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Slow spin of older stars key in the search for life

This is our sun.  It spins on its axis once in about 25 days.  Image via NASA
This is our sun. It spins on its axis once in about 25 days. According to this new research, two billion years ago, our sun would have spun faster, in about 18 days. Image via NASA

If you wanted to search for alien civilizations outside our solar system, it’d be helpful to look at stars at least as old as our sun. That’s because life as we know on Earth has taken a long time to reach the level of complexity we find today. That’s why astronomers want to have as accurate a stellar clock as they can. They want to be able to identify stars with planets that are as old as our sun or older. Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) say they’ve now taken a significant step forward in building that clock. The CfA researchers are presenting their results today (January 5, 2015) at the 225th meeting of the American Astronomical Society in Seattle, Washington.

Soren Meibom of CfA said:

Our goal is to construct a clock that can measure accurate and precise ages of stars from their spins.

A star’s spin rate depends on its age because, like a top spinning on a table top, stars slow down steadily with time. A star’s spin also depends on its mass; astronomers have found that larger, heavier stars tend to spin faster than smaller, lighter ones. The new work by the CfA astronomers shows that there’s a close mathematical relationship between a star’s mass, spin, and age so that by measuring the first two, scientists can calculate the third.

Sydney Barnes of the Leibniz Institute for Astrophysics in Germany, who is a co-author on the study, said:

We have found that the relationship between mass, rotation rate, and age is now defined well enough by observations that we can obtain the ages of individual stars to within 10 percent

Barnes first proposed this method in 2003, building on prior work, and called it gyrochronology from the Greek words gyros (rotation), chronos (time/age), and logos (study).

To measure a star’s spin, astronomers look for changes in its brightness caused by dark spots on its surface — the stellar equivalent of sunspots. Even through telescopes, distant stars appear as pinpoints of light, which means that astronomers can’t directly see a sunspot cross a star’s disk. Instead, they watch for the star to dim slightly when a sunspot appears, and brighten again when the sunspot rotates out of view.

These changes are very difficult to measure because a typical star dims by much less than 1 percent, and it can take days for a sunspot to cross the star’s face. The team achieved the feat using data from NASA’s Kepler spacecraft, which provided precise and continuous measurements of stellar brightnesses.

For gyrochronology ages to be accurate and precise, astronomers must calibrate their new clock by measuring the spin periods of stars with both known ages and masses. Meibom and his colleagues previously studied a cluster of billion-year-old stars. This new study examines stars in the 2.5-billion-year-old cluster known as NGC 6819, thereby significantly extending the age range. However, Meibom pointed out:

Older stars have fewer and smaller spots, making their [rotation periods harder to detect.

The team examined stars weighing 80 to 140 percent as much as our sun. They were able to measure the spins of 30 stars with periods ranging from 4 to 23 days, compared to the present 26-day spin period of the Sun. The eight stars in NGC 6819 most similar to our sun have an average spin period of 18.2 days, strongly implying that the sun’s period was about that value when it was 2.5 billion years old (about 2 billion years ago).

The team then evaluated several existing computer models that calculate the spin rates of stars based on their masses and ages, and determined which model best matched their observations. Meibom said:

Now we can derive precise ages for large numbers of cool field stars in our galaxy by measuring their spin periods. This is an important new tool for astronomers studying the evolution of stars and their companions, and one that can help identify planets old enough for complex life to have evolved.

Bottom line:

Via CfA

More results from this week’s AAS meeting:

Super-Earths may have long-lasting oceans

How to make an Earth

January 5, 2015
Science Wire

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