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ASTROPHYSICS

The hole in Eta Carinae

A smaller star digs a hole in a larger one, allowing us to see below the surface

castelli_eta-carinae_final-invSandro CastelliEta Carinae, the most studied star in the Milky Way after the Sun and one of the largest and brightest ever seen, continues to surprise us.  First, astronomers confirmed that it was actually made up of two very large stars: the principal and largest, Eta Carinae A, with a mass 90 times that of the Sun, and the second, Eta Carinae B, two-thirds smaller and 10 times dimmer. Next, they noticed that the larger star stopped shining for about 90 consecutive days in certain bands of the electromagnetic spectrum, especially the X-ray band, every five and a half years. Now, specialists in Brazil, the United States, and other countries, after examining the information obtained during the 2014 blackout — the most recent — have described a new phenomenon: the formation of a hole on the surface of the larger star caused by the smaller one.

The collision of the strong winds of the two stars, which had already been described (see Pesquisa FAPESP Issue nº 191), and the formation of a hole in Eta Carinae A, shed light on the intense — and not yet entirely explained — emission of light produced by one of the forms of the chemical element Helium, observed during the blackouts.  This form is doubly ionized Helium, or He++, so called because it has lost its two electrons and consists solely of the nucleus, which contains two protons and two neutrons. In 2014, data obtained by ground telescopes located in Chile, Brazil, the United States, Australia and New Zealand, and by the Hubble space telescope, provided detailed data on the variation in the intensity of light emitted at a specific frequency by the transformation of He++ into He+, an ion with just one electron.

“The collision of the solar winds alone was not enough to produce a quantity of He++ sufficient to explain the intense release of light at this frequency,” says astrophysicist Mairan Teodoro, a researcher at the NASA Goddard Space Flight Center.  Shortly after the 2009 blackout, while still at the University of São Paulo (USP), he began to plan to gather information about the 2014 blackout with Augusto Damineli, then his postdoctoral supervisor.  Teodoro published an international call for professional and amateur astronomers, established a site with information on the project, and coordinated the work methods of the groups interested in participating.

In 2012, already in the United States, and seeking explanations for the phenomena observed, Teodoro worked with his NASA colleague Thomas Madura.  A theoretical physicist, Madura formulated the hypothesis that the smaller star, when drawing close to the larger every five and a half years due to its elliptical orbit, would dig a hole that could reach the innermost layers of the larger star, where He++ is abundant. However, there was a lack of evidence to support or refute this possibility. The information collected during the 2014 blackout confirmed and adjusted this hypothesis and indicated that He++ light emission is the result of the collision of solar winds and the formation of a hole in the larger star.

Near the end
“The solar wind acts like a blanket, covering the primary star,” says Teodoro, the first author of an article published in the March, 2016 issue of Astrophysical Journal, describing these results.  According to this study, the smaller star overcomes the resistance of the solar winds of the larger, since the winds of the former are faster. It dives in a shallow arc and digs a hole in the larger star, exposing some of the high-temperature gas, or plasma, of which it is made.

“The light leaving the hole is only about 100 times the luminosity of the Sun, 50,000 times weaker than the light of the star.  It would be like seeing a match lit in front of the sun,” says Damineli. In addition to being tenuous, the signal is sparse because the opening in the star from which the light is emitted is open for only just over a month every five and a half years.

Damineli predicted that Eta Carinae, which he has been studying since 1989, would suffer an eclipse or blackout — a reduction equivalent to the brightness of 60 Suns in a single day — in 2003.  His predictions were confirmed, attracting a growing number of astronomers interested in observing the star (see Pesquisa FAPESP Issue nº 54, 94 and 154). Now the phenomenon is clearer. “The blackout begins when the small star dips into the larger and ends when the hole closes up, when signs of electromagnetic emissions disappear,” he says. “This is a completely new phenomenon in astrophysics.”

The researchers hope to determine the size of the hole during the next blackout in 2020.  It will also be an opportunity to understand a bit more about the luminosity and the variation in temperature of the two stars; the smaller has not yet been observed directly.

Gigantic and very bright, with luminosity five million times greater than that of the Sun, Eta Carinae is “a representative of the first generation of stars, formed 200 million years after the Big Bang and that died shortly afterwards, when raw materials were abundant,” explains Damineli.  “Eta Carinae and the other larger stars, initially always binary, illuminated the Universe, which was opaque, during what was called the Dark Ages.”

As the only star of this size and with these singular features in our galaxy, Eta Carinae, he says, “is like a live dinosaur in our back yard.”  The high density and composition of the winds indicate that it is losing mass rapidly. Astronomers predict that the star will explode and generate a black hole, perhaps within a few decades.  “Eta Carinae could already have died and we do not know it yet,” notes Teodoro, “because the light leaving there takes 7,500 years to arrive on Earth.”

Project
STELES: A high-resolution spectrograph for SOAR (nº 2007/02933-3) Grant Mechanism Thematic Project; Coordinator Augusto Damineli Neto (IAG-USP); Investment R$1,373,456.33.

Scientific article
MAIRAN, T. et al. He II λ4686 emission from the massive binary system in η Car: Constraints to the orbital elements and the nature of the periodic minima. The Astrophysical Journal. V. 819, No. 2, p. 131-55. 2016.

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