🔭 astrophysics

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

This study presents a comprehensive chemical abundance analysis of 23 elements in the open cluster NGC 5822 using optical and near-infrared spectra, revealing insights into the cluster's thin disk characteristics, the origin of lithium-enriched giants, and the mass of polluting companions responsible for its barium stars.

Original authors: N. Holanda, V. Loaiza-Tacuri, A. Sonally, S. Bijavara Seshashayana, M. P. Roriz, C. F. Martinez, M. Borges Fernandes, C. B. Pereira, O. J. Katime Santrich, S. Daflon

Published 2026-02-04

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: N. Holanda, V. Loaiza-Tacuri, A. Sonally, S. Bijavara Seshashayana, M. P. Roriz, C. F. Martinez, M. Borges Fernandes, C. B. Pereira, O. J. Katime Santrich, S. Daflon

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Milky Way galaxy as a giant, bustling city. Within this city, stars aren't just scattered randomly; they often form in "neighborhoods" called open clusters. These clusters are like stellar families: all the stars were born at the same time, from the same cloud of gas, and share the same chemical DNA. By studying these families, astronomers can learn how the city of the galaxy has evolved over billions of years.

This paper is a detailed "chemical autopsy" of one specific neighborhood called NGC 5822. The researchers, led by N. Holanda, used powerful telescopes to take a close-up look at the "elderly" stars (red giants) in this cluster to see what they are made of.

Here is a breakdown of their findings using simple analogies:

1. The Tools: Two Different Cameras

To get the full picture, the team didn't just use one camera. They used two different types of "spectrographs" (devices that split starlight into a rainbow to reveal chemical ingredients):

The Optical Camera (FEROS): This sees visible light, like what our eyes see.
The Infrared Camera (IGRINS): This sees heat radiation (near-infrared). This is crucial because red giant stars are cool and dusty; the infrared camera can see through the "fog" and spot chemical ingredients that the optical camera misses.

By combining these two, they analyzed 23 different chemical elements (like Lithium, Fluorine, and Lead) and even the ratios of different "flavors" of Oxygen.

2. The Mystery Guests: The "Ba-Stars"

In this stellar neighborhood, they found two stars (#002 and #201) that are chemically peculiar, known as Barium stars.

The Analogy: Imagine a family where everyone has a standard diet, but two members suddenly have a table full of heavy, rare spices (Barium and other heavy elements) that no one else has.
The Cause: These stars didn't cook these spices themselves. They are like "vampires" that stole them from a neighbor. In the past, these stars were in a binary system (a pair). Their companion star, which was once a giant, cooked up these heavy elements and dumped them onto our two stars before dying and becoming a white dwarf.
The Discovery: By analyzing the "stolen" spices, the team calculated that the "chef" companion stars were likely about 3 to 3.75 times the mass of our Sun.

3. The Lithium Enriched Stars: The "Energy Drink" Giants

The team also found three stars (#006, #102, and #240) that are unusually rich in Lithium.

The Mystery: Normally, as stars age and expand into red giants, they swallow their own Lithium and destroy it. Finding a giant with high Lithium is like finding an old person who suddenly has the energy of a teenager.
The Investigation: The team looked for clues: Are they spinning fast? Are they magnetic? Are they eating a companion star?
The Result: They found some signs of activity (like a faint magnetic hum), but nothing that fully explains the Lithium boost. The stars aren't spinning fast enough to be the sole cause.
The Likely Culprit: The team suspects these stars are in a specific phase of life called the Red Clump (where they are burning helium in their cores). The Lithium boost likely happened during a "helium flash" (a sudden burst of energy) or perhaps a merger with a small white dwarf companion. It's a unique event in their life story that we are just beginning to understand.

4. The Fluorine Gradient: The City's "Smog"

The researchers also measured Fluorine, a rare element.

The Analogy: Think of Fluorine as a specific type of "smog" or exhaust fume produced by different types of factories (stars).
The Finding: They found that Fluorine is more abundant in younger clusters and less abundant in older ones.
The Conclusion: This confirms that Fluorine is produced by a mix of "factories": massive stars that explode quickly (Supernovae) and older, dying stars (AGB stars) that take billions of years to release their goods. The mix changes as the galaxy gets older.

5. The Neighborhood Map

Finally, the team double-checked who actually belongs to this neighborhood. Using data from the Gaia satellite (which maps star positions and movements), they confirmed that most of the stars they studied are indeed part of the NGC 5822 family.

They calculated the cluster's age (about 1 billion years old), distance (about 800 light-years away), and location in the galaxy.
They found that the cluster's chemical makeup fits perfectly with the "Thin Disk" of the Milky Way, the flat, crowded part of the galaxy where our Sun lives.

Summary

This paper is like a forensic investigation of a stellar family. By using both visible and infrared light, the team:

Confirmed the family's history (age and location).
Identified two "thieves" (Barium stars) who stole heavy elements from a dead neighbor.
Found three "energy boosters" (Lithium-rich stars) that likely had a dramatic life event (a merger or helium flash) that we don't fully understand yet.
Mapped out how the galaxy produces Fluorine over time.

It highlights that even in well-studied neighborhoods, there are still chemical mysteries waiting to be solved by looking at the stars with the right "glasses."

