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astrophysics

Unique brilliance

How the Eta Carinae star took on importance and attracted the attention of physicists all over the world

EDUARDO CESARAn amateur group’s spectrographEDUARDO CESAR

A solitary journey has finally ended for a man who for over 15 years held his own ideas and challenged solid scientific reputations to prove that his conclusions could be right. The astrophysicist from Paraná, Augusto Damineli, a professor at the University of São Paulo (USP), had started in 1989 to observe Eta Carinae, the largest and most luminous star of our galaxy, the Milky Way, and in the years that followed came across a phenomenon that had not yet been described: a brutal reduction in the brilliance of this star, which loses every day a quantity of light equivalent to that emitted by hundreds of Suns. In the beginning, the specialists from the United States and from Italy who had been working for decades on this star did not take him seriously. They seemed to be indignant with the possibility of a Brazilian, with what they called a telescope in the jungle, having observed what they themselves had never seen, with far more powerful equipment. Damineli observed the fading the luminosity loss in some bands of radiation for the first time in June 1992, and in the following year he concluded that the phenomenon ought to repeat itself every five and a half years.

At the end of 1997, when the fading was repeated, attesting to his forecasts, he won the support of a few American and Brazilian physicists, but it was only this year that he managed to turn the game around. At the end of June, at least 50 astrophysicists from Brazil, the United States and Argentina, based on the results verified by eight terrestrial and five space telescopes, confirmed the brutal loss of Eta Carinae’s brilliance and gave strength to the model that the Brazilian researcher had formulated to explain the fading: located 7,500 light-years from the Earth, equivalent to 68 quadrillion kilometers, Eta Carinae may not be one, but two stars one smaller and hotter, with a temperature of about 30,000 degrees, and another, three times larger, colder (15,000 degrees), and at least ten times brighter. The two are enveloped by winds that collide and generate temperatures of 60 million degrees and by a dense cloud of gases and dust a nebula which extends for 4 trillion kilometers, equivalent to400 times the diameter of the solar system.

With a radius equivalent to the distance between the Sun and the Earth, this star can be seen normally using binoculars, to the right of the Southern Cross, although the fading can only be registered by more sophisticated equipment, as it occurs in specific channels of light the spectral lines , in the radio, X-ray and infrared bands. It is now cogitated that the loss of luminosity may be a consequence of the two stars drawing closest together, known as the periastron, which would happen every five and a half years: it is when the smaller star occults almost half of the other. The physicists thought for years that they were dealing with an eclipse, but instead of all the bands of light emitted by different chemical elements disappearing at the same time, as happens in an eclipse, only some vanish, and some before others, as if only some television channels were to go off the air.

Plunge into the winds
Thinking on the basis of the binary model, the smaller star, surrounded by a rarified atmosphere, plunges into the dense winds that form a kind of atmosphere swathed around the larger star. As the smaller star enters the winds of the larger star, the high-energy signals, those emitted by atoms of the smaller star itself, disappear, but the low energy channels coming from the main star remain unaltered. His calculations indicate that the smaller star, usually 4 billion kilometers away from the larger one, comes within 300 million kilometers at the moment when it gets the closest. “Apparently”, says he, “at this moment of closest proximity, the attraction of the gravity between the two stars is so strong that waves and eruptions of gas arise on the surfaces of both of them, as a consequence of a tidal effect of one on the other.” All this movement of going into the atmosphere of the larger star and leaving it lasts some two months. This year’s fading, for example, began sluggishly in March, reached its apex on June 25, and ended in September, after the brightness of the star had diminished the equivalent of 20,000 Suns. But the high-energy channels revived slowly, like a person who faints and recovers his consciousness little by little. For Damineli, the fact that the star does not go back to shining as it used to indicates that it is dragging bits of the other one with it.

