HST Observations Reveal Coolest and Oldest White Dwarf Stars in the Galaxy

1/15/1996 - 2-96r
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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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