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