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TESS Asteroseismology of Red Giants in the Old Metal-Rich Open Clusters NGC 188 & NGC 6791

This study utilizes TESS photometry to perform asteroseismic analysis of red giants in the old, metal-rich open clusters NGC 188 and NGC 6791, successfully deriving precise seismic masses, confirming low mass loss at high metallicities, identifying binary interaction candidates, and establishing a cluster age of 7.0±0.9 Gyrs despite systematic offsets in frequency measurements for fainter stars.

Original authors: Madeline Howell, Jennifer A. Johnson, Marc H. Pinsonneault, Leslie M. Morales, Jamie Tayar, John D. Roberts, Dennis Stello, Madeleine McKenzie

Published 2026-05-01

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CC BY 4.0

Original authors: Madeline Howell, Jennifer A. Johnson, Marc H. Pinsonneault, Leslie M. Morales, Jamie Tayar, John D. Roberts, Dennis Stello, Madeleine McKenzie

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

The Big Picture: Cosmic Timekeepers

Imagine the universe as a giant library. Inside this library are "open clusters," which are like tightly packed families of stars. All the stars in a single family were born at the same time, from the same cloud of gas, and are roughly the same distance from us. Because they are so similar, they are the perfect "test subjects" for astronomers trying to understand how stars live, age, and die.

This paper is about two specific star families: NGC 188 and NGC 6791. They are old and rich in heavy elements (like gold and iron), making them "metal-rich" families. The researchers used a space telescope called TESS to listen to these stars "sing."

How They "Heard" the Stars

Stars aren't silent; they vibrate like giant bells. These vibrations create sound waves that travel through the star. By measuring these vibrations (a field called asteroseismology), astronomers can figure out a star's mass, size, and age, just like a doctor uses a stethoscope to check a heart.

However, TESS has a "blurry" view compared to previous telescopes. Its pixels are large, meaning it often sees a group of stars as one big blob of light. This is like trying to hear one person's voice at a crowded party while standing next to a wall of speakers.

The Solution: Instead of using the telescope's automatic software (which might mix up the voices of neighbors), the team used a "boutique method." They manually crafted custom "listening masks" for each star, carefully cutting out the noise from nearby stars to isolate the pure sound of their target. This gave them a much clearer signal, proving that with careful work, TESS can hear these stars just as well as the older, more powerful Kepler telescope.

The Two Families: A Tale of Two Clusters

1. NGC 6791: The "Faint" Family
This cluster is far away and its stars are dim. The researchers compared TESS's new "listening" to old, high-quality recordings from the Kepler telescope.

The Result: They heard the same "notes" (frequencies), but the TESS signal was a bit fuzzier. There was a small, consistent error of about 2.2% in the measurements.
The Takeaway: Because the stars are so faint, TESS struggles a bit more here. The team decided not to update the known data for this cluster but used it to show where TESS's limits lie.

2. NGC 188: The "Bright" Family
This cluster is closer and brighter. Since no one had ever measured the "songs" of these specific stars before, the team used their boutique method to get brand-new measurements for 17 red giant stars.

The Result: They got very precise data. They found that the average mass of these stars is consistent with what we expected from other studies, proving that TESS is a powerful tool for studying bright clusters.

Key Discoveries

1. The "Weight Loss" Mystery
As stars age, they burn fuel and lose mass (like a person losing weight as they get older). The team compared the "weight" of stars that are just starting to age (Red Giants) with those that are further along (Red Clump stars).

The Finding: They found that these stars lost very little mass—only about 0.02 solar masses.
Why it matters: This supports a theory that stars with high metal content (like these two clusters) lose less mass than metal-poor stars. It's like a heavy coat (high metallicity) protecting the star from shedding weight as easily as a thin shirt would.

2. The "Oddballs" (Binary Interactions)
In a family where everyone is supposed to be the same age and size, finding a sibling who is too heavy or too light is a clue that something unusual happened. The team found three "oddballs":

Two "Underweight" Stars: These stars are lighter than they should be. The team suspects they were "stripped" of their outer layers by a partner star in a binary system, like a bully taking a backpack.
One "Overweight" Star: This star is heavier than average. The team suspects it was formed by two stars crashing into each other and merging into one giant star.
The Smoking Gun: The overweight star also showed signs of "dipole-mode suppression." Imagine a bell that usually rings with a complex, rich sound, but this one only rings with a simple, dull thud. This happens when a star has a strong magnetic core, which often occurs if two stars merged.

