Feature Story

GALEX Studies a Stellar Bully in Our Milky Way Galaxy

Written by Linda Vu (Spitzer Science Center)
December 13, 2006

Unlike our Sun, a relative "cosmic loner," many of the Milky Way galaxy's stars actually lurk in groups of two or more. In most of these systems, the inhabiting stars have no problem co-existing peacefully with their stellar companions. However, astronomers have found a few exceptions.

Three hundred light-years away in a binary (two star) cataclysmic variable system called EF Erindanus, or EF Eri for short, a "stellar bully" has pulled so much material from its closely orbiting companion star, that the companion no longer resembles any known star type. The bully in this case is a white dwarf, or dead star with nearly 60 percent of our Sun's mass, crammed into an Earth size ball. Astronomers say that more than 1 million Earths can fit inside our Sun.

Currently the nature of white dwarfs in cataclysmic variables is a mystery. Astronomers cannot directly study the white dwarf while it is pulling material off of its partner star because light from the accreted debris typically outshines the system's stars. However, if the white dwarf takes a "bullying break," astronomers can study the system's stellar inhabitants directly with telescopes like NASA's Galaxy Evolution Explorer (GALEX).

"Because white dwarfs are hot and faint, we need space-based telescopes like GALEX with far-ultraviolet capabilities to see them," said Dr. Paula Szkody of the University of Washington, Seattle, Wash. "No other space telescope flying has the wide-field imaging capability and high time-resolution of GALEX."

Szkody is the principal investigator of the GALEX EF Eri observations.

In 1997, the white dwarf in EF Eri stopped accreting material from its stellar partner. Except for a few weeks in early 2006, the bully has been relatively inactive ever since. Without the accretion blocking the view of EF Eri's stars, Szkody and team were able to conduct observations with GALEX.

"We currently don't know what causes accretion to turn on and off in systems like EF Eri," said Szkody. "Since there is so little known about the nature of the stars in these systems, I wasn't exactly sure what I'd find."

Although most observations with other telescopes obtained during this time did not detect any mass transfer in EF Eri, data from GALEX's sensitive ultraviolet instruments indicated a large hot spot on the white dwarf, hinting that the star was probably still taking material from its partner.

"Using GALEX's unique ability to get good time resolution, we were able to construct very well defined light curves, which provided insights into the extent and structure of the hot spot," said Szkody. "We saw huge fluctuations in the light curve, which indicated that some kind of transfer is still going on."

Szkody suspects that the hot spot is caused by material being funneled into one of the white dwarf's poles. The material most likely comes from the partner star's winds and is being picked up by the white dwarf's strong magnetic fields. Team members believe that the material is probably following the white dwarf's magnetic field and funneling into both poles, but the fact that they only detect a very hot spot on one of them indicates that the white dwarf is tilted, with its "south" end pointing toward the companion.

"More observations definitely need to be done," Szkody adds. She notes that the nature of the white dwarf's spot remains a mystery because fluctuations in the near-ultraviolet and far-ultraviolet light curves taken by GALEX did not match any known spot models.

"We know something is there, because if there wasn't, the curve would just be a flat line," she says. "We just don't have the right model for it yet."

Szkody also notes that GALEX observations were vital in determining the real temperature of EF Eri's white dwarf. Astronomers typically use a temperature model to determine the age of their white dwarfs. The cooler the star, the older astronomers believe the object to be.

Szkody's findings were recently published in the August 2006 issue of Astrophysical Journal Letters.