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Alternative route

The discovery of a two-star, high-mass system undergoing fusion indicates that they underwent a different evolutionary path

060_061_Astrofisica_236The discovery of one of the rarest and least common objects in the Cosmos, made by an international group of researchers including Brazilians, could lead to a more refined understanding of how high-mass stars much larger and more luminous that the Sun evolve. The object is known by the acronym VFTS 352 and is located in the Tarantula nebula, also known as 30 Doradus, which is part of the Large Magellanic Cloud, one of the satellite galaxies of the Milky Way, about 160,000 light-years from Earth. It is composed of two type-O blue stars, with a combined mass 58 times greater than that of the Sun, which are in a phase called “overcontact.” This expression means that one star is basically glued to the other, sharing its envelope, its most external layer. According to the astrophysicists, this situation means that the stars must be merging.

“We know of only three other systems with this configuration,” explains Leonardo Almeida, who is doing postdoctoral research at the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP) and is the first author of the article reporting the discovery, to be published in October 2015 in Astrophysical Journal.  “VFTS 352 is the most interesting and important, because it is the most massive and the hottest.” The binary-system stars are already sharing about 30% of their envelopes. As time passes, they will probably become a single star. But this will not happen any time soon. “Although the system is expected to evolve very quickly on the star time scale, we will not see anything radical in the coming centuries,” affirms astrophysicist Augusto Damineli, also from USP and co-author of the article.

The system is interesting because it allows practical study of an alternative evolutionary path for high-mass stars. Until recently, it was believed that they formed in isolation, and not also in pairs as seems to be the case of the VFTS 352 system. “The theory of stellar development was based on less-massive, isolated stars,” says Cássio Leandro Barbosa, an astronomer specializing in high-mass stars who did not participate in the study. “In this case, we have a double violation of these conditions, so they should be different from what theory predicts.” Early measurements indicate that the system’s stars are hotter than would be expected according to the traditional model. Their temperature surpasses the expected 40,000 Kelvin (K). “Recent discoveries such as this show that at least 70% of type-O stars interact in dual systems,” says Damineli. An important aspect is that stars of this type are the main sources of oxygen in the Universe.

Stars are classified according to temperature. This, in turn, can be associated with mass, at least when stars are in what is called the principal sequence and use hydrogen to feed the nuclear reactions that make them shine. Type-O stars are the largest, followed by types B, A, F, G, K and M. The Sun, of modest size, is type G.

The discovery of the binary system was made by two simultaneous studies, the VLT-Flames Tarantula Survey and the Tarantula Massive Binary Monitoring project. Both used the Very Large Telescope at the European Southern Observatory (ESO) for their observations. The finding was considered so important that it even led to observations made with the much sought-after Hubble Space Telescope.

Element factory
The Big Bang, the event marking the beginning of the Universe, produced only two elements in significant quantities: hydrogen and helium. The first stars are believed to have arisen from these primordial atoms, clumped into gaseous clouds by gravitational forces. As the mass begins to contract due to gravitational forces at the center of the star, internal pressure and temperatures become so high that the hydrogen nuclei begin to fuse, forming helium. This is the reaction that produces the star’s energy. However, when the hydrogen at the star’s center runs out, the process begins again with increasingly heavier elements. First helium, then oxygen, and then up the periodic table until reaching iron.

The greater the star’s mass, the greater its internal pressure and temperature, and the greater its ability to produce heavy elements. At the end of its life, when nuclear fusion is no longer possible, type-O stars disappear in violent explosions known as supernovas. These events produce all of the elements heavier than iron. The atoms making up Earth and its inhabitants arose thanks to this cycle, carried out by stars more than 13 billion years ago.

The details of the proportions of production of these elements, however, are still far from known. “Astronomers tend to do chemical element production accounting assuming that stars are isolated,” says Damineli. “Discoveries like VFTS 352 force us to redo our calculations, taking into consideration the high level of duality of high-mass stars.”

In its final stage, the VFTS 352 system could produce what astronomers call a long gamma-ray burst. “When these objects explode 12 billion light-years away, they can interrupt telecommunications if their axis of rotation is aligned with Earth,” says Damineli. “If it dies in this way, since it is less than 200,000 light-years from us, it will be more than a spectacle, the system will be a potential problem for any inhabited planets in the path of the gamma-rays.

If emitted in Earth’s direction, the gamma rays would be unable to pass through our atmosphere, but could destroy the Ozone layer and, thus, expose living beings to harmful ultraviolet rays from the Sun. This demonstrates how certain astrophysical events can be hostile to life even from a huge distance. However, Leonardo Almeida reminds us that this event will only take place millions of years in the future: “And the likelihood that the gamma-ray beam will come towards Earth is very, very small.”

Accurate distances to young clusters through massive eclipsing binaries (nº 2012/09716-6); Grant Mechanism Postdoctoral research grant: Principal investigator Augusto Damineli (IAG-USP); Recipient Leonardo Almeida; Investment R$239,299.28 and US$44,400.05.

Scientific article
ALMEIDA, L. A. et al. Discovery of the massive overcontact binary VFTS 352: Evidence for enhanced internal mixing. Astrophysical Journal. In production.