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Astronomy

The Hercules beacon

Team from Santa Catarina unravels the binary star enigma

MARK GARLICK/SCIENCE PHOTO LIBRARYThe dwarf and the giant: the main star, smaller and denser, attract matter from the disk formed by the gas of its companion starMARK GARLICK/SCIENCE PHOTO LIBRARY

In the early hours of December 13, 1934, amateur astronomer John Philip Manning Prentice saw one of the Northern Hemisphere’s brightest stars appear in the skies over Suffolk, England. Passionate about meteors, Prentice knew he had witnessed a major event. He got into his car and drove for a few hours to the Greenwich Observatory, where he reported the event to professional researchers. Prentice had not witnessed the birth of a new star, but rather, the rare sighting of a binary system that currently defines an entire category of celestial objects, the nova stars. The brightness of nova stars suddenly increases hundreds of thousands of times due to a massive explosion. The behavior of these paired stars, which has been studied continuously for the last 75 years, has now become much clearer, thanks to the work of two astrophysicists from the State of Santa Catarina.

Raymundo Baptista, from the Federal University of Santa Catarina, and Roberto Saito, currently a visiting researcher at the Universidad Catolica de Chile, spent the last five years analyzing images of this binary star obtained from the Palomar Observatory in California. The star was named the Herculis nova because it is in the Hercules constellation, so named in honor of the mythological Greek hero. The two researchers employed a sophisticated technique to evaluate the variation of the brightness during the eclipse caused by the movement of the bigger, duller star into the shadow of the smaller, brighter one. Thus, they managed to reconstruct the stars’ three-dimensional configurations and establish the origin of the rapid light pulses detected by land telescopes.

Today, scientists know that the strong brightness observed in 1934 was the result of a fleeting event that only happens once every hundreds of thousands of years, making binary stars – usually invisible to the naked eye – 200 thousand times brighter than they are normally. The event that Prentice witnessed in the skies of England was the most recent explosion of the Herculis nova, a phenomenon of catastrophic proportions that is common to small binary star systems at the end of their life. In this explosion, which had actually occurred some 1,500 years earlier (the light of the explosion took that long to reach the Earth), the main star, called a white dwarf, whose mass, comparable to that of the Sun, is compressed into a volume 60 times smaller and similar to that of the Earth, expelled its outermost layer at an extremely high speed into the interstellar environment. The remodeled white dwarf then began to emit light pulses every 71 seconds; though these pulses were much weaker than those of the explosion, the white dwarf still overshadowed the brightness of its companion.

Ever since the earliest observations of the Herculis nova, astronomers and astrophysicists the world over have tried to understand one of its peculiarities: the fluctuation in the pace of its light pulses. From time to time, the pulses, normally seen every 71 seconds, speed up or slow down. Measurements have already shown that during any given decade, the white dwarf emits light pulses once every 69.5 seconds, while at other times this rises to 71.5 seconds. Half a second less or more makes little difference to any human being leading today’s hurried life, but it means a lot to a white dwarf, which spins at very high speeds, completing a full rotation in a mere 71 seconds; if the Earth spun at this speed, the day would begin and end before anyone could go from the living room to the kitchen to get a cup of coffee.

The explanations proposed until recently seemed too elaborate, requiring such very special conditions that they were unlikely; one such explanation stated that the rise and fall of the pace of the bright pulses depended on the presence of far more mass than exists in that part of outer space to speed up or slow down the spinning motion of the white dwarf. “It was an uncomfortable situation,” says Baptista, a star-imaging expert. This is no longer the case, however.

Cannibalism
Based on the reconstruction of this system, Baptista and Saito managed to identify the source of light of the Herculis nova and provide a much simpler and more acceptable explanation for the fluctuation in the pace of its luminosity. After the explosion, during which it gets rid of its outermost layer, the main star starts to devour its companion: the white dwarf’s field of gravity yanks the outer layers of the secondary star (a bigger but less dense star than the main one) which then begins to disintegrate as it goes around the main star in 4 hours and 40 minutes. This outer space cannibalism forms a gas disc comprised of electrically charged particles that nourish the white dwarf. Magnetic fields thousands of times stronger than those of the Sun pull the gas in the disc to the poles of the primary star, where the shock with the atmosphere results in a conical beam of X-rays.

Still, there were some points to be clarified. Scientists did not know whether there was only one beam of light or two – one at each pole – sweeping in opposite directions, like a port beacon. Scientists also believed that the light detected by the telescopes had originated directly from the white dwarf’s poles. In an article published this year in Astrophysical Journal, Baptista and Saito answer the two questions with a single model that is simpler than those that preceded it. “Our findings indicate that the Herculis white dwarf emits two X-ray beams, which sway at an angle that is very close to the plane of the gas disc,” says Saito. “However, on Earth, one can only see the light emitted by one of them.” They also discovered that the radiation detected here is not the radiation emitted by the pole, but that which is reflected by the gas disc that feeds the white dwarf. The energy of the X-ray beams warms the disk, which then starts to emit another kind of light. “It is as if, instead of seeing the beacon’s actual light, we were seeing the light reflected by the sea that is lit by the beacon,” Baptista compares.

This new model also allows scientists to understand the variation in the frequency of light pulses. “The mass provided by the secondary star to the white dwarf determines the speed of the light pulses,” says Baptista. When the companion star loses a higher amount of matter, the disc’s outer rim becomes thicker. When the secondary star’s pace of degradation diminishes, the disc becomes thinner and narrower. As the gas spirals at different speeds in the disc’s different bands – the closer it is to the white dwarf, the faster the gas spins – the pace of the light pulses varies. “This is a much simpler and more natural explanation,” says Baptista. This answer, which seems to have solved a 75-year old mystery, also helps one to understand what happens to small binary stars similar to the Herculis nova – which, according to estimates, account for one-third of the bright spots in the skies – before they die out forever.

Scientific article
SAITO, R. K.; BAPTISTA, R. Spin-cycle eclipse mapping of the 71 s oscillations in dq herculis: reprocessing sites and the true white dwarf spin period. The Astrophysical Journal. v. 693, p. L16-L18. Mar 1. 2009.

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