An international team of researchers, including three Brazilians, has produced the most detailed three-dimensional radiograph of the cloud of gas and dust that prevents direct observation of the mysterious massive star Eta Carinae, located 7,500 light-years from Earth. The details about the structure of the entire nebula are in an article scheduled to be published in early July 2014 in the scientific journal Monthly Notices of the Royal Astronomical Society. “We are even providing a file that anyone can download from the journal’s website and use to produce a replica of the cloud on a 3D printer,” says Mairan Teodoro, one of the authors, an astrophysicist who studied at the University of São Paulo (USP) and is now engaged in postdoctoral research at the NASA Goddard Space Flight Center. For example, the Renato Archer Information Technology Center (CTI), in Campinas, printed a 15 cm replica of the Homunculus Nebula, whose full extension, from one end to the other, is on the order of 3 trillion kilometers.
The study reveals details of a structure that has formed around the star in just over 170 years. In the early 1840s, Eta Carinae, which suffers a sort of blackout for about three months every five and a half years, began to display a peculiar feature: a thick cloud of gas and dust, with a format similar to two balloons connected by a common entrance, formed around it. Called the Homunculus, this envelope of expanding material actually covers a binary system. Today there is a consensus that Eta Carinae is composed of two stars, one with 90 solar masses and another with 30, instead of a single star as was previously believed. The origin of the nebula is attributed to a series of major eruptions beginning in 1843 that caused the star system to eject huge amounts of matter, temporarily increasing its brightness.
Bulges and holes
The new three-dimensional model of the Homunculus confirms some cloud features that had already been mentioned in other works and highlights some previously unknown peculiarities. The two halves of the nebula are very similar, almost symmetrical. What is called the blue lobe, which is easier to observe since it in the Earth’s line of sight, has a bulge in its central region. This bulge forms an angle of 55 degrees with respect to the equatorial plane that divides the nebula (see figure). The red lobe, which is partially hidden from Earthly observers, also has a bulge at the same angle, but in the opposite direction.
In addition to these bulges at the heart of the Homunculus, the 3D model shows irregularities at the poles, or the extremities, of each half of the cloud of gas and dust. The blue lobe has a principal hole and a sort of relatively plane valley or depression that occupies a region around its pole. The red lobe also has a large hole, but, additionally, has smaller holes and a valley with a more variable format. The researchers believe that the nebula has these anatomical features because it formed around a binary system. “The interaction between the two Eta Carinae stars, the primary and the secondary, and their respective stellar winds must have molded these Homunculus features,” says Augusto Damineli, professor at the USP Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG), one of the prominent scholars of this system and also the author of the new study.
Within the nebula, which is almost hollow inside with a denser shell, the two Eta Carinae stars orbit around their joint center of mass. Each star produces a strong stellar wind, a stream of ionized particles that constantly emanates from its surface. The dynamics of collisions between the winds, a kind of power cord between the two streams of electrically charged particles, changes as the stars pass through the closest point of their orbit (the periastron) and the furthest (the apastron). Recently, Damineli and Teodoro showed that the periodic blackout of Eta Carinae is prolonged by the interaction of the stellar winds (see Pesquisa FAPESP Issue No. 191). Now they have gathered evidence that the irregularities in the shape of each lobe of the homunculus, described by Damineli as two Chinese lanterns expanding, appear to be a “fingerprint” of the binary high mass system concealed within the cloud of gas and dust.
Teodoro and his colleagues used a new technique to model the contours of the Homunculus in three dimensions. With the aid of the XShooter spectrograph installed at the Very Large Telescope (VLT), one of the apparatuses maintained by the European Southern Observatory (ESO) in Chile, they measured the infrared emissions of the nebula at the molecular hydrogen (H2) wavelength. “At this wavelength, we can see the back of the Homunculus, which cannot be observed in the visible spectrum,” says German astrophysicist Wolfgang Steffen, of the National Autonomous University of Mexico (UNAM). “These emissions provide information on the nebula’s speed of expansion, which increases as the dust and gas move away from the center of this almost symmetrical formation.” Data on the speed at different points in the cloud are entered into a software program developed by Steffen, called Shape, which generates a 3D structural model of the entire Homunculus. “This allowed us to uncover the geometry of the nebula,” states Teodoro.
STEFFEN, W. et al. The three-dimensional structure of the Eta Carinae Homunculus. Monthly Notices of the Royal Astronomical Society. V. 422, n. 2, p. 3316-3328. jul. 2014.