The megaoperation of following the fading this year brought a little bit of peace, besides international scientific recognition, for this obstinate citizen of Paraná, accustomed to challenging his own luck. As a child, he lived in a tumbledown shack of planks and a dirt floor, on a little farm by the side of a river and a forest, in the municipality of Ibiporã, in the north of Paraná and home to 20,000 inhabitants. He learnt to work on the smallholding at the age of 7, but he refused to accept the prospect of spending his life with a hoe in his hands, and struggled until he joined the nuns? school in town, where he only learnt to read at the age of 9. Damineli arrived in São Paulo in 1968, at the age of 21, to finish high school. He worked on building sites, jotting down the production of the workmen and arrival of materials, and he battled to get a scholarship at a night school and changes jobs: he went into an office and was able to study more.

Solidarity
Damineli stumbled across the first intriguing signs of high-mass stars, in 1989. In 1992, he detected the fading, still without knowing what it was, noting that one of the high-energy channels, the helium one, disappeared and came back to normal two months later. He told the episode to an Italian physicist, Roberto Viotti, who had already studied these oscillations, and proposed that they should join in publishing their observations. The reaction was far from a friendly one. “He said that he couldn’t run the risk of having shame brought on him for presenting data that would hardly be repeated”, says the physicist from Paraná. For him, the observations and other studies that showed the fading in different wavebands since 1981 led to the conclusion that the phenomenon ought to repeat itself every 2,014 days. “At the congresses, everyone would say that the results were interesting, just to be polite”, recalls Damineli, who has enjoyed the support of FAPESP since 1992, through projects that add up to R$ 60,000.

The conversations were not making much headway, because Eta Carinae broke one of the rules the models that governs the behavior of the stars, the Eddington limit, according to which very large stars would have no other fate than to evaporate which is why more luminous stars do not exist. Eta Carinae would not come out of the so-called prohibited zone of the Eddington limit, which stars perchance enter, when they expel matter, and then go back to the normal situation, but in irregular and unforeseeable cycles. “To show that there was a periodicity in the variation of luminosity was as if a meteorologist were to say that it is going to rain every Sunday”, likens Damineli. The mood only improved in 1996, with the publication of an article with these ideas, in the Astrophysical Journal Letters.

“Nobody wanted to sign this article with me”, he says, apparently without resentment. “From that moment onwards, I had no reason for believing more in data from renowned scientists than in the data in my own hands.” In the following year, the Brazilian researcher joined up with Peter Conti, from the University of Colorado, and Dalton Lopes, from the National Observatory, in Rio de Janeiro, and, together, presented the binary star model in an electronic magazine, New Astronomy . Two issues later, Kris Davidson, an astrophysicist from the University of Minnesota, United States, who has been accompanying Eta Carinae since the 60s, drew up arguments counter to this idea. Soon afterwards, Mario Livio, from the Space Telescope Science Institute, United States, proposed another theory: Eta Carinae was not a system of two stars, but of three.

How is it that Damineli managed to see what the others couldn’t? He says that he just used the most suitable technique for the telescope with a 1.6 meter diameter mirror at the National Astrophysics Laboratory (LNA), in Brasópolis, in the south of Minas Gerais the so-called jungle telescope, adapted a camera that captures infrared, a wavelength close to visible light, and shows the star through the dust of the nebula. This year, over almost 30 nights running in June and July, Damineli, now at the age of 56, sat down at this same telescope to observe his favorite star, which appeared at the beginning of the night, radiant in a cloudless sky. On these same nights, glued to a small observatory built in Mairinque, 120 kilometers from São Paulo, and equally fascinated by the strange star, were two amateur astronomers: chemical engineer Tasso Napoleão, aged 54, and a 30 year-old physicist, Rogério Marcon, a optics technician with the State University of  Campinas (Unicamp). In conjunction with Damineli, they had planned to accompany the star by means of a piece of equipment capable of differentiating the light emitted by each chemical element a spectrograph that they themselves had built. Even a piece of equipment smaller than the jungle telescope recorded the fading.

