Astronomers Discover New Method of Estimated Dwarf Novae Distances

Astronomers using the Fine Guidance Sensors of the Hubble Space Telescope to
study dwarf novae unexpectedly discovered a new method of estimating the
distances to these strange double-star systems, using their orbital periods and
outburst brightness. In the process they learned that dwarf novae tend to be
farther away and much brighter than previously thought.

"This sends everybody back to the drawing board," said New Mexico State
University astronomer Joni Johnson. "The models for dwarf novae have to be
adjusted."

The findings of Johnson and her collaborators from NMSU, the University of
California at Riverside, the University of Washington and the University of
Texas at Austin were reported May 26 to the American Astronomical Society
meeting in Nashville.

The researchers used the orbiting Hubble Space Telescope to directly calculate
the distances to six dwarf novae and determine the star systems’ brightness.
Their original intention was to check the accuracy of commonly used methods of
estimating distances to dwarf novae, but "along the way serendipity intervened,"
Johnson said.

The researchers not only found that dwarf novae estimates have been off the
mark. They also discovered a remarkably tight correlation between the orbital
period of a dwarf nova system and the system’s brightness during its occasional
outbursts — the longer it takes the pair of stars to orbit each other, the
brighter the outburst, Johnson said.

"What this means is that we now have a better method of estimating the distances
to these systems," she said. "If we know the orbital period of the binary
system, we can predict its true luminosity at outburst and calculate the
distance to the system."

Dwarf nova systems are a type of cataclysmic variable — stars that sometimes
increase in brightness dramatically. They are binary systems in which the
primary, more massive star is a white dwarf — a star about the size of Earth
but with the mass of the sun — and the secondary star is often a cool red dwarf.

The two stars are locked in a tight orbit, so close to each other that the
powerful gravity of the white dwarf strips material away from the red dwarf.
This material swirls around the white dwarf in what is known as an accretion disk.

"At times the system becomes unstable and the accretion disk dumps a lot of
material onto the surface of the white dwarf," Johnson said. "During these
outbursts, the system gets anywhere from a few times brighter to 250 times
brighter."

The two stars in a dwarf nova are so close they orbit each other in a matter of
hours, compared with the month it takes the moon to orbit Earth or the year it
takes Earth to orbit the sun. Known orbital periods range from 78 minutes to 48
hours, Johnson said.

A key to learning more about these systems is knowing how far away they are, she
said. Previous distance estimates have been based primarily on comparing the
light spectrum of the secondary star in a dwarf nova to that of a standard star
of the same type a known distance away.

For more accurate measurements, the astronomers used the Fine Guidance Sensors
on the HST to obtain high-precision parallaxes for six dwarf novae, including RU
Pegasi, WZ Sagittae and YZ Cancri. They also examined existing data on four
other similar objects. Parallaxes provide a way to accurately calculate
distances by observing an object from two vantage points and measuring its shift
against its background — the greater the shift, or parallax angle, the closer
the object is to the observer.

With these measurements in hand, the researchers could calculate the brightness
of the dwarf novae without having to make any assumptions.

"It immediately became obvious that the secondary stars were not normal by any
means," Johnson said. "They didn’t have the normal luminosity for a star of the
measured temperature and in fact were brighter than expected. So the distance
measurements to dwarf nova systems were often wrong."

The serendipity came in the unexpected discovery of a tight correlation between
the orbital period of the binary stars and their true luminosity when the system
is in outburst, she said. This correlation gives astronomers a new tool for
measuring the distances to dwarf novae that are too far away to measure by
parallaxes.

Johnson’s collaborators on the project are Tom Harrison of NMSU, Steve Howell of
UC-Riverside, Paula Szkody of the University of Washington, and Barbara McArthur
and Fritz Benedict of UT-Austin. The project was funded by NASA’s HST Guest
Observer Program.