Wednesday, May 25, 2022

Middle-aged stars can experience their own kind of midlife crisis, study

Magnetic fields implicated in the mysterious midlife crisis of stars.

Astronomers have known that stars experience magnetic breaking. Magnetic breaking is a process in which solar wind escapes from the star over time. The particles within solar wind carry small amounts of the star’s angular momentum. This slow drain causes star to slow down their rotation over billions of years.

The slower rotation hence results in altered magnetic fields and less stellar activity. After some time, this decreased activity and slower rotation, expected to become smooth because of the gradual loss of angular momentum.

This thought brought forth the tool known as ‘stellar gyrochronology,’ which has been generally utilized to estimate the age of a star from its rotation period.

The recent observations demonstrate that this intimate relationship separates around middle age. Scientists documented that middle-aged stars can experience a kind of midlife crisis. As a result, there cause dramatic breaks in their activity and rotation rates at about the same age as our Sun.

The review gives a new hypothetical supporting the unexplained breakdown of established methods for estimating ages of stars past their middle age and the transition of solar-like stars to a magnetically inactive future.

 the spinning interior of a star
Artist’s impression of the spinning interior of a star, generating the stellar magnetic field. This image combines a dynamo simulation of the Sun’s interior with observations of the Sun’s outer atmosphere, where storms and plasma winds are generated. CESSI / IISER Kolkata / NASA-SVS / ESA / SOHO-LASCO

For the study, scientists used dynamo models of magnetic field generation. The models show that stars’ magnetic field generation mechanism suddenly becomes sub-critical or less efficient at the age of the Sun. This allows stars to exist in two different states: low activity and active modes.

Prof. Dibyendu Nandy at the Indian Institute of Science Education and Research (IISER) Kolkata, India, said“This hypothesis of sub-critical magnetic dynamos of solar-like stars provides a self-consistent, unifying physical basis for a diversity of solar-stellar phenomena, such as why stars beyond their midlife do not spin down as fast as in their youth, the breakdown of stellar gyrochronology relations, and recent findings suggesting that the Sun may be transitioning to a magnetically inactive future.”

The study offers significant clues on the existence of low activity episodes in the recent history of the Sun known as grand minima. It could also highlight recent observations indicating that the Sun is comparatively inactive.

Journal Reference:

  1. B. Tripathi, D. Nandy, S. Banerjee, Monthly Notices of the Royal Astronomical Society: Letters (2021). Stellar Mid-life Crisis: Subcritical Magnetic Dynamos of Solar-like Stars and the Break-down of Gyrochronology. DOI: 10.1093/minerals/slab035

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