Friday, December 1, 2023

Inferring Stellar Age through Spin-Down

Stellar ages are a fundamental aspect of our understanding of the universe, from planetary evolution to galactic dynamics. However, accurately determining the age of a star can be challenging, as direct measurements are not possible. Gyrochronology is one method used to infer the age of a star based on its spin-down, which is a function of its rotation period and mass.

A recent study by Luke G. Bouma, Elsa K. Palumbo, and Lynne A. Hillenbrand presents an interpolation-based gyrochronology framework that reproduces the time and mass-dependent spin-down rates implied by the latest open cluster data while matching the rate at which the dispersion in initial stellar rotation periods decreases as stars age. The researchers validated their technique for stars with temperatures of 3800-6200 K and ages of 0.08-2.6 gigayears (Gyr) and used it to reexamine the empirical limits of gyrochronology.

Their findings suggest that the uncertainty floor in gyrochronology varies strongly with both stellar mass and age. For Sun-like stars (5800 K), the statistical age uncertainties improve monotonically from ±38 at 0.2 Gyr to ±12 at 2 Gyr and are caused by the empirical scatter of the cluster rotation sequences combined with the rate of stellar spin-down. For low-mass K-dwarfs (4200 K), the posteriors are highly asymmetric due to stalled spin-down, and ±1σ age uncertainties vary non-monotonically between 10% and 50% over the first few gigayears. High-mass K-dwarfs (5000 K) older than 1.5 Gyr yield the most precise ages, with limiting uncertainties currently set by possible changes in the spin-down rate (12% systematic), the calibration of the absolute age scale (8% systematic), and the width of the slow sequence (4% statistical).

The study suggest that their technique offers an improved approach to gyrochronology, taking into account a range of factors that affect spin-down rates beyond ordinary magnetized braking. They also note that their findings have implications for understanding the evolution of stars and their planetary systems.

However, the study is not without its limitations. The authors note that their model assumes a fixed scatter of rotation periods around the slow sequence, which may not hold true in all cases. Additionally, their technique may not be applicable to stars outside the temperature range studied, and may require further refinement for use in other contexts.

Despite these limitations, the study offers valuable insights into the challenges of determining the age of stars using gyrochronology. By accounting for a range of factors that can affect spin-down rates, the researchers have developed a more accurate approach that can be used to improve our understanding of the evolution of stars and planetary systems. Additionally, the study provides a useful framework for future research in this area, with its open-source implementation, gyro-interp, available online for further analysis and refinement.










Source: Bouma, L.G., Palumbo, E.K., & Hillenbrand, L.A. (2023). The Empirical Limits
of Gyrochronology.

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