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ASTROPHYSICS

Cannibalistic stars

Group from Brazil and Argentina explains how a class of pulsars evolves until it consumes other celestial objects.

064-065_C_Sistemas estelares_238NOVAOuter space is a zoo containing curious species. Among them, one of the most intriguing is the pulsar, a compact object that rotates quickly and emits regular radio wave pulses. A model developed by researchers from Brazil and Argentina helps explain how some of the most exotic varieties of pulsars evolve. As would be appropriate in a zoo, they were given animal names: the redback spider and the black widow.

Pulsars have been fascinating astronomers since their discovery in 1967. When astronomers Jocelyn Bell and Antony Hewish observed the pulsed emissions that they referred to when naming these objects for the first time, they found them so intriguing that they were unable to dismiss the possibility of their being transmissions from extraterrestrial civilizations. Humorously, Bell and Hewish named the object they discovered LGM-1, short for Little Green Men. But it did not take long before pulsars were discovered to be a category of neutron stars, a type of massive star cadaver that, after exhausting its nuclear fuel, explodes as a supernova.

These massive stars—eight times larger than the Sun—explode and eject their outer layers. At the same time, their nuclei are compacted so much that their electrons dive towards the protons and convert them into neutrons—thus the name neutron star. They are very compact objects, in which the remaining mass, equivalent to that of two Suns, is compacted into a sphere with a diameter of 10 to 30 kilometers. When the powerful magnetic field of one of these stars is not aligned with its axis of rotation, the radiation beam emitted gyrates in a precession movement. From the Earth, this radiation is seen intermittently, in the form of the pulses that characterize these objects.

Many of these pulsars have companion stars orbiting around them. Some are accompanied by a star whose mass corresponds to 20% to 40% of the mass of the Sun and form systems known as redbacks, an Australian spider that has a red stripe on its black abdomen. Similarly, pulsars accompanied by smaller stars, with 5% of the Sun’s mass, are called black widows.

The systems were called this because, in them, the more massive, denser star—the pulsar—contributes to “evaporating” the smaller one. This is similar to what occurs with these spiders: the much larger female kills the male after copulating. “Americans and Australians used these nicknames and they stuck,” recalls astrophysicist Jorge Horvath, of the University of São Paulo Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG-USP). “Now these systems are known as spiders.”

The work Horvath carried out with Argentine colleagues Omar Benvenuto and María Alejandra De Vito, both from the National University of La Plata, takes an important step towards understanding the evolution of these systems. Their model shows that there is an evolutionary relationship between the redback and black-widow systems.

In both cases, the pulsar consumes part of the mass of its companion through a mechanism called accretion. Much denser, pulsars have a strong gravitational field that attracts the mass of the companion star. They act like a vacuum cleaner that sucks up the pieces of the neighbor as it crumbles. But these spider systems can also form a much more interesting configuration: the orbits of the two stars may evolve to a point at which the distance between them is less than that between the Earth and the Moon.

In these cases, when the mass of the companion star becomes very small (5% of the Sun’s mass), typical of black-widow systems, it ends up being consumed via a second mechanism: evaporation. The radiation and particles emitted by the pulsar sweep part of the mass of the companion star away, like blowing dust off a table. “In the simulations, we found that in some cases there would be enough time for the pulsar to completely evaporate its companion,” says Horvath. “We also saw that, in other cases, a ‘core’ with a mass similar to that of a planet could remain at a greater distance from the pulsar,” he says.

The researchers described these evolutionary trajectories in an article in the journal Astrophysical Journal Letters. In this paper, they also showed that the behavior of these systems depends both on the initial distance between the pulsar and the companion star and the initial mass of the latter. When the companion is in an orbit close to the pulsar, with an orbital period of an Earth day or less, its mass is consumed by accretion and some of these systems evolve to become redbacks. If the distance is smaller, equivalent to an orbital period of less than three hours, the companion star is consumed via evaporation, typical of black-widow systems. The researchers also saw that, under certain conditions, the former system can become the latter. “In these systems, the pulsar’s mass increases greatly, which is important for understanding the nature of the matter of which it is composed,” says Horvath.

In their model, Horvath and his colleagues included the effects of pulsar radiation and particle emission on the system’s evolution. “The emission influences the system in two ways: it can peel layers of gas off the companion star via evaporation and the matter attracted to the pulsar generates intense X-rays, enough to affect the companion’s structure,” says Marcelo Allen, professor at the Federal Institute of São Paulo, who did not participate in the study.

Full understanding of redbacks and black widows will require new efforts. “We are far from a satisfactory theoretical formulation to explain the behavior observed over long time scales,” says Flavio D’Amico, an astrophysicist at the National Institute for Space Research.

Project
Superdense matter in the Universe (nº 2013/26258-4); Grant Mechanism Thematic Project; Principal Investigator Manuel Máximo Bastos Malheiro de Oliveira (ITA); Lead Investigators Jorge Ernesto Horvath (IAG-USP) and João Braga (INPE); Investment R$222,701.00.

Scientific article
BENVENUTO, O. G., DE VITO, M. A. e HORVATH, J. E. Understanding the evolution of close binary systems with radio pulsars. Astrophysical Journal Letters. V. 786 (L7). May 2014.

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