Eta Carinae - ISO tells the true story
2 December 1999In 1843 the stellar system Eta Carinae suffered a violent explosion which caused it to become, in just a few decades, an amazingly beautiful nebula with two huge round blobs of material symmetrically distributed. For years astronomers have been looking for the cause of the explosion, and to explain the strange hourglass shape. A team of astronomers using ESA's infrared space telescope, ISO, have now succeeded, putting the blame firmly on a previously undetected very massive 'donut' of dust which squeezes the nebula at its centre. They publish their discovery in the current issue of the journal Nature (2 December).
"Everything seems to fit more clearly now. We certainly can explain the double-lobe shape of the system, and we may also have a good idea of the cause of the explosion itself", says main author Pat Morris, of the University of Amsterdam.
Eta Carinae, in the constellation of the same name in the southern hemisphere, has puzzled scientists ever since the famous nineteenth-century British astronomer William Herschel noticed the enormous change in the object's brightness, marking the explosion. It could not have been a supernova explosion - which happens when a very massive star ends its life - because the exploding star survived. (In fact, Eta Carinae is still unique in this respect because no other stellar object, apart from a final supernova explosion, has been known to lose so much mass so quickly and violently).
Modern astronomers have constructed several hypotheses to explain the event. One of these theories involved the presence of a disk of dust squeezing the exploded star like a tight belt, and thus pushing the expelled material into the two famous lateral blobs now seen in Eta Carinae. However, the problem was that no telescope could find this disk. Until now.
ESA's infrared space telescope, ISO, has done just that. Eta Carinae is the brightest object in the infrared - outside the Solar System - and the Amsterdam team used both spectrometers on board ISO (called SWS and LWS) to observe it. They clearly found a huge amount of mass that had gone undetected before. Then, to find how this mass was distributed, they turned to a ground-based infrared telescope at the La Silla observatory (Chile, European Southern Observatory). These additional observations confirmed their suspicions: the material was concentrated in a central torus, like a 'donut'.
The mass of the huge central torus seen by ISO is equivalent to 15 solar masses and its radius is about 1/5 light-year. The Amsterdam group, led by Rens Waters, also analysed the torus' chemical composition and compared it with that of the symmetric blobs. Now they can reconstruct the true story of Eta Carinae, as Morris explains:
"First of all, there is recent evidence that the object at the centre of the Eta Carina nebula is actually two stars. These two stars now have very elliptical orbits and they might have come close to each other at a time when one or the other was changing its size due to changes in nuclear fuel in its core. Because of the gravitational forces between them, one of the stars probably lost a huge amount of material, which then formed the torus. That event is likely to have caused the explosion seen by Herschel last century. It is as if the star got quite upset about its lost material".
It comes as a surprise that, if this explanation is true, the explosion in Eta Carinae had its real roots two millennia ago, since that's when the formation of the massive torus must have taken place. That's what the group of Amsterdam estimates, and this is supported by the data about the torus' chemical composition: it is made of material coming from the outer layers of the star, while the material in the two symmetric blobs comes from the central layers and must therefore have been expelled afterwards.
Footnote about ISO
The European Space Agency's infrared space observatory, ISO, operated from November 1995 to May 1998, almost a year longer than expected. An unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO made nearly 30 000 scientific observations.
Martin F. Kessler (ISO Project Scientist):
Tel: +34 91 813 1254
University of Amsterdam
Tel: +31 (0) 20 5925126