a single enrichment mechanism, highlighting the heterogeneous nature of Li-rich giants in open clusters. Overall, the results reinforce the use of open clusters as laboratories for chemically peculiar stars, informing Galactic chemical evolution and nucleosynthesis in AGB stars.

Abstract

We present a chemical abundance study of giant stars in the Galactic open cluster NGC 5822, which hosts two barium stars (#002 and #201) and three lithium-enriched giants (#006, #102, and #240). Using high-resolution optical and near-infrared ($H$ and $K$ band) spectra from FEROS and IGRINS, we determine atmospheric parameters and abundances for 23 elements (Li, C, N, O, F, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, Cr, Fe, Ni, Y, Ce, Nd, Yb, and Pb). This includes species not yet studied in this cluster, such as F, P, K, Yb, and Pb, as well as oxygen isotopic ratios $^{16}$O/$^{17}$O and $^{16}$O/$^{18}$O. Membership was assessed using astrometry and chemical abundances, providing insight into the evolutionary stages of Li-enriched giants and cluster parameters (age, distance, extinction). However, the identification of Ba-stars remains challenging due to their binary nature and less reliable astrometric solutions. The cluster's abundances are broadly consistent with expectations for the Galactic thin disk. The mean fluorine abundance agrees with chemical evolution models predicting that young clusters (<2 Gyr) exhibit elevated [F/Fe], with production from SN II, SN Ia, AGB, and Wolf-Rayet stars. No distinct chemical or rotational features were found to explain the lithium enrichment, likely occurring either during the red clump phase or near the RGB tip. For the Ba-stars, nucleosynthesis models combined with the cluster's turn-off mass suggest polluting companion masses of 3.00 and 3.75 $M_{\odot}$ for stars #002 and #201. These results highlight the importance of open clusters as laboratories for chemically peculiar stars.

Figures (11)

• Figure 1: Proper motion distribution for bright stars around of the centre of NGC 5822 (top left), where the colour coded and light blue points (field contaminants) represent stars with probability to belonging to cluster of P $\geq$ 0.5 and 0.0 $\leq$ P $<$ 0.5, respectively. Colour–magnitude diagram (top right) is shown with an isochrone fitting bressan2012 for stars with proper motions and parallaxes characteristics of the cluster ($\log\,t$ = 9.018).The bottom left panel shows magnitude versus parallax, where blue dashed line represents the mean parallax, while the bottom right panel represents the plane distribution of stars around the cluster. The stars analyzed in this work are marked with "star" symbols. Data were taken from gaia2023.
• Figure 2: Examples of best-fitting results obtained from synthetic spectra (red lines) and observed IGRINS spectra (grey points) of stars. Additional lines are included to illustrate variations in abundance for the same spectral features (blue dashed-lines; do not represent uncertainties).
• Figure 3: Abundance ratios [X/Fe] versus [Fe/H] for the analysed sample: stars with high astrometric probability are represented in red and the stars with low probability are denoted by yellow (#002 and #201; Ba-stars) and gray (#001) colour symbols. The light blue circles represent the samples of giant stars from mishenina2006, ryde2020, nandakumar2022, from montelius2022, and from contursi2024. We also include dwarf stars analyzed by lucertini2022, bensby2014, battistini2015, and battistini2016, selected with Age $<$ 7.0 Gyr and TD/D $<$ 0.5. The gray dashed lines indicate the solar values, while the blue dashed lines represent the mean values of the cluster members.
• Figure 4: Mean elemental abundances for NGC 5822 derived from the optical (light blue) and NIR (red) spectral regions. Triangles indicate mean abundances corrected for NLTE effects. Abundance ratios are normalized to the solar abundances of asplund2009.
• Figure 5: Normalized Ca ii H & K spectra of the three Li-enriched giants in NGC 5822.