3. The Age of the Family
By combining the mass measurements with the chemical "fingerprints" of the stars, the team calculated the age of the NGC 188 cluster.

The Result: They determined the cluster is about 7 billion years old. This matches perfectly with previous estimates, confirming that their new "listening" technique is accurate.

The Bottom Line

This paper is a success story for the TESS telescope. It shows that even though TESS has a "blurry" view compared to its predecessors, if you use a careful, custom-made approach (the "boutique method"), you can get high-quality data.

The study confirms that:

TESS can measure star masses and ages in clusters as well as the old Kepler telescope did.
Metal-rich stars lose very little mass as they age.
We can find "secret" stars that have merged or been stripped by partners, helping us understand how binary stars interact.

The researchers are now excited to apply this same careful listening technique to many other star clusters to solve the remaining mysteries of how stars lose weight and age across the galaxy.

1. Problem Statement

Open clusters serve as fundamental laboratories for stellar evolution and Galactic archaeology because their stars share common distances, ages, and metallicities. While asteroseismology (the study of stellar oscillations) has revolutionized the measurement of stellar masses and radii using data from the Kepler mission, the Transiting Exoplanet Survey Satellite (TESS) offers an all-sky view of many more clusters. However, TESS data presents specific challenges for cluster asteroseismology:

Lower Spatial Resolution: TESS has larger pixels ( $21''$ ) compared to Kepler ( $4''$ ), leading to significant photometric contamination from neighboring stars in dense cluster environments.
Systematics: TESS light curves often suffer from scattered Earth/Moon light and background noise not fully removed by standard automated pipelines.
Data Gaps: TESS observations are split into 27-day sectors, creating gaps that complicate frequency analysis compared to Kepler's continuous baselines.
Scientific Gaps: There is a lack of asteroseismic mass loss measurements for metal-rich open clusters to test the "mass loss-metallicity trend," which currently shows conflicting behaviors between metal-poor and metal-rich populations. Additionally, identifying stars with non-standard evolutionary paths (e.g., binary mergers or mass stripping) in these clusters remains challenging.

2. Methodology

The authors employed a "boutique" analysis method to overcome TESS limitations and study red giants in two old, metal-rich clusters: NGC 188 and NGC 6791.

Target Selection:
- Candidates were selected from the Gaia DR3-based membership catalog (Hunt & Reffert 2024).
- Membership was confirmed via spectroscopic radial velocities and metallicities from APOGEE DR17 (or Gaia XP spectra for stars lacking APOGEE data).
- Detection limits were estimated using the tess-atl package, setting a 50% detection probability threshold ( $T_{mag} \approx 12.3$ for NGC 188 and $13.5$ for NGC 6791).
Photometric Analysis (The "Boutique" Method):
- Instead of using pipeline-generated light curves (e.g., QLP or SPOC), the team extracted Full Frame Images (FFIs) using TESSCut.
- Custom Aperture Masks: Manually defined masks were created for each star and sector to minimize contamination from neighbors within a 2-pixel radius.
- Detrending: A custom pipeline (LightKurve) was used to correct for background noise (using PCA to remove Earth/Moon scattered light), remove outliers, fill short gaps (<1.5 hrs), and apply high-pass filters (4–12 days) to remove long-term trends.
- Power Spectra: Lomb-Scargle periodograms were generated from the stitched light curves to identify solar-like oscillations.
Asteroseismic Analysis:
- Frequency of Maximum Power ( $\nu_{max}$ ): Measured using the pyMON pipeline, which is optimized for lower Signal-to-Noise (SNR) data and stars with fewer radial orders.
- Large Frequency Spacing ( $\Delta\nu$ ): Measured only for a "gold" subsample (9 stars in NGC 188) with clear oscillation patterns using the pySYD pipeline.
- Scaling Relations: Seismic masses were derived using $\nu_{max}$ , spectroscopic effective temperatures ( $T_{eff}$ ), and Gaia-derived radii. Corrections ( $f_{\nu_{max}}$ ) were applied to align seismic radii with the Gaia scale.
Stellar Parameters & Ages:
- $T_{eff}$ and $[C/N]$ abundances were taken from APOGEE DR17.
- Radii were calculated from Gaia photometry, distances (Bailer-Jones et al. 2021), and individual dust extinction estimates ( $E(B-V)$ ).
- Ages were derived via two methods: (1) Isochrone fitting to seismic masses, and (2) The chemical clock $[C/N]$ -age relation.

