Rotational clock for stars needs recalibration

January 4, 2016
This is the "South Pillar" region of the star-forming region called the Carina Nebula. Like cracking open a watermelon and finding its seeds, the infrared telescope "busted open" this murky cloud to reveal star embryos tucked inside finger-like pillars of thick dust. Credit: NASA

New work from a team of astronomers led by Carnegie's Jennifer van Saders indicates that one recently developed method for determining a star's age needs to be recalibrated for stars that are older than our Sun. This is due to new information about the way older stars spin, as spin rate is one of the few windows into stellar ages. Their findings, published in Nature, have implications for our own Solar System, as they indicate that our own Sun might be on the cusp of a transition in its magnetic field.

Just like planets, stars rotate around an axis. As stars age, this spin slows down due to the star's magnetic field acting on its stellar wind, which is a flow of gas moving away from the star. The loss of mass, as the flowing gases get caught in the magnetic field and spin outward until they are ejected, affects the Sun's angular momentum and causes the slowdown. In this way, the magnetic field acts like a brake.

About a decade ago, it was discovered that this phenomenon can be used to calculate the age of a Sun-like star if its rotation rate and mass are known. The process is called gyrochronology. However, in their new paper van Saders and her team demonstrate that stars don't spin down exactly as expected when they get older. The correction affects the gyrochronological calculation for older stars.

Prior to gyrochronology, scientists would use properties of a star that change over time like the surface temperature and luminosity to infer its age. However, the changes are so slow and minimal that precise inferences of age are very difficult. Another, newer, method involves studying sound waves in the interiors of stars, which can detect changes in a star's deep core where hydrogen is burned to helium. However, these kinds of observations are very expensive and require very bright targets, so they can't be used for many stars. Measuring rotation, on the other hand, is fairly straightforward—if we can validate and calibrate gyrochronology, it could be a powerful tool for measuring ages in large numbers of stars

"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," van Saders explained. "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 correct."

Data from the Kepler spacecraft now make it possible to test the calibrations of gyrochronology for stars older than our Sun, which is what van Saders and her team set out to do. What they found is that standard models predict more slowdown as stars age than actually occurs; leading the team to conclude that the magnetic field's braking action is weaker in intermediate-aged and old stars—stars older than the Sun.

Their findings imply that something fundamentally changes in the magnetization of stellar winds as stars get older. If so, our own Sun could be close, in an astronomical sense, to a critical transition to a weakened braking power for its magnetic field. This is something that would likely occur on timescales that seem long to humans, but very fast in comparison to the lifetime of the Sun. It's impossible to say how quickly based on current data, but the team is now working on answering this and related questions.

More concretely, it means that using current gyrochronologic calculation techniques will not be as accurate for stars that are more than halfway through their lifetimes as they are for younger stars.

"I think this is a very important result that will greatly improve our ability to understand the stellar aging process," said Carnegie Observatories Director John Mulchaey.

Explore further: Sun-like stars reveal their ages

More information: Jennifer L. van Saders et al. Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars, Nature (2016). DOI: 10.1038/nature16168

Related Stories

Sun-like stars reveal their ages

July 10, 2014

(Phys.org) —Defining what makes a star "Sun-like" is as difficult as defining what makes a planet "Earth-like." A solar twin should have a temperature, mass, and spectral type similar to our Sun. We also would expect it ...

Magnetic fields on solar-type stars

December 12, 2014

The Sun rotates slowly, about once every 24 days at its equator although the hot gas at every latitude rotates at a slightly different rate. Rotation helps to drive the mechanisms that power stellar magnetic fields, and in ...

The ages of sun-like stars

February 6, 2015

The mass of a star is perhaps its most significant feature. It determines how brightly it shines (a star ten times more massive than the Sun will, during its normal lifetime, shine about forty million times brighter than ...

Stars' spins reveal their ages

January 5, 2015

When you're a kid every birthday is cause for celebration, but as you get older they become a little less exciting. You might not want to admit just how old you are. And you might notice yourself slowing down over the years. ...

Mysterious, massive, magnetic stars

September 11, 2015

A Canadian PhD student has discovered a unique object – two massive stars with magnetic fields in a binary system. Matt Shultz of Queen's University, Ontario, Canada found the system – Epsilon Lupi – and will publish ...

Recommended for you

Cassini transmits first images from new orbit

December 7, 2016

NASA's Cassini spacecraft has sent to Earth its first views of Saturn's atmosphere since beginning the latest phase of its mission. The new images show scenes from high above Saturn's northern hemisphere, including the planet's ...

ExoMars orbiter images Phobos

December 7, 2016

The ExoMars Trace Gas Orbiter has imaged the martian moon Phobos as part of a second set of test science measurements made since it arrived at the Red Planet on 19 October.

Earth's days getting longer: study (Update)

December 7, 2016

Earth's days are getting longer but you're not likely to notice any time soon—it would take about 3.3 million years to gain just one minute, according to a study published on Wednesday.

Curiosity rover team examining new drill hiatus

December 6, 2016

NASA's Curiosity Mars rover is studying its surroundings and monitoring the environment, rather than driving or using its arm for science, while the rover team diagnoses an issue with a motor that moves the rover's drill.

Cassini makes first ring-grazing plunge

December 6, 2016

NASA's Saturn-orbiting Cassini spacecraft has made its first close dive past the outer edges of Saturn's rings since beginning its penultimate mission phase on Nov. 30.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.