{"id":216627,"date":"2016-05-03T15:31:00","date_gmt":"2016-05-03T18:31:00","guid":{"rendered":"http:\/\/revistapesquisa.fapesp.br\/en\/?p=216627"},"modified":"2016-05-03T15:31:00","modified_gmt":"2016-05-03T18:31:00","slug":"cannibalistic-stars","status":"publish","type":"post","link":"https:\/\/revistapesquisa.fapesp.br\/en\/cannibalistic-stars\/","title":{"rendered":"Cannibalistic stars"},"content":{"rendered":"

\"064-065_C_Sistemas<\/a>Outer 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.<\/p>\n

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.<\/p>\n

These massive stars\u2014eight times larger than the Sun\u2014explode 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\u2014thus 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.<\/p>\n

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\u2019s mass, are called black widows.<\/p>\n

The systems were called this because, in them, the more massive, denser star\u2014the pulsar\u2014contributes to \u201cevaporating\u201d the smaller one. This is similar to what occurs with these spiders: the much larger female kills the male after copulating. \u201cAmericans and Australians used these nicknames and they stuck,\u201d recalls astrophysicist Jorge Horvath, of the University of S\u00e3o Paulo Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG-USP). “Now these systems are known as spiders.”<\/p>\n

The work Horvath carried out with Argentine colleagues Omar Benvenuto and Mar\u00eda 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.<\/p>\n

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.<\/p>\n

In these cases, when the mass of the companion star becomes very small (5% of the Sun\u2019s 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. \u201cWe also saw that, in other cases, a \u2018core\u2019 with a mass similar to that of a planet could remain at a greater distance from the pulsar,\u201d he says.<\/p>\n

The researchers described these evolutionary trajectories in an article in the journal Astrophysical Journal Letters<\/em>. 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\u2019s mass increases greatly, which is important for understanding the nature of the matter of which it is composed,” says Horvath.<\/p>\n

In their model, Horvath and his colleagues included the effects of pulsar radiation and particle emission on the system\u2019s evolution. \u201cThe 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\u2019s structure,\u201d says Marcelo Allen, professor at the Federal Institute of S\u00e3o Paulo, who did not participate in the study.<\/p>\n

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.<\/p>\n

Project<\/strong>
\nSuperdense matter in the Universe (n\u00ba 2013\/26258-4<\/a>); Grant Mechanism<\/strong>\u00a0Thematic Project; Principal Investigator<\/strong>\u00a0Manuel M\u00e1ximo Bastos Malheiro de Oliveira (ITA); Lead Investigators <\/strong>Jorge Ernesto Horvath (IAG-USP) and Jo\u00e3o Braga (INPE); Investment<\/strong>\u00a0R$222,701.00.<\/p>\n

Scientific article<\/em>
\nBENVENUTO, O. G., DE VITO, M. A. e HORVATH, J. E. Understanding the evolution of close binary systems with radio pulsars<\/a>. Astrophysical Journal Letters<\/strong>. V. 786 (L7). May 2014.<\/p>\n","protected":false},"excerpt":{"rendered":"Group explains how a class of pulsars consumes other celestial objects","protected":false},"author":19,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"footnotes":""},"categories":[159],"tags":[205],"coauthors":[111],"class_list":["post-216627","post","type-post","status-publish","format-standard","hentry","category-science","tag-astronomy"],"acf":[],"_links":{"self":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/216627","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/users\/19"}],"replies":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/comments?post=216627"}],"version-history":[{"count":0,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/216627\/revisions"}],"wp:attachment":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media?parent=216627"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/categories?post=216627"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/tags?post=216627"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/coauthors?post=216627"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}