Technical Summary: A Near-Infrared and Optical Study of NGC 5822

Problem and Context
Open clusters (OCs) serve as critical benchmarks for understanding stellar evolution and Galactic structure due to their shared distances, ages, and initial chemical compositions. While optical spectroscopy has traditionally dominated chemical abundance studies in OCs, near-infrared (NIR) spectroscopy offers distinct advantages, including reduced sensitivity to interstellar extinction and access to molecular and atomic lines (particularly for red giants) that are inaccessible in the optical domain. The open cluster NGC 5822 is a well-studied system hosting chemically peculiar stars, specifically two barium (Ba) stars (#002 and #201) and three lithium-enriched giants (#006, #102, and #240). However, prior to this work, no study had explored its red giants using NIR spectroscopy covering both the $H$ and $K$ bands, leaving elements such as fluorine (F), phosphorus (P), potassium (K), and ytterbium (Yb) uninvestigated in this cluster. Furthermore, the mechanisms driving lithium enrichment in giant atmospheres and the precise nature of the Ba-star companions remain topics of active investigation.

Methodology
The authors conducted a comprehensive chemical abundance study of 13 giant stars in NGC 5822, combining high-resolution optical and NIR data.

Observations: Optical spectra were obtained using the FEROS spectrograph ( $R \approx 48,000$ ) on the 2.2 m ESO telescope, while NIR spectra ( $H$ and $K$ bands, $R \approx 45,000$ ) were acquired using the IGRINS spectrograph on the 8.1 m Gemini South telescope.
Membership Analysis: Using Gaia DR3 astrometric data, the authors employed the UPMASK code to identify cluster members. Stars with high Renormalized Unit Weight Error (RUWE $\ge$ 1.2), such as the Ba-stars #002 and #201, were flagged as potential binaries, complicating their astrometric membership probabilities. Isochrone fitting (Bressan et al. 2012) was used to derive cluster parameters (age, distance, extinction).
Spectroscopic Analysis: Atmospheric parameters ( $T_{\text{eff}}$ , $\log g$ , $\xi_t$ , [Fe/H]) were derived using the MOOG code under Local Thermodynamic Equilibrium (LTE) assumptions, employing a grid of Castelli & Kurucz (2004) models. Chemical 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, Pb) and isotopic ratios ( $^{12}\text{C}/^{13}\text{C}$ , $^{16}\text{O}/^{17}\text{O}$ , $^{16}\text{O}/^{18}\text{O}$ ) were determined via equivalent width measurements (optical) and spectral synthesis (optical and NIR). Non-LTE (NLTE) corrections were applied where available (Li, Na, Mg, Al, Pb).

Key Contributions
This study presents the first NIR chemical analysis of NGC 5822 covering both $H$ and $K$ bands. Key contributions include:

First Measurements: Determination of abundances for F, P, K, and Yb, and isotopic ratios $^{17}\text{O}/^{16}\text{O}$ and $^{18}\text{O}/^{16}\text{O}$ in this cluster.
Robust Membership: A refined membership list distinguishing true cluster members from field contaminants, while addressing the challenges posed by binary Ba-stars.
Multi-wavelength Consistency: A comparative analysis of optical and NIR results, demonstrating general consistency while highlighting specific discrepancies (e.g., Al abundances) driven by NLTE effects.

Results

Cluster Parameters: The derived age ( $\log t = 9.018 \pm 0.080$ Gyr), distance ( $817 \pm 52$ pc), and metallicity ([Fe/H] $\approx -0.06$ ) are consistent with recent literature. The turn-off mass is estimated at $2.05 \pm 0.10 M_\odot$ .
Chemical Abundances: The cluster's mean abundances are consistent with Galactic thin disk expectations.
- Fluorine: The mean fluorine abundance agrees with chemical evolution models predicting elevated [F/Fe] in young clusters (< 2 Gyr) due to production from SN II, SN Ia, AGB, and Wolf-Rayet stars. A Galactic gradient of $-0.05 \pm 0.03$ dex kpc $^{-1}$ was derived for the inner disk.
- Lithium-Enriched Giants: Stars #006, #102, and #240 show slight lithium enrichment ( $\log \epsilon(\text{Li}) \gtrsim 1.30$ ). Isochrone fitting places them in the red clump phase. No distinct chemical or rotational features (e.g., high $v \sin i$ or strong chromospheric activity) were found to definitively explain the enrichment, though #006 and #102 show Ca II H&K emission. The authors suggest mechanisms such as mergers involving helium white dwarfs or helium-flash-induced mixing.
- Barium Stars: Stars #002 and #201 exhibit strong s-process enhancements (Y, Ce, Nd, Yb). Nucleosynthesis models combined with the cluster's turn-off mass suggest polluting AGB companion masses of $3.00 M_\odot$ and $3.75 M_\odot$ for #002 and #201, respectively. The absence of sodium enrichment in these stars helps constrain the companion mass to be below $4.0 M_\odot$ .
- Isotopic Ratios: The $^{12}\text{C}/^{13}\text{C}$ and oxygen isotopic ratios generally agree with first dredge-up (1DUP) and thermohaline + rotation (TH+ROT) mixing models, though a significant discrepancy was noted for $^{16}\text{O}/^{18}\text{O}$ compared to model predictions.

Significance
The paper underscores the utility of open clusters as laboratories for studying chemically peculiar stars. By combining optical and NIR spectroscopy, the study provides a more complete chemical inventory of NGC 5822, filling gaps in the literature regarding heavy elements and isotopic ratios. The results validate the use of isochrone fitting to constrain the evolutionary stages of peculiar stars in the absence of asteroseismic data. Furthermore, the derived constraints on the masses of AGB companions to Ba-stars and the investigation of lithium enrichment mechanisms contribute to a deeper understanding of stellar evolution and binary interaction scenarios. The study also reinforces the role of open clusters in tracing Galactic chemical evolution, particularly regarding fluorine production and distribution.