“Even under Luminous Blue Variable [stars]η Auto is unusual and the parameters are extreme.”
That bit of scientific language roughly translates to “Even among the largest, most energetic stars, Eta Carinae has done things we can’t explain, but find incredibly impressive.” The main entry in η Carinae’s (η is the Greek letter eta) list of extreme behavior involves producing a decades-long outburst that caused it to become the second brightest star in the sky. This outburst released as much energy as a supernova and ejected many times the mass of the sun. Yet somehow η Carinae remained intact.
Now researchers have used a series of Hubble images to create a timeline of the debris left behind by this puzzling eruption. The new data shows that this was just the latest in a series of eruptions, and we still can’t explain why they are happening.
Eta Carinae is actually a binary star system. Eta Carinae B, the smaller of the two stars, is still huge, with up to 60 times the mass of the Sun. However, η Carinae A is at least 90 solar masses and possibly much larger. Both are expected to be hundreds if not thousands of times brighter than the Sun. The two share an extreme orbit of 5.5 years: their distance varies from 1.6 astronomical units (an AU is the average distance between the Earth and the Sun) to as much as 30 AU.
It’s difficult to say much more about the two stars, as they are embedded in a dense, lobed cloud of gas and debris known as the Homunculus Nebula. That material was brought in by what is called the Great Eruption. Although the stars are not currently visible to the naked eye, this was not always the case. Astronomers noticed a brightening that began in the 1820s and continued until the Great Eruption in the 1840s, after which Eta Carinae became the second brightest star in the sky. A second, smaller outburst occurred later that century, and the system has continued to fluctuate in brightness ever since.
The cause of that brightening has been determined to be a dizzying burst of matter from the surface of η Carinae A. The star is estimated to have ejected between 10 and 15 solar masses of material into space, sending it away with a kinetic energy equal to 1050 bad (just over 2 x 1027 megaton). That’s nearly the amount of energy produced in a core-collapse supernova, which normally tears its star apart. Somehow η Carinae A is still there.
So what could cause such an event? A rare class of large stars called Wolf-Rayet stars also experience sporadic matter ejection. But these tend to eject only about 10 percent of the sun’s mass — not several hundred percent.
A number of possible explanations have been proposed, all as extreme as the event itself. These include the idea that Eta Carinae started out as a three-star system and that the merger of two of the stars caused the Great Eruption. There is also the idea that η Carinae A was physically larger before its mass loss, allowing η Carinae B to collide with the outer layers of the stars at its closest approach to its orbit.
These different models have implications for the behavior of both the stars and the matter released by the Great Eruption. So three astronomers (Megan Kiminki, Megan Reiter and Nathan Smith) joined forces to take a closer look at the Homunculus Nebula to see if the debris could help differentiate between these ideas.
The three astronomers were helped by the impressive longevity of the Hubble Space Telescope. More than a decade ago, they made some observations of η Carinae using Hubble. Recently, they were given observation time specifically when the Hubble would be oriented in much the same way it was then, which allowed them to minimize perspective differences between the images. They also rummaged through the Hubble archives to find other images containing η Carinae. The astronomers adjusted these for the difference in location and aiming angle.
All told, they were able to track the material released during the Great Eruption over a 21-year period using 11 different images. This allowed them to find the relative motion of 792 individual clumps of matter in the nebula. Once adjusted for perspective, each of these objects appears to be moving directly away from the star that ejected it. And, at least within the time window examined, there was no trend in the velocity, suggesting that the material is still moving at about the same speed as when it was first ejected. (It also moves pretty fast, with speeds in excess of 900 miles per second.)
The astronomers tested this conclusion by modeling where the material would have been 50 years ago and then comparing that to an archival photograph of the galaxy. It’s all checked out.
So if you know how fast the material is moving and where it is now, you can calculate when it left its point of origin, η Carinae A. Large swathes of the material can be traced back to the early 1800s, suggesting that the major eruption actually occurred slightly earlier than the clearing. This could be explained if the erupted material caused the brightening by colliding with other dust and gas near the stars.
But some key features date back to the 13th century. And curiously, all of the material from this period is on one side of the star, suggesting that the earlier outburst was radically asymmetric. There is also evidence of a smaller eruption in the 1500s that was equally one-sided.
This quite likely rules out a merger as the cause of the Great Eruption. After all, it’s not clear that a single fusion event could trigger eruptions centuries later. In their forthcoming paper, Dr. Reiter and her colleagues: “Non-repeating mechanisms such as the concatenation of a close binary into a triple system would require additional complexity to explain the observations.” Whatever causes these events must also be able to cause asymmetric outbursts to explain the distribution of matter in the nebula.
What the mechanism might be, the three astronomers have no suggestions. The last line of their paper simply lays out the hurdle for theorists to overcome: “Models for this still-mysterious star must therefore account for the recurrence of these major mass-loss events, along with their timescales of hundreds of years and their various asymmetries.” For now, η Carinae remains a mystery and a challenge.
The arXiv. Abstract number: 1609.00362 (About the arXiv). Will be published in Monthly communications from the Royal Astronomical Society.