Some stars in double-star systems have found a quick way to lose weight by dumping their extra pounds onto their companions. Astronomers using NASA's Hubble Space Telescope have discovered such a case in the double-star system Phi Persei. A "rapid diet" program has trimmed an aging, once massive star to a lean one-solar mass, while the once mild-mannered, moderately massive companion has bulked up to a hefty nine-solar masses and is spinning so violently that it's flinging gas from its surface.
This observation has allowed astronomers to catch a glimpse of an unusual, fleeting moment in the life of a massive star in a double-star system.
"We have seen massive star binaries at the end of their lives, when one star has collapsed to become a neutron star," says Douglas Gies, of the Center for High Angular Resolution Astronomy at Georgia State University in Atlanta, Ga. "There are about six dozen of those known in our galaxy. But what we hadn't seen before is the phase just prior to the collapse of the aging star. The Hubble observations dramatically show how severely a star's material can be removed through the gravitational influence of a nearby companion."
What Gies and his team have seen is an opportunistic star that has taken advantage of an aging, ailing partner. After consuming practically all of its hydrogen - the fuel that keeps its thermonuclear furnace running - the aging star swelled up and began jettisoning its excess mass until only the bare core was left. Seizing upon an opportunity, the companion began cannibalizing the discarded material, thereby increasing in size. Gies calls the stripped-down star a subdwarf, a type of aging star that has passed the expansion phase - by swelling and puffing away its outer layers - and is on its way to becoming a fading white dwarf. And yet this aging, stripped-down star, which has the same mass as the Sun, is very hot. At 95,000 degrees Fahrenheit (53,000 K), the star is nine times hotter than the Sun.
Although very hot and bright, the subdwarf is lost in the glare of its brighter, more massive companion. In fact, until Gies aimed the Hubble telescope at this stellar object, no telescope could provide definitive evidence of its existence. The subdwarf would be the brightest object of its class in the sky (a sixth-magnitude star) if it could be seen alone. If placed at the Sun's distance, the subdwarf would appear 200 times brighter than the Sun. But the beefed-up companion is 10 times brighter in visible light than the subdwarf; its brilliance dominates our view. Phi Persei is 720 light-years away in the constellation Perseus. It is visible in the autumn evening sky in the northern hemisphere, just north of the Andromeda Galaxy, M31. The double-star system is visible as a fourth-magnitude star.
For years, the subdwarf's light had completely eluded detection. Astronomers had suspected the star was there, because they had detected a slight wobble in the motion of the bright, beefed-up companion. The wobbling motion was due to the gravitational attraction between the two stars. Astronomers finally fingered the subdwarf with the help of the Hubble telescope's Goddard High Resolution Spectrograph (which was removed last February during the Second Servicing Mission). The Hubble telescope was aimed at Phi Persei on five occasions between November 1995 and October 1996. The spectrograph collected the ultraviolet light emitted by this very hot, stripped-down star, allowing scientists to identify the spectral signature of this once massive stellar object. Gies' analysis of these observations will appear in the Jan. 20, 1998 issue of the Astrophysical Journal.
By collecting information on the subdwarf, Gies was able to piece together a better picture of life in this double-star system, especially how the beefed-up companion gained its extra mass. The stellar aging process may not have been kind to the stripped-down subdwarf, but it has been very generous to the opportunistic companion. By puffing away most of its mass, the aging, stripped-down star has given new life and a new identity to its companion. Here is a case where gaining weight is healthy. The roughly 10-million-year-old companion has potentially doubled its lifetime because it has gained a vast amount of hydrogen fuel, which is needed to maintain its thermonuclear furnace. By beefing up, the companion also has changed its identity from a normal, moderately massive star to a "Be" star, a type of hot star with a broad, flattened disk of hydrogen gas swirling around it, much like the rings of Saturn. Based on measurements taken by astronomers at the U.S. Naval Observatory, the disk is eight times wider than the star.
The disk formed from gas spun off the rapidly rotating "Be" star. What causes the fast spin of "Be" stars has been a mystery to astronomers. Now the Hubble telescope observations of Phi Persei offers at least a partial explanation: The gas discarded from a nearby swelling star strikes the companion off-center, causing it to spin faster.
"The companion is now rotating so fast (1 million mph or 450 kilometers per second at its equator) that the star is distorted into a flattened oblate figure in which gravity can barely maintain its hold on the star's outer layers," Gies explains.
This new information about the stripped-down star and its companion leads scientists to guess about this unusual duo's past. Before the exchange of material, the stripped-down subdwarf was the more massive stellar object, about six times more massive than the Sun. Its companion was slightly less bulky, about five solar masses. Such massive stars usually race through life at a faster pace than most stellar objects, ending their lives in one big supernova explosion. But stars in binary systems live differently. When the once-massive subdwarf entered its twilight years about 1 million years ago, for example, it swelled in size as it began using up its hydrogen fuel. A single, massive star would have eventually exploded. But the presence of the companion prevented the once-massive subdwarf from suffering such a violent fate. Instead, the once-massive star dumped most of its outer layers onto its companion, and now is heading to a quiet demise.
"Inhabitants of an imaginary planet orbiting these stars would be witnessing one star's demise and the other's rejuvenation," Gies says. "The Hubble observations have allowed us to become distant witnesses to this cosmic drama."
The Hubble telescope, however, may have captured only "Part 1" of this drama. The enigmatic pair of stars may act out "Part 2" sometime in the future. That's because a curious destiny may await this pair. As long as the "Be" star doesn't break apart, it will live for another 10 million years because of the hydrogen fuel it acquired from its companion. Then it will swell during the expansion phase and possibly dump some of its mass back onto the subdwarf, which will have evolved into a white dwarf. (The pair is now safely separated by about one astronomical unit, the distance between Earth and the Sun.) Then, things could get interesting. The white dwarf might grow in mass and eventually explode as a peculiar supernova. Or, the companion might swell up so much that it would engulf the white dwarf, eventually tossing out its material in mix-master action. Such is the life of massive stars living with companions.
"The beefed-up 'Be' star has won a new lease on life, but its ultimate fate will be determined by the corpse of its former companion, which remains in orbit uncomfortably nearby," Gies concludes.
Space Telescope Science Institute, Baltimore, MD
University Relations, Georgia State University (Atlanta, GA)