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TESS observations of the Pleiades cluster: a nursery for delta Scuti stars

This study utilizes TESS photometry and Gaia data to analyze 89 Pleiades members, confirming that over 80% of A- and F-type stars in the instability strip exhibit delta Scuti pulsations while revealing that their varied pulsation spectra challenge the applicability of a universal nu_max relation for young stars.

Original authors: Timothy R. Bedding, Simon J. Murphy, Courtney Crawford, Daniel R. Hey, Daniel Huber, Hans Kjeldsen, Yaguang Li, Andrew W. Mann, Guillermo Torres, Timothy R. White, George Zhou

Published 2026-02-18

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Original authors: Timothy R. Bedding, Simon J. Murphy, Courtney Crawford, Daniel R. Hey, Daniel Huber, Hans Kjeldsen, Yaguang Li, Andrew W. Mann, Guillermo Torres, Timothy R. White, George Zhou

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 Pleiades (also known as the Seven Sisters) not just as a pretty cluster of stars, but as a bustling cosmic nursery. In this nursery, the stars are young, energetic, and mostly the "teenagers" of the stellar world—hot, blue, and spinning fast.

This paper is like a detailed report card written by a team of astronomers using two powerful space telescopes: Gaia (which acts like a high-precision ruler and mapmaker) and TESS (which acts like a super-sensitive microphone listening to the stars' "voices").

Here is the story of what they found, broken down into simple concepts:

1. The "Singing" Stars (Delta Scuti Variables)

Some stars in this nursery have a special talent: they pulsate. They rhythmically expand and contract, like a giant heart beating or a drum being hit. Astronomers call these $\delta$ Scuti stars.

The Big Question: Why do only some stars sing, while others of the exact same age and size stay silent?
The Old Mystery: In the past, astronomers found that only about half the stars in the "singing zone" (called the instability strip) were actually pulsing. It was a bit like walking into a choir room where only half the people are singing, even though everyone has the same sheet music.

2. The New Discovery: A Choir of 36

The researchers looked at 89 stars in the Pleiades. Using TESS, they listened closely and found that 36 of them are pulsating.

The Surprise: In the middle of the "singing zone," over 80% of the stars are actually singing! This is much higher than the usual 50% seen in older star clusters.
The Metaphor: Imagine a classroom where you expect only half the students to raise their hands to answer a question. Instead, almost everyone raises their hand. This suggests that in this young "nursery," the conditions are perfect for stars to start pulsing.

3. The "Spinning Top" Effect

The team also measured how fast these stars were spinning.

The Analogy: Think of a spinning top. As it spins faster, it flattens out at the poles and bulges at the equator.
The Finding: They confirmed that the faster a star spins, the more it bulges. This spinning makes the stars look slightly different in color and brightness, which "smears out" the main line of stars in their charts. It's like a spinning dancer blurring into a circle; the spin is a major reason why the stars don't all line up perfectly neatly.

4. The "Noisy" Patterns

While they found many singers, the songs were all over the place.

The Expectation: Scientists hoped to find a simple rule, like "Hotter stars sing at a higher pitch, cooler stars at a lower pitch." They wanted a formula to predict the song based on the star's temperature.
The Reality: The songs were chaotic. Some hot stars sang low notes; some cool stars sang high notes. There was no clear pattern.
The Metaphor: It's like walking into a jazz club where everyone is playing the same instrument, but there is no sheet music. The musicians are improvising wildly. This suggests that for young stars, the "rules" of how they choose which notes to sing are still a mystery.

5. The "Foggy Window" (Interstellar Reddening)

The team also looked at stars in other parts of the galaxy (observed by the Kepler telescope) and compared them to the Pleiades.

The Discovery: They realized that dust in space acts like fog or a dirty window. As stars get farther away, this dust makes them look redder than they really are.
The Lesson: If you don't clean the "window" (correct for the dust), you might think a star is a different color (and therefore a different type of star) than it actually is. The Pleiades helped them prove that distance and dust can trick our measurements.

6. The "Runaway" Stars

The team also looked at five stars that had "escaped" the nursery (they were drifting away from the cluster).

The Result: Even though they were leaving, these runaway stars were still singing the same songs as the stars that stayed behind. This confirmed that they really were born in the Pleiades, just like a child who moves away from home but still has the same family traits.

The Bottom Line

This paper is a success story of combining two tools: Gaia (to know exactly where the stars are and how they move) and TESS (to listen to their beats).

In short:

The Pleiades is a young star nursery where most stars in the right zone are pulsating.
Spinning makes the stars look messy and spread out.
The songs (pulsation patterns) are chaotic and don't follow a simple temperature rule yet.
Dust in space can trick us into seeing stars as the wrong color, but we now know how to fix that.

It's a reminder that even in a place as well-studied as the Pleiades, the universe still has plenty of surprises, and the "rules" of how stars sing are still being written.

1. Problem Statement

The excitation and mode selection mechanisms in $\delta$ Scuti stars (A- and F-type pulsators) remain a major unsolved challenge in stellar astrophysics. Key questions include:

Why do only a subset of stars within the theoretical instability strip exhibit detectable $\delta$ Scuti pulsations?
How can stars with nearly identical positions in the Hertzsprung-Russell (H-R) diagram differ in their pulsational behavior?
Is there a universal scaling relation for the frequency of maximum power ( $\nu_{max}$ ) in $\delta$ Scuti stars, similar to that used for solar-like oscillators?
Previous studies (e.g., using Kepler) suggested a pulsator fraction of only ~50–60% in the center of the instability strip, leaving the cause of non-pulsation in the remaining stars unclear.

