Gyrochronology -- a Powerful New Method to Determine Stellar Ages (Forwarded)

Lowell Observatory

Contact:
Steele Wotkyns, Public Relations Manager
(928) 233-3232

For Immediate Release: April 24, 2007

How Old Are Stars?

Enter Gyrochronology -- a Powerful New Method to Determine Stellar Ages

Flagstaff, Ariz. -- Gyrochronology, a new method for accurately determining
the ages of field stars based on their rotational rates, is being announced
today by Sydney Barnes, Lowell Observatory astronomer. This fundamental
research, "Ages for illustrative field stars using gyrochronology:
viability, limitations and errors," is accepted for publication in an
upcoming issue of the Astrophysical Journal and is now available at
http://arxiv.org/abs/0704.3068 .

"Gyrochronology transforms a rotating star into a clock which is set using
the Sun and keeps time well," said Barnes.

The age of a star is its most fundamental attribute apart from its mass. A
star's age tells astronomers how astrophysical phenomena change over time.
"For example, the ages of the host stars of planetary systems are needed to
understand how these systems change over time," said Barnes.

By showing that the rotation period of a star is a steadily changing and
tight function of its age and color, gyrochronology allows the age to be
determined by measuring the two other properties -- the rotation period and
the color. "If you know the relationship between three quantities, measuring
two of them allows you to calculate the third," said Barnes. "The
relationship between age, color, and rotation period has particular and
useful mathematical properties that simplify the analysis and allow the
uncertainties to be calculated easily." A star's color is a proxy for its
mass or surface temperature. The uncertainties in gyrochronology ages are
typically 15 percent; with preexisting stellar aging methods the
uncertainties range from 50 to 100 percent.

Gyrochronology can be calibrated using the known age of the Sun (4.6 billion
years). Another distinguishing characteristic of the technique is that it
works well for the vast majority of stars including field stars, or those
not found in star clusters. For the first time, this new technique makes
possible the derivation of accurate ages for solar- and late-type main
sequence stars using only their rotation periods and colors. In his new
paper, Barnes calculates ages for sun-like and other low mass stars that
burn their hydrogen fuel at a relatively steady rate on what is known as the
main sequence. Barnes derives ages for sample stars where rotation and color
are known, but the stellar ages using other methods are not known.

The technique builds on an insight of Skumanich in 1972 who noticed that
another measure of stellar rotation changes steadily with the ages of star
clusters. However, the related imprecision greatly compromises the accuracy
of ages derived using this insight alone. Measurements made at Lowell
Observatory in the late 1980s showed that rotation also depends on the
color/mass of a star. Gyrochronology combines and develops these two
insights into a precise way of deriving stellar ages, and shows that it
works even for single field stars. The paper shows that the rotation period
of a star (whether in a cluster or in the field) can be written as a simple
product of two separable functions of its age and color. This mathematical
behavior provides the key simplification that makes gyrochronology unique.

Preexisting methods for attempting to determine stellar ages are the
isochrone and chromospheric techniques. The isochrone method was first named
by Demarque and Larson in an elegant 1964 refinement on pioneering work by
Allan Sandage. Isochrone ages are derived through computations of the
evolutionary tracks of stars. Barnes' study points out that, while the
isochrone method works well for star clusters, it does not work well for
individual (field) stars because it requires the distance to measured. And
that is difficult. The study shows that another reason isochrone ages are
not satisfactory is that they do not work well for stars on the main
sequence, where the majority of a star's life is spent. "However,
gyrochronology is independent of distance and works well on main sequence
stars," said Barnes.

Another method for determining ages of stars is the chromospheric method, a
breakthrough developed by Olin Wilson and others since the 1960s.
Chromospheric ages are calculated using the measured chromospheric emission
from stars. These ages are not dependent on distance and can be used on main
sequence stars. However, chromospheric ages have large uncertainties of up
to 50 percent. The uncertainties in gyrochronology ages are typically 15
percent.

In additional analysis, Barnes used gyrochronology to demonstrate that,
unlike results using other techniques, the individual components of three
wide binary stars have essentially the same ages. (See Gyrochronology
Background, http://www.lowell.edu/press_room/gyrobkgrnd.pdf, for detail.)

Gyrochronology does not work well for the youngest stars, those that have
not begun burning fuel at the steady rate indicative of the majority of
stars (main-sequence) stars, or for those that have left the main sequence.
However, future work might be able to extend the method for these stars as
well.

For the vast majority of stars, gyrochronology ages are shown to be more
consistent than other stellar aging techniques and will have applications
across the field of astronomy. For example, NASA's Kepler Mission, being
readied for launch, is likely to yield not only discovery of new planets
through observations of the transits of these new planets moving in orbits
across the disks of their host stars, but also the rotation periods of those
host stars. Kepler is likely to yield rotation periods for orders of
magnitude more stars than planetary transits. "No matter what, stellar
rotation periods will be determined routinely as time domain astronomy comes
into its own," said Barnes. "A very significant portion of time domain work
on stars will yield the stellar rotation period (it is a by-product of all
searches for planetary transits), and since this measurement can be used to
derive a precise stellar age, it would permit us to address many problems
involving chronometry that are not presently solvable."

"The age of a star is its most fundamental attribute apart from its mass,
and usually provides the chronometer that permits the study of the time
evolution of astronomical phenomena," said Barnes. "Gyrochronology refines
the use of stars as clocks, revealing their own ages, and the ages of
associated astronomical bodies."

About Lowell Observatory

Lowell Observatory is a private, non-profit research institution founded in
1894 by Percival Lowell. The Observatory has been the site of many important
findings including the discovery of the large recessional velocities
(redshift) of galaxies by Vesto Slipher in 1912-1914 (a result that led
ultimately to the realization the universe is expanding), and the discovery
of Pluto by Clyde Tombaugh in 1930. Today, Lowell's 19 astronomers use
ground-based telescopes around the world, telescopes in space, and NASA
planetary spacecraft to conduct research in diverse areas of astronomy and
planetary science. Lowell Observatory currently has four research telescopes
at its Anderson Mesa dark sky site east of Flagstaff, Arizona, and is
building a 4-meter class research telescope, the Discovery Channel
Telescope, in partnership with Discovery Communications, Inc.

For more information:

* Astrophysical Journal online abstract
http://arxiv.org/abs/0704.3068
* Gyrochronology Background (pdf)
http://www.lowell.edu/press_room/gyrobkgrnd.pdf
* Brief biographical information on Sydney Barnes
http://www.lowell.edu/People/bios/barnes.html