Using the Hubble Space Telescope,
astronomers at the Naval Research Laboratory (NRL) have detected
five optical companion stars orbiting millisecond pulsars. Only
two other such systems are known. Three of the companions are
among the coolest and oldest white dwarf stars known. Interpretation
of the properties of these systems has provided insight into
the final fate of stars in binary systems and may eventually
set limits on the age of the Milky Way galaxy. Drs. Scott C.
Lundgren and Roger S. Foster of NRL in Washington, D.C., and
Dr. Fernando Camilo of the University of Manchester at Jodrell
Bank, England, reported these findings at the 187th Meeting of
the American Astronomical Society on January 15, 1996.
Previous observations suggested
that millisecond pulsars may be some of the oldest objects in
the Milky Way Galaxy. However, only upper limits to their ages
were known until now. When asked about the significance of the
new Hubble data, Dr. Scott Lundgren, a National Research Council
Associate at NRL said, "our optical observations of pulsar
companions provide definitive evidence that some of these systems
were formed when the Galaxy was much younger, five to seven billion
years ago. This study is only the first in a series, which we
expect will detect a large number of the oldest white dwarf stars,
eventually giving important constraints on the age of the Milky
Way galaxy."
The pulsars were originally discovered
within the past few years in radio pulsar searches. The pulsar
rotates hundreds of times per second and emits a powerful beam
of radio waves, which is detected as pulses as the pulsar spins,
similar to the way a light-house beacon flashes as it rotates.
Each of the pulsars was known to have a companion from observations
of changes in its pulse period. As the pulsar is tugged closer
and farther from Earth by the companion orbiting around it, the
doppler effect modulates the pulse period. However, the radio
observations of the pulsar give only limited information about
the companion: the orbital period, the distance and weak constraints
on the mass. Since the companions are cool (as cool as 3500 degrees
Kelvin), compact (0.01-0.02 times the radius of the Sun) and
distant (1000-4000 light years), most are too faint to detect
with the largest telescopes on the ground. The Wide Field Planetary
Camera on the Hubble Space Telescope was used to search for the
companions in observations made in July 1995. Five out of six
companions studied were found; PSR J0034-0534 in the constellation
Cetus, PSR J1022+1001 in Leo, PSR J1640+2224 in Hercules, PSR
J1713+0747 in Ophiuchus, and PSR J2145-0750 in Aquarius.
The white dwarf detected in each
of these binary pulsar systems is the fossil relic of an earlier
period. The white dwarf was formed from a star in the range of
one to six times the mass of the Sun. When the hydrogen fuel
in the progenitor star's core was used up, the outer envelope
expanded to form a red giant, while hydrogen burning continued
in a shell around the helium core. When the envelope got large
enough, it dumped mass onto the orbiting neutron star, making
it spin faster and causing it to spiral into an orbit closer
to the giant. Eventually the hydrogen burning ceased and the
giant-star envelope was ejected, leaving the compact core astronomers
observe as a white dwarf star in orbit around the neutron
star.
The ages of the white dwarfs
were determined from their temperatures. The observed white dwarfs
are basically cooling embers. The nuclear fire of the stars burned
out billions of years ago. The light emitted comes from the heat
remaining from the earlier nuclear burning. By measuring the
spectrum of the light, the brightness in various colors, the
temperatures of the stars were determined. The two coolest of
the white dwarfs studied, PSR J0034-0534 and PSR J1713+0747,
are 3400 degrees Kelvin (5600 F), making them the coolest known
white dwarfs. For comparison, the surface of the sun measures
5800 degrees Kelvin and the coolest previously known white dwarfs
are 4000 degrees Kelvin. These cool white dwarfs will be important
for calibrating the white dwarf temperature scale and the ages
of the oldest stars in the Milky Way galaxy.
Models for white dwarf cooling
have been developed, which take into account the light emitting
and absorbing properties of the surface and the atmosphere of
the star. Using these cooling models, the ages have ben estimated
for the white dwarfs studied. Most of the ages are 2 4 billion
years. The oldest of the five white dwarfs detected, PSR J2145-0750,
has a cooling age of 5-7 billion years, making it one of the
oldest know white dwarfs in the Galaxy. The white dwarf stars
around other millisecond pulsars. Future observations may reveal
even older white dwarfs, improving our limits on the age of the
Galaxy.
"We are observing dead stars
orbiting each other," says Dr. Roger S. Foster, a radio
astronomer at NRL. "From our observations we are able to
piece together how these stars evolved. You might say that we
are 'stellar pathologists.' By studying how these stars died,
we can understand how they lived."
By measuring the white dwarf
properties such as temperature, luminosity, mass, and age using
Hubble observations, astronomers have found clues about the previous
evolution of these systems. In the standard binary pulsar evolution
theory described above, the mass of the white dwarf is determined
by the size of the orbit, and hence the orbital period. The expected
white dwarf mass is derived from the core mass of a progenitor
giant start that is just large enough in the observed orbit for
the pulsar gravity to cause mass to transfer from the giant envelope.
By measuring the temperature and brightness with the Hubble data
and using a distance estimate, the mass of each companion has
been determined and compared with the theoretical prediction.
This work was supported by the
National Aeronautics and Space Administration, the Office of
Naval Research, and the National Research Council.
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