‘Light Echo’ Reveals Eta Carinae Puzzle

by Paul Gilster on February 16, 2012

Luminous Blue Variables are large, bright stars that give rise to periodic eruptions, like the so-called “Great Eruption” of Eta Carinae that was first noted in 1837 and continued to be observed for an additional 21 years. Things must have been lively around the companion star thought to orbit in the nebula around Eta Carinae, for the LBV blew off about 20 solar masses in this era, mimicking a supernova as it became the second brightest star in the sky. We’ve witnessed similar ‘supernova impostor’ events in other galaxies, but at 7500 light years, the Eta Carinae system is relatively nearby, allowing close study by Hubble and other telescopes.

What brings Eta Carinae’s 1837 event back into the news is the use of so-called ‘light echoes’ to study what happened at a time when astronomy was in a much earlier state. Armin Rest (Space Telescope Science Institute) notes how useful the work is turning out to be:

“When the eruption was seen on Earth 170 years ago, there were no cameras capable of recording the event. Everything astronomers have known to date about Eta Carinae’s outburst is from eyewitness accounts. Modern observations with science instruments were made years after the eruption actually happened. It’s as if nature has left behind a surveillance tape of the event, which we are now just beginning to watch. We can trace it year by year to see how the outburst changed.”

Rest is referring to light bouncing off dust clouds, echoing the cataclysm of 170 years ago so that astronomers can study it with the latest technology. The light echo technique has been employed before in the study of supernovae, culling information about the speed and temperature of the material ejected from the star. You may recall, too, that the anomalous object called Hanny’s Voorwerp, discovered by a Dutch school teacher using the Galaxy Zoo project, turned out to be a gas cloud illuminated by a beam of intense optical and ultraviolet emission from the center of a nearby galaxy, an event researchers dubbed the ‘first quasar light echo.’

Image: The color image at left shows the Carina Nebula, with the massive double-star system Eta Carinae near the top of the image. The star system, about 120 times more massive than the Sun, produced a spectacular outburst that was seen on Earth from 1837 to 1858. But some of the light from the eruption took an indirect path and is just now reaching our planet. The light bounced off dust clouds (the boxed region about 100 light-years away at the bottom of the image) and was rerouted to Earth, a phenomenon called a light echo. The three black-and-white images at right show light from the eruption illuminating dust clouds near the doomed star system as it moves through them. The effect is like shining a flashlight on different regions of a vast cavern. Credit: NASA, NOAO, and A. Rest (Space Telescope Science Institute, Baltimore, Md.).

In the case of Eta Carinae, the echo showed up when Rest and colleagues compared visible-light observations of the system with earlier observations from the Cerro Tololo Inter-American Observatory (CTIO) in Chile. An intense spectroscopic follow-up allowed the astronomers to measure the speed of the outflow (about 195 kilometers per second), and to determine its temperature (5000 K). Among Luminous Blue Variables, Eta Carinae seems unusual in that the outflow from the central region is cooler than observed in other erupting stars, a fact we’ll return to in a moment.

Having this kind of cosmic play-back gives us all kinds of interesting possibilities, including the fact that we already know that a year after its 1843 peak in brightness (which is what the current work measures), another brightening was observed in 1844. Rest’s team is thus awaiting the light echo of that event, which should be observable in about six months and will offer a more complete view of the eruption. Massive Eta Carinae, thought likely to explode as a supernova within the next million years, has much to teach us about the behavior of Luminous Blue Variables. “This star really seems to be an oddball,” Rest adds. “Now we have to go back to the models and see what has to change to actually produce what we are measuring.”

A ‘Lesser Eruption’ occurring around 1890 was observed spectroscopically and shows a strikingly different light spectrum, indicating to the authors of the paper on this work that two distinct physical processes may have been involved in the two events. Whatever the case, Eta Carinae is extraordinarily useful to astronomers because LBV giant eruptions are a rarity, with only the Great Eruption of this star and a giant eruption of the star P Cygni in the 17th Century recorded in our galaxy in the last 400 years. All other supernova ‘impostor’ events have been extragalactic. The challenges posed by this intriguing star are made clear in the paper:

η Car’s Great Eruption has been considered the prototype of the extragalactic SN imposters or η Car analogues, even though it is actually an extreme case in terms of radiated energy (1049:3 erg), kinetic energy (>1050 erg), and its decade-long duration. The spectra of the light echo indicates now that it is not only extreme, but a different and rather unique object. It is difficult to see how strong emission lines could be avoided in an opaque wind where the continuum photosphere is determined by a change in opacity, and its temperature and broad absorption lines are more consistent with the opaque cooling photosphere of an explosion. The cause that triggered such an explosion and the mass-loss without destroying the star is still unknown, but predictions from future radiative transfer simulations trying to explain η Car and its Great Eruption can now be matched to these spectral observations. Other alternative models that were proposed, e.g. the ones that use mass accretion from the companion star during periapsis passage as a trigger for the eruption, can be either verified or dismissed.

The work points to the conclusion that Eta Carinae is too cool to qualify as the same kind of supernova impostor observed in other galaxies because such stars are thought to be far hotter. This article in Nature quotes Augusto Damineli (University of São Paulo) as saying the findings have surprised everyone. “All well educated astronomers would have bet that they would find the spectrum of a 7,000-kelvin star.” Rest and colleagues are now looking for further Eta Carinae light echoes in different parts of the sky to build up a fuller picture of the eruption.

The paper is Rest et al., “Light echoes reveal an unexpectedly cool η Carinae during its 19th-century Great Eruption,” Nature 482, 375–378 (16 February 2012). Abstract / Preprint.

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{ 2 comments }

Alex Tolley February 16, 2012 at 23:28

I am fascinated by this concept of detecting past events by reflections. I would never have guessed that was possible. Very cool.

Quiet Professional February 17, 2012 at 2:34

Light echos. Wow. Humans are damn clever creatures. I wouldn’t bet against them!

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