3. Key Contributions

Validation of Boutique Methods: Demonstrated that custom aperture photometry and manual detrending significantly improve SNR in TESS power spectra compared to standard pipeline products, making asteroseismology viable for fainter cluster members.
TESS vs. Kepler Benchmark: Provided a direct comparison of $\nu_{max}$ measurements for NGC 6791, quantifying the systematic offsets introduced by TESS's lower SNR.
Mass Loss Constraints: Delivered a precise measurement of integrated RGB mass loss for a metal-rich cluster, contributing to the debate on the mass loss-metallicity relationship.
Binary Interaction Candidates: Identified specific stars with anomalous masses and chemical signatures, providing evidence for binary interactions (mass transfer and mergers) in old clusters.

4. Key Results

A. NGC 6791 (Validation Study)

$\nu_{max}$ Comparison: While oscillation modes were recovered in both Kepler and TESS data, TESS measurements showed a systematic offset of 2.2% with a scatter of 9% compared to Kepler.
Cause: The discrepancy is attributed to the lower SNR of TESS data for the relatively faint stars in NGC 6791.
Conclusion: The authors did not re-derive seismic parameters for NGC 6791 but used it to validate the limitations of TESS for faint targets.

B. NGC 188 (Primary Analysis)

Seismic Masses:
- RGB: $M_{RGB} = 1.13 \pm 0.04 (\text{rand}) +0.12_{-0.19} (\text{sys}) M_\odot$
- RC: $M_{RC} = 1.11 \pm 0.01 (\text{rand}) +0.11_{-0.19} (\text{sys}) M_\odot$
- These values are consistent with independent estimates from isochrones and eclipsing binaries, with precision comparable to previous Kepler studies.
Integrated Mass Loss:
- The difference between RGB and RC masses yields an integrated mass loss of $\Delta M = 0.02 \pm 0.04 (\text{rand}) \pm 0.01 (\text{sys}) M_\odot$ .
- This low mass loss supports the trend that metal-rich stars experience less mass loss than metal-poor stars, contradicting simple Reimers' law predictions which suggest higher mass loss for metal-rich stars.
Cluster Age:
- Seismic Age: $7.0 \pm 0.9$ Gyrs (using MIST models).
- Chemical Age ( $[C/N]$ ): $6.3 \pm 0.5$ Gyrs.
- Both are in excellent agreement with previous literature ages for NGC 188.
Anomalous Stars (Binary Interactions):
- Under-massive Stars: Two stars (one RGB, one RC) were found with masses significantly below the cluster median.
  - The RGB star ( $0.73 M_\odot$ ) has an enhanced $[C/N]$ ratio, suggesting mass loss before the first dredge-up (likely a sub-subgiant progenitor).
  - The RC star ( $0.88 M_\odot$ ) has normal $[C/N]$ , suggesting mass loss after the first dredge-up (post-common envelope stripping).
- Over-massive/Merger Candidate: One RGB star (TIC 461601491, $1.35 M_\odot$ $1.35 M_{⊙}$ ) exhibits:
  - Anomalous mass (highest in sample).
  - Suppressed dipole-mode visibility (indicating a convective core, likely from a merger).
  - Low $[C/N]$ abundance (suggesting mass gain before the first dredge-up).
  - Spectroscopic binary status.
  - Hypothesis: This star is likely the remnant of a merger between two main-sequence stars in a triple system.

5. Significance

TESS Potential: The study proves that with careful, individualized analysis ("boutique" methods), TESS can deliver asteroseismic data for open clusters with precision rivaling Kepler, despite lower spatial resolution.
Stellar Evolution Physics: The measured low mass loss in NGC 188 ( $\sim 0.02 M_\odot$ ) challenges current mass loss prescriptions (like Reimers' law) for metal-rich stars, highlighting a dichotomy in mass loss trends that requires new theoretical modeling.
Binary Evolution: The identification of under-massive and over-massive giants provides direct observational evidence of binary interaction channels (mass stripping and mergers) in old, metal-rich environments, offering insights into the formation of blue stragglers and sub-subgiants.
Future Outlook: The methodology is scalable to other TESS clusters, promising to resolve the mass loss-metallicity trend across a wider range of metallicities and ages, and to expand the census of binary interaction remnants in the Galaxy.