Magnetic fields implicated in the mysterious midlife crisis of stars


Date:
July 28, 2021
Source:
Royal Astronomical Society
Summary:
Middle-aged stars can experience their own kind of midlife crisis,
experiencing dramatic breaks in their activity and rotation rates
at about the same age as our Sun, according to new research. The
study provides a new theoretical underpinning for the unexplained
breakdown of established techniques for measuring ages of stars
past their middle age, and the transition of solar-like stars to
a magnetically inactive future.



FULL STORY ========================================================================== Middle-aged stars can experience their own kind of midlife crisis,
experiencing dramatic breaks in their activity and rotation rates at
about the same age as our Sun, according to new research published today
in Monthly Notices of the Royal Astronomical Society: Letters. The study provides a new theoretical underpinning for the unexplained breakdown of established techniques for measuring ages of stars past their middle age,
and the transition of solar-like stars to a magnetically inactive future.


========================================================================== Astronomers have long known that stars experience a process known as
'magnetic braking': a steady stream of charged particles, known as the
solar wind, escapes from the star over time, carrying away small amounts
of the star's angular momentum. This slow drain causes stars like our
Sun to gradually slow down their rotation over billions of years.

In turn, the slower rotation leads to altered magnetic fields and less
stellar activity -- the numbers of sunspots, flares, outbursts, and
similar phenomena in the atmospheres of stars, which are intrinsically
linked to the strengths of their magnetic fields.

This decrease in activity and rotation rate over time is expected
to be smooth and predictable because of the gradual loss of
angular momentum. The idea gave birth to the tool known as 'stellar gyrochronology', which has been widely used over the past two decades
to estimate the age of a star from its rotation period.

However recent observations indicate that this intimate relationship
breaks down around middle age. The new work, carried out by Bindesh
Tripathi, Prof.

Dibyendu Nandy, and Prof. Soumitro Banerjee at the Indian Institute of
Science Education and Research (IISER) Kolkata, India, provides a novel explanation for this mysterious ailment.

Using dynamo models of magnetic field generation in stars, the team show
that at about the age of the Sun the magnetic field generation mechanism
of stars suddenly becomes sub-critical or less efficient. This allows
stars to exist in two distinct activity states -- a low activity mode
and an active mode. A middle aged star like the Sun can often switch to
the low activity mode resulting in drastically reduced angular momentum
losses by magnetized stellar winds.

Prof. Nandy comments: "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 new work provides key insights into the existence of low activity
episodes in the recent history of the Sun known as grand minima -- when
hardly any sunspots are seen. The best known of these is perhaps the
Maunder Minimum around 1645 to 1715, when very few sunspots were observed.

The team hope that it will also shed light on recent observations
indicating that the Sun is comparatively inactive, with crucial
implications for the potential long-term future of our own stellar
neighbour.

========================================================================== Story Source: Materials provided by Royal_Astronomical_Society. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Bindesh Tripathi, Dibyendu Nandy, Soumitro Banerjee. Stellar
mid-life
crisis: subcritical magnetic dynamos of solar-like stars
and the breakdown of gyrochronology. Monthly Notices of the
Royal Astronomical Society: Letters, 2021; 506 (1): L50 DOI:
10.1093/mnrasl/slab035 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210728105700.htm

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