Eta Carinae is a massive, bright stellar binary system. The more massive component is one of the largest and most luminous stars known. In the central region of the binary, the powerful stellar winds from both stars collide at speeds up to 10 million km per hour. An international research team led by Gerd Weigelt from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, including SA Astronomical Observatory postdoctoral fellow, Dr Nicola Clementel has for the first time studied Eta Carinae using near-infrared interferometric imaging techniques. The team obtained unique images of the wind collision regions between the two stars. These discoveries improve our understanding of this enigmatic stellar monster. The observations were carried out with the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO).
“There is a long history of South African interest in this truly remarkable star, which continues to astonish us,” says Prof Patricia Whitelock (SAAO & UCT). During the ‘Great Eruption’ it would have been the second brightest star in our skies and its changes were followed and recorded by several well known people, including Burchell, Herschel and Maclear. Very much more recently, observations that led to our understanding that Eta Carinae was actually a binary star were made from SAAO at Sutherland in the Northern Cape.
The more massive of the two stars in the Eta Carinae system, called the primary star, is a monster because it is about 100 times more massive and five million times more luminous than our Sun. In late phases of the evolution, such massive stars lose huge amounts of gas before they explode as a supernova. Studies of this dramatic mass-loss process are important to improve our understanding of stellar evolution.
Both stars of the Eta Carinae binary system are so bright that the powerful radiation they produce drives matter from their surfaces in the form of massive, fast stellar winds. These high-velocity stellar winds violently collide in the space between the two stars. Extreme physical processes occur in this innermost region, where the very fast stellar wind from the less massive but hotter companion star crashes into the dense primary star wind with a velocity of about 3 000 km per second (more than 10 million km per hour). In this collision region, temperatures reach many tens of millions of degrees, hot enough to emit X-rays. In the past, it was not possible to resolve this violent collision zone, because its extension is too small even for the largest telescopes.
For the first time, an international team of astronomers led by Gerd Weigelt from the Bonner Max Planck Institute for Radio Astronomy has obtained extremely sharp images of Eta Carinae (see Fig. 1) by using a new imaging technique based on long-baseline interferometry. This technique combines the light from three or more telescopes to obtain multi-telescope images called interferograms. From a large number of interferograms, extremely sharp images can be reconstructed using sophisticated image reconstruction techniques. This interferometric imaging method can achieve a resolution that is proportional to the distance between the individual telescopes.
The new Eta Carinae observations were carried out with the AMBER interferometry instrument of ESO’s Very Large Telescope Interferometer(VLTI; Fig. 2). . The team combined the infrared light from three of the movable VLTI telescopes with 1.8-metre mirror diameter. Because the largest distance between the telescopes was about 130 metres, an angular resolution was obtained that is about 10 times higher than the resolution of the largest single telescope.
“Our dreams came true, because we can now get extremely sharp images in the infrared regime. The ESO VLTI provides us with a unique opportunity to improve our physical understanding of Eta Carinae and many other key objects,” says Gerd Weigelt.
The applied high-resolution imaging technique allowed the team to obtain, for the first time, both direct images of the stellar wind zone surrounding the primary star and the collision zone in the central region between the two stars (Fig. 1). Because this technique provides both high spatial and spectral resolution, it was possible to reconstruct images at more than 100 different wavelengths distributed across the Brackett Gamma emission line of hydrogen. This is of great importance for astrophysical studies of Eta Carinae, because these multi-wavelength images show both the intensity and the velocity distribution of the collision region. Velocities can be derived from the multi-wavelength images because of the Doppler effect. These results are important to improve physical models of the wind collision zone and to better understand how these extremely massive stars lose mass as they evolve.
“The unprecedented level of details of this VLTI multi-wavelength observations is at the same time fascinating and challenging. The high-quality data allow for better understanding of the physical properties, but also place stronger constrains which require an increased effort in modelling this fascinating object. These techniques and new instruments also provide new possibilities for studying stellar outflows,” explained Dr Clementel.
Original paper: Weigelt et al.: VLTI-AMBER velocity-resolved aperture-synthesis imaging of Eta Carinae with a spectral resolution of 12 000, 2016, Astronomy & Astrophysics, Online Publication October 19 (DOI: 10.1051/0004-6361/201628832) : www.aanda.org/10.1051/0004-6361/201628832