🔭 astrophysics

A Multi-Method Age Determination for the Ursa Major Moving Group

By analyzing the largest sample of candidate Ursa Major Moving Group members to date using Gaia DR3 data and three independent age-determination methods, this study resolves previous controversies to establish a consistent age of approximately 418 Myr for the group's dominant population, providing a crucial benchmark for understanding stellar and planetary evolution.

Original authors: Julia Sheffler, Max Clark, Melinda Soares-Furtado, Adam Distler, Ritvik Sai Narayan, Jenna Karcheski, Kenneth Nordsieck

Published 2026-01-15

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Julia Sheffler, Max Clark, Melinda Soares-Furtado, Adam Distler, Ritvik Sai Narayan, Jenna Karcheski, Kenneth Nordsieck

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 Ursa Major Moving Group (UMa) as a massive, drifting school of fish in the ocean of space. For over a century, astronomers have been trying to figure out exactly how old this school is. Some said they were teenagers (200 million years old), while others insisted they were middle-aged adults (up to 1 billion years old). This confusion made it hard to use them as a "benchmark" for studying how stars and their planets grow up.

In this paper, the authors act like a team of forensic detectives who finally solved the mystery of the school's age by gathering the biggest group of suspects they ever had and using three different "clocks" to tell the time.

Here is how they did it, broken down simply:

1. Gathering the Suspects (The Kinematic Search)

First, the team needed to find the fish. They used data from the Gaia satellite, which acts like a giant, ultra-precise 3D map of the stars near our Sun. They looked for stars that were moving in the exact same direction and speed as the core of the Ursa Major group.

The Catch: Just because stars are swimming in the same direction doesn't mean they were born at the same time. It's like seeing a group of people walking down a street together; some might be neighbors, but others might just be passing through.
The Result: They found 1,172 stars that fit the movement criteria. This is the largest list of potential members ever assembled. However, they knew this list was "contaminated" with strangers (field stars) who just happened to be moving the same way.

2. The Three Clocks (Determining the Age)

To figure out the true age of the group, they didn't rely on just one method. Instead, they used three independent "clocks" to see if they all agreed. If three different watches all show the same time, you can be sure of the time.

Clock #1: The Lithium Test (The "Battery" Clock)

Stars are born with a chemical element called lithium in their atmospheres. As stars get older, they burn through this lithium like a battery draining.

The Analogy: Think of lithium as a fuel tank. Young stars have full tanks; old stars have empty ones.
The Finding: By measuring how much lithium was left in 23 of the stars, they calculated an age of roughly 394 million years.

Clock #2: The Spin Test (The "Spinning Top" Clock)

Stars spin when they are young, but as they age, they slow down, much like a spinning top losing momentum.

The Analogy: Imagine a group of children running a race. The fastest runners are the youngest; the slow ones are the older ones.
The Finding: Using data from the TESS satellite, they measured how fast 76 of the stars were spinning. This method suggested an age of about 428 million years.

Clock #3: The "Fidgety" Test (The Variability Clock)

Young stars are often "fidgety." They have big sunspots and magnetic storms that make their brightness flicker. Older stars are calmer and steadier.

The Analogy: A toddler is constantly moving and changing expression (fidgety), while a grandparent is usually calm and steady.
The Finding: By looking at how much the stars' light flickered in the Gaia data, they estimated an age of about 449 million years.

3. The Verdict: The "Sweet Spot"

When the team combined the results from all three clocks, they converged on a very specific answer: The dominant group of stars is approximately 418 million years old.

This is a "Goldilocks" age. It's not too young (like a newborn) and not too old (like a grandparent). It's a time when stars are settling down, and their planetary systems are becoming stable.

4. The "Strangers" in the Group

The paper admits that their list of 1,172 stars wasn't perfect. About 40% of them were likely "strangers"—stars that just happened to be moving in the same direction but were actually much older or younger.

The Analogy: Imagine a high school reunion. You might see a few people from the neighborhood who aren't alumni but are wearing the same colors. The authors used a computer algorithm to separate the real alumni (the 418-million-year-old group) from the neighborhood visitors.

5. Why This Matters: The "Time Machine" Planet

The paper highlights a specific star in this group called HD 63433. It has three planets orbiting it, including one that is Earth-sized.

Because the authors now know the star is exactly 418 million years old, we know exactly how old its planets are.
This allows scientists to study what Earth-like worlds looked like when our own Solar System was in its "Hadean" phase (a time when the Earth was a molten, volcanic rock). It's like having a time machine to see what our own planet looked like billions of years ago.

6. Bonus: The "Ghost" Stars (White Dwarfs)

The team also found 225 "ghost" stars called white dwarfs (the dead, cooling cores of stars). Because these stars are so dense, they can't be timed using the standard movement rules. Instead, the team used their cooling speed (how long they've been "dead") to confirm they also belong to the 418-million-year-old group. This helps scientists understand how massive stars live and die.

