Stellar Age Recalibration May Imply Weakening Solar Magnetic Field

Summary: Stellar age recalibration for older stars such as Earth's sun may imply weakening solar magnetic field braking power, according to Jan. 4's Nature Letter.

artist's concept of gyrochronology, which determines stellar age as calculable via mass and spin rate: Michael Bachofner, no usage restrictions, via EurekAlert!

Gyrochronology, a method proposed in 2003 for calculating stellar age via a star’s rotation period, needs to be recalibrated for dating older stars such as Earth’s sun, according to findings published online Jan. 4, 2016, in Nature by an international team of eight researchers.

“The ability to determine a star’s age is important for improving our understanding of the life cycles of astronomical systems -- for cataloging how the star and the objects near it have changed through history and for predicting how they might change in the future,” explains Jennifer van Saders, lead author and Princeton University postdoctoral fellow at Carnegie Observatories in Pasadena, California, in Carnegie Science’s press release on Jan. 3, 2016. “Gyrochronology has the potential to be a very precise method for determining the ages of the average Sun-like star, provided we can get the calibrations right.”

In their Nature letter, the eight co-authors share their finding that slowdowns in spin predicted by standard gyrochronology models for stars that are older than Earth’s sun exceed the actual rotation rate. The research team attribute “the presence of unexpectedly rapid rotation in stars that are more evolved than the Sun” to the dramatic weakening in braking power by stellar magnetic fields.

Tracking the progress of dark spots, known as sunspots on Earth’s aun and as starspots on other stars, across a star’s surface yields the stellar spin rate. The surface dims, typically by less than 1 percent, at the spot’s site and brightens with the spot’s rotation out of view. Instead of using the traditional method of visually monitoring the sometimes painstakingly slow progress, over days or even over weeks, of rotated dimness, geochronology uses data from NASA’s Kepler spacecraft. Launched on March 7, 2009, the spacecraft carries a photometer for continuous monitoring of stellar brightness.

As with planets, stars also spin around an axis, an imaginary line passing through its north and south poles. Geochronology recognizes the spin rate as a window into stellar age because interactions between the star’s magnetic field and its stellar wind cause the star’s rotation to slow with stellar age. As an almost perfectly round sphere of gas and plasma, the sun displays differential rotation. Spin rate decreases with increasing latitude. Equatorial regions rotate in 25 to 27 Earth days while polar regions complete the spin in 36 Earth days.

A star’s magnetic field captures outflows of gas, known as stellar wind, and carries the highly ionized, or charged, materials along field lines, farther and farther away from the stellar surface until the wind escapes into space. The escape of stellar wind into space reduces the star’s mass, which in turn observably slows down the star’s spin rate. The interaction between the magnetic field and stellar wind that leads to loss of surface mass is known as magnetic braking.

The team’s letter in Nature suggests that ageing ushers in a fundamental change in the magnetization of stellar winds that critically weakens the magnetic field’s braking power. The sun has not yet transitioned to weakened magnetic braking.

“Our findings might suggest a fundamental change in the nature of ageing stellar dynamos, with the Sun being close to the critical transition to much weaker magnetized winds. This weakened braking limits the diagnostic power of gyrochronology for those stars that are more than halfway through their main-sequence lifetimes,” note the international team representing research institutions in Denmark (Aarhus University), France (Paris Diderot University), and the United States (Carnegie Observatories, Ohio State University, Princeton University, Space Science Institute, University of California-Santa Barbara).

Sun image, taken Jan. 12, 2007, by Hinode's Solar Optical Telescope reveals filamentary nature of plasma connecting regions of different magnetic polarity in chromosphere, thin layer of solar atmosphere between visible surface, photosphere, and corona. The Hinode spacecraft is a Japan Aerospace Exploration Agency solar mission, in collaboration with United States and United Kingdom: Hinode JAXA/NASA, Public Domain, via NASA

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

artist's concept of gyrochronology: Michael Bachofner, no usage restrictions, via EurekAlert! @ http://www.eurekalert.org/multimedia/pub/84615.php

filamentary plasma connecting sun's differently polarized magnetic regions: Hinode JAXA/NASA, Public Domain, via NASA @ www.nasa.gov/mission_pages/hinode/solar_017.html

For further information:

Botkin-Kowacki, Eva. "Is that star older than we thought? Time to recalibrate, scientists say." The Christian Science Monitor > Science > First Look. Jan. 5, 2016.
Available @ http://www.csmonitor.com/Science/2016/0105/Is-that-star-older-than-we-thought-Time-to-recalibrate-scientists-say

Hays, Brooks. "New research suggests sun's magnetic field may soon change." UPI > Science News. Jan. 4, 2016.
Available @ http://www.upi.com/Science_News/2016/01/04/New-research-suggests-suns-magnetic-field-may-soon-change/7661451938417/

Laguipo, Angela. "Magnetic Field Of The Sun May Soon Change,  Study Of Red Giants Reveal." Tech Times > Science. Jan. 7, 2016.
Available @ http://www.techtimes.com/articles/121599/20160107/magnetic-field-of-the-sun-may-soon-change-study-of-red-giants-reveal.htm

"Rotational Clock For Stars Need Recalibration." Carnegie Science > Press release > Astronomy. Jan. 3, 2016.
Available @ https://carnegiescience.edu/node/1969

"Rotational clock for stars needs recalibration." EurekAlert! > Public releases. Jan. 4, 2016.
Available @ http://www.eurekalert.org/pub_releases/2016-01/ci-rcf123115.php

"Rotation of the Sun." Windows to the Universe > Sun > Solar interior > Sun layers.
Available @ http://www.windows2universe.org/sun/Solar_interior/Sun_layers/differential_rotation.html

"Stars' spins reveal their ages." EurekAlert! > Public releases. Jan. 5, 2015.
Available @ http://www.eurekalert.org/pub_releases/2015-01/hcfa-ssr010215.php

van Saders, Jennifer L., et al. "Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars." Nature, vol. 529 (Jan. 14, 2016): 181-184. Published online Jan. 4, 2016. DOI: 10.1038/nature16168
Available @ http://www.nature.com/nature/journal/vaop/ncurrent/full/nature16168.html