2. Methodology

The authors conducted a comprehensive survey of the Pleiades open cluster (Messier 45), leveraging data from Gaia and TESS to create the most detailed sample of $\delta$ Scuti stars in a young cluster to date.

Sample Selection:
- Identified 89 likely A- and F-type members (spectral types A0 V to F8 V) using Gaia DR2/DR3 astrometry and the BANYAN- $\Sigma$ code (membership probability >90%).
- Included 5 "escaped" members identified by Heyl et al. (2022) that lie outside the core selection criteria.
- The sample includes 12 known spectroscopic binaries and 10 previously named variables.
Rotational Velocity Measurements:
- Measured projected rotational velocities ( $v \sin i$ ) for 49 stars using high-resolution spectroscopy (Tillinghast Reflector Échelle Spectrograph) and Gaia RVS data.
- Used Least-Squares Deconvolution (LSD) to extract line profiles and model broadening.
Photometric Analysis:
- Utilized TESS time-series photometry (120s cadence) from Sectors 42–44 (and 18/19 for some stars).
- One star (HD 23028) was analyzed using Kepler/K2 long-cadence data.
- Performed amplitude spectrum analysis to detect pulsations, setting a detection threshold of peaks >10 $\times$ the mean noise level.
Stellar Parameters:
- Constructed a Color-Magnitude Diagram (CMD) and H-R diagram using Gaia DR3 parallaxes and photometry.
- Corrected for interstellar extinction/reddening ( $A_G = 0.11$ , $E(BP-RP) = 0.055$) to compare with theoretical PARSEC isochrones (110 Myr, $Z=0.017$ ).

3. Key Contributions

High-Resolution Pulsation Census: The study detected $\delta$ Scuti pulsations in 36 out of 89 stars, including 30 new detections. This represents the most extensive search for $\delta$ Scuti stars in the Pleiades to date.
Rotational Broadening Analysis: The paper explicitly links the observed broadening of the main sequence in the CMD to stellar rotation, quantifying this effect via $v \sin i$ measurements.
Instability Strip Mapping: The authors mapped the observed instability strip against theoretical models and Kepler data, analyzing the effects of interstellar reddening on the strip's position.
Testing the $\nu_{max}$ Relation: The study critically evaluates the applicability of the $\nu_{max}$ scaling relation (common in solar-like oscillators) to young $\delta$ Scuti stars.

4. Key Results

Pulsator Fraction: The fraction of pulsating stars in the middle of the instability strip ($0.20 < BP-RP < 0.40$) is 84% ( $21/25$ ), and 72% ( $36/50$ ) overall in the relevant color range. This is significantly higher than the ~50–60% observed in the older Kepler field, suggesting that young, chemically homogeneous clusters are "nurseries" where pulsation is the norm rather than the exception.
Rotation and CMD Broadening: The CMD shows a clear spread in magnitude at a given color. The authors confirm that rapid rotators (high $v \sin i$ ) are responsible for this broadening, shifting stars to the red and brighter in the CMD, distinct from the effects of binarity.
Instability Strip Position:
- The observed strip for the Pleiades aligns well with theoretical models (Dupret et al. 2005) when corrected for reddening.
- Comparison with Kepler data reveals that the instability strip shifts to the red (redder colors) with increasing distance due to interstellar reddening. The shift is approximately 0.15 mag in $BP-RP$ per kiloparsec.
Pulsation Spectra Diversity:
- Despite the high pulsator fraction, the oscillation spectra are highly diverse and do not show a strong correlation with effective temperature ( $T_{eff}$ ).
- There is no clear trend of shifting to higher radial orders with increasing temperature, contradicting some theoretical predictions.
The $\nu_{max}$ Question: The study finds no evidence for a useful $\nu_{max}$ scaling relation for young $\delta$ Scuti stars. The lack of a defined power envelope (unlike solar-like oscillators) and the weak correlation between the strongest mode frequency ( $f_1$ ) and stellar parameters suggest that $\nu_{max}$ is not a robust diagnostic for this class of young stars.
Chemical Peculiarity: While Am stars (chemically peculiar, slow rotators) are often non-pulsators, the authors found that chemical peculiarity can only account for a small fraction of the non-pulsating stars in the sample.

5. Significance

Resolution of the "Missing Pulsators" Problem: The high pulsation fraction in the Pleiades suggests that the low detection rate in older field stars (like those in the Kepler field) may be due to evolutionary effects, chemical inhomogeneity, or detection limits in older populations, rather than a fundamental lack of excitation in young stars.
Challenge to Mode Selection Theories: The lack of correlation between pulsation characteristics and stellar temperature in young stars challenges current theoretical models of mode selection and excitation, indicating that rotation or other unmodeled factors play a complex role.
Methodological Benchmark: The paper demonstrates the power of combining Gaia (for precise astrometry and membership) and TESS (for high-precision, high-cadence photometry) to study open clusters. It provides a critical baseline for understanding how pulsation properties evolve from the Zero Age Main Sequence (ZAMS) to older field populations.
Re-evaluation of Scaling Relations: The findings urge caution in applying solar-like oscillation scaling relations (like $\nu_{max}$ ) to $\delta$ Scuti stars, particularly young ones, highlighting the need for distinct theoretical frameworks for different pulsation regimes.