Summary

The paper concludes that the Ursa Major Moving Group is a 418-million-year-old family of stars. While there are some "strangers" mixed in, the core family is real, consistent, and provides a perfect laboratory for studying how young planetary systems evolve. This resolves decades of confusion and gives astronomers a solid foundation for studying the early history of worlds like our own.

Technical Summary: A Multi-Method Age Determination for the Ursa Major Moving Group

Problem Statement
The Ursa Major Moving Group (UMa), also known as Collinder 285, is one of the closest stellar associations to the Sun, containing systems like the multi-planet host HD 63433. Despite its proximity and importance as a laboratory for studying early planetary system evolution, the group's age has remained a subject of significant controversy. Published estimates have historically spanned nearly an order of magnitude, ranging from approximately 200 Myr to 1 Gyr. This ambiguity stems from the group's dispersed nature, potential contamination by field stars sharing similar space motions but different formation epochs, and methodological limitations in previous studies. Resolving this age discrepancy is critical for accurately interpreting the evolutionary state of its member stars and their planetary systems.

Methodology
The authors present a comprehensive age analysis utilizing the largest sample of candidate UMa members assembled to date. The study employs a multi-step approach:

Kinematic Selection: Using Gaia Data Release 3 (DR3) astrometry and radial velocity (RV) measurements, the team identified 1,172 stars within 100 pc of the Sun. They utilized a modified version of the FriendFinder package to select stars with 3D kinematic motions consistent with the UMa nucleus (using HD 115043 as a reference), applying strict cuts on tangential velocity offsets ( $\pm 5$ km s $^{-1}$ ) and RV differences ( $\le 4$ km s $^{-1}$ ).
Multi-Method Age Dating: To isolate the dominant coeval population from kinematic interlopers, the authors applied three independent age diagnostics to the kinematic sample:
- Lithium Equivalent Widths (EW $_{Li}$ ): Archival EW $_{Li}$ measurements for 23 main-sequence candidates were analyzed using the EAGLES (Estimating Ages from Lithium Equivalent Widths) software, which models the relationship between lithium depletion, effective temperature, and age.
- Gyrochronology: Rotation periods were derived from TESS photometry for 147 candidates. After applying cuts for reddening, temperature (3800–6200 K), and binarity (RUWE < 1.25), the remaining 76 stars were analyzed using gyro-interp. To address contamination, a spectral clustering algorithm was applied to the posterior age distributions to isolate the dominant population.
- Photometric Variability (EVA): The Excess Variability Age (EVA) method was applied to 539 candidates using Gaia G, BP, and RP band scatter. This method leverages the correlation between photometric variability and age, calibrated against clusters of known ages.
White Dwarf Analysis: A separate sample of 225 white dwarf candidates was identified via proper motion. Due to gravitational redshift uncertainties in RVs, membership was assessed using wdwarfdate to derive cooling ages, filtering for those consistent with the main sequence population's age.

Key Results
The three independent methods converged on a consistent age for the dominant population within the kinematic sample:

Lithium Analysis: $393.6^{+85.1}_{-80.9}$ Myr (based on 14 reliable sources).
Gyrochronology: $428 \pm 93$ Myr (based on 45 stars identified as the dominant cluster).
Photometric Variability (EVA): $449^{+114}_{-79}$ Myr.

Combining these results, the authors determine a joint age for the dominant UMa population of $418^{+32}_{-34}$ Myr.

The analysis also revealed significant contamination within the kinematic sample. The spectral clustering in the gyrochronology analysis indicated that approximately 41% of the kinematic candidates are field interlopers or members of distinct populations, a finding consistent with previous chemical homogeneity studies (e.g., G. Dopcke et al. 2019). The study identified eight white dwarf candidates whose total ages (cooling age + progenitor lifetime) are consistent with the $418$ Myr estimate, providing empirical constraints on intermediate-mass stellar evolution.

Significance and Claims
The paper claims to resolve the historical age controversy for UMa by demonstrating that while the kinematic selection includes field stars, a distinct, dominant coeval population formed approximately 400 Myr ago. This age determination is in excellent agreement with the precise interferometric estimate of $414 \pm 23$ Myr provided by J. Jones et al. (2015).

The authors emphasize that this refined age is essential for:

Stellar Evolution: Providing a benchmark for studying stellar rotation, magnetic activity evolution, and lithium depletion at this specific epoch.
Planetary System Evolution: Offering a critical context for interpreting young exoplanets, such as those in the HD 63433 system. A precise age allows for more realistic modeling of atmospheric escape rates and the transition of planetary systems from active, gas-rich phases to stable architectures.
Methodological Rigor: The study underscores that kinematic membership alone is insufficient for characterizing young moving groups due to high contamination rates. The authors advocate for multi-dimensional validation (kinematics + lithium + rotation + variability) to establish reliable membership lists for future demographic and planetary studies.

The work establishes a foundation for future spectroscopic follow-up to refine membership, map the group's structure and chemistry, and search for additional young exoplanets within the group.