More surprises
During the day, astrophysicist Zulema Abraham and postdoctoral student Tânia Dominici, from USP, followed Eta Carinae in another band, of radio waves, by means of the Itapetinga radio telescope, in Atibaia, 60 kilometers from the São Paulo state capital. Analyzing the data, Zulema discovered something out of the ordinary: the emission in radio waves resulting from the clash of the winds of the stars, which became visible because of the fading of the giant disc of ionized (electronically charged) gas that encircles the star, with an extension in the order of one hundred times the orbit of Pluto. “Other stars too have ionized regions, but not as intense”, comments Zulema, who has been collaborating with Damineli since the 1998 fading. In another line of research at the Astronomy, Geophysics and Atmospheric Sciences Institute (IAG), Vera Jatenco-Pereira and her pupil for a doctorate Diego Falceta Gonçalves simulated the collision of the winds to explain the production of dust. According to what they discovered, the dust may form closer to the stars and in much less time than used to be believed: the collision makes the temperature of the gases of the winds exceed 60 million degrees, and the particles of dust solidify, at 1,000 degrees, in less than one day. They are mulling over the possibility that this process of dust formation may be one of the reasons for the fading of Eta Carinae.

Information that is arriving is also fueling the studies into the dense cloud of gas and dust the nebula than was given the name of Homunculus, for being reminiscent of a primitive doll. In fact is there nothing simple in this star there are two homunculi: the larger, formed from a gigantic eruption of matter that took place in 1843, and the smaller one, one quarter of the former in size, created in another eruption, far more discrete, in 1890. Also at the IAG, Elisabete Gouveia Dal Pino, one of her postdoctoral pupils, Ricardo Gonzalez, and a colleague from the University of Mexico, Alex Raga, concluded, by means of numerical simulations, how the smaller homunculus was formed, around the central region of the star, taking advantage of the spaces left by the bigger homunculus, which seems to have resulted from the collision of the gases from the great eruption 1843 with the gas that already surrounded the star. What no one can explain is how Eta Carinae survives such intense eruptions without turning inside out. In 1843, the star released a quantity of matter equivalent to three Suns at least and shined so much that it became visible in daylight, rivaling Sirius, the brightest star in the sky, just 10 light-years from the Earth. In accordance with the theory, the nucleus would have to explode to release so much energy. The star continued to shine, though.

Eta Carinae is dying. Formed about 2.5 million years ago, it should fade out within 500,000 years at the most. The estimate is that the larger star of Eta Carinae was born with 120 solar masses, and that today, after so many eruptions, it has some 70, with a margin of error of 20 masses more or less. The astrophysicists want to discover its mass with the greatest possible precision, because from that point they can estimate how much life is still left for it. It is dies soon perhaps 10,000 years hence still with plenty of mass, it may emit a brutal load of gamma rays, capable of finishing off life in the Southern Hemisphere of the Earth.

Clocks and hearts
It is probable that more astrophysicists will be interested in the star and decided to study the next fading, whose start date Damineli has already set in stone: January 15, 2009, or one day before or after. The study of the largest star of the Milky Way is now part of a priority a Treasury project at Nasa, the American space agency, in which Damineli and by another 12 astrophysicists, from the USA, are taking part, and which has a budget of US$ 10 million to cover the costs of the 72 orbits of the space telescope Hubble in the course of the year 2003. What arrived from Hubble forms a database with 240 gigabytes of information, equivalent to 340 CDs packed with figures, about the behavior of the light, in all its wavelengths, emitted by each kind of the star’s chemical elements a database that, little by little, is becoming public and should provide input for studies about this star.

Damineli called attention to this star, but the fact that his ideas have been recognized in other countries may, ironically, generate an effect that is opposite to what was expected. The principal researcher of Nasa’s project is Kris Davidson, who wants to prove precisely the opposite: the fadings are not periodical and precise, like a clock, but just cyclical and repetitive like heartbeats a subtle difference in the focus adopted until now, but capable of overthrowing the double star hypothesis. For him, only stars smaller and younger than Eta Carinae could show such rhythmical oscillations in luminosity. A star with a very great mass would quickly lose the beat as he is said to have told Damineli in 1997, it would be like an elephant trying to dance samba.

The Project
Computing Equipment for Infrared Astrology Project; Modality Regular Line of Grants for Research Projects; Coordinator Augusto Damineli Neto – IAG/USP; Investment R$ 9,856.00

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