Gyrochronology

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Gyrochronology (from Greek γύρος , jíros or [ ˈʝirɔs ], gyroscope, round and χρόνος, chrónos, "time" and λόγος, lógos, "teaching") is an empirical method of astrophysics for determining the age of stars of medium and low mass using their period of rotation .

The age of a star cannot be directly measured in contrast to other variables that are essential for its evolution, such as its mass and chemical composition. The methods of isochronous fitting, asteroseismology and the cooling sequence of white dwarfs are dependent on assumptions about the physics of the stars, such as the mixing length in convective energy transport. The Lithium Depletion Boundary Technique , magnetic activity and gyrochronology are independent of assumptions about stellar physics . These empirical methods can be used for stars with a mass of less than 1.3 solar masses , since they require convective energy transport in the photosphere .

Physical basics

Stars with a convective photosphere create a global magnetic field. A stellar wind accelerates ionized matter away from the star, and the electrical charge carriers of the stellar wind are frozen in the magnetic field. They follow the magnetic field lines, which also rotate around the star due to the rotation of the star. As a result, the star has to drag the matter with it from the stellar wind and this in turn slows the rotation of the star. Correspondingly, a mean rotation period of the stars that decreases with age can be observed in star clusters .

Gyrochronology

The rotation time for stars of low mass can be measured by photometry or spectroscopy , since star spots or flares modulate the brightness or the spectrum in the course of the rotation. However, the duration of a star's rotation is not a strict function of the star's mass and age. It is therefore necessary to measure the rotation period of at least 25 stars. From the distribution of the periods, the age can be determined with an accuracy of around 30 percent. The measurable age range is around 20 million to 10 billion years, with the accuracy improving at an age of 100 million years. In younger stars, accretion from a circumstellar disc can accelerate the rotation of the stars. A major flaw in gyrochronology is undiscovered binary stars , which due to bound rotation rotate considerably faster than single stars of the same age.

literature

  • RD Jeffries: Using rotation, magnetic activity and lithium to estimate the ages of low mass stars . In: Astrophysics. Solar and Stellar Astrophysics . 2014, arxiv : 1404.7156v1 (English).
  • PA Cargile, DJ James, J. Pepper, RB Kuhn, R. Siverd, KG Stassun: Evaluating Gyrochronology on the Zero-Age-Main-Sequence: Rotation Periods in the Southern Open Cluster Blanco 1 from the KELT-South Survey . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1312.3946v1 (English).
  • Saurav Dhital et al .: A Gyrochronology and Microvariability Survey of the Milky Way's Older Stars Using Kepler's Two-Wheels Program . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1309.1172v2 (English).
  • P. Delorme, A. Collier Cameron, L. Hebb, J. Rostron, TA Lister, AJ Norton, D. Pollacco, RG West: Stellar rotation in the Hyades and Praesepe: gyrochronology and braking timescale . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1101.1222v1 (English).