Meanwhile, on Centauri B…

by Paul Gilster | Dec 28, 2006 | Exoplanetary Science | 1 comment

With COROT on its way, the search for exoplanets moves into a new phase with an active, space-based transit study. Launched from Baikonur (Kazakhstan) yesterday, the mission’s status reports will be available online and should provide fascinating reading. After all, COROT will monitor 120,000 stars with its 30-centimeter telescope, looking for the signatures of planetary transits. That means the kind of ‘hot Jupiters’ we’ve already found around many stars, but it should also involve smaller rocky worlds, some perhaps not all that much larger than Earth.

But notice what COROT stands for: ‘Convection Rotation and Planetary Transits.’ The first part of that phrase refers to asteroseismology, the study of stellar interiors by examining the acoustic waves that move across the surface of stars. That means COROT will be able to detect so-called ‘starquakes’ that well up from deep inside the star. Examining their strength and duration tells astronomers much about the star’s mass and composition, and it can also help pin down the star’s age.

That last point is worth noting in relation to the Centauri stars. Yesterday we looked at the odd dimming of Centauri A in x-ray wavelengths, implying some sort of coronal cycle that had not been observed by earlier researchers. Asteroseismology is one way we can learn more about the Centauri stars, but here too there is an anomaly. It’s spelled out in an interesting paper by Mutlu Yildiz (Ege University, Turkey), who notes that our values for the age of these stars vary depending on the observing method.

Here’s the problem: We can use ground-based spectrography to gather data about the seismic properties of the Centauri stars. Indeed, the internal structure of both Centauri A and B has been widely tackled, and age estimates of from 4.85 to 7.6 billion years have emerged from this work. However, using parameters like mass, radius, luminosity, metallicity and so on instead of seismic data gives a different value indeed, suggesting they are as much as 8.88 billion years old. The seismic studies, in other words, give different values than these observations.

Here’s Yildiz on the issue:

The reason for such a great age for the system is that the observed luminosity of ? Cen A is much greater than that of ? Cen B according to their masses. Because ? Cen A evolves faster than ? Cen B, an old age is required for a simultaneous agreement between the models and observations. However, we should also question the accuracy of the observed values…

So it comes down to this. The latest seismic values that Yildiz plugs in show an age of 5.6 to 5.9 billion years for the Centauri stars. But the luminosities of Centauri A and B don’t match such a young age. In fact, based on the seismic studies’ age estimate, the expected luminosity of Centauri B is 15 percent larger than what we actually see.

Centauri B, in other words, is not bright enough for its apparent age. If it is as young as the seismic studies suggest, it should be brighter. If the seismic studies are correct, that seems to be telling us we need to reduce our estimate of Centauri B’s mass or change our value for its radius. Or — and this is the most probable answer — it may point to seismic processes that don’t fit existing models. Figuring this out may bring the age estimates into agreement, and should help to refine our understanding of seismic properties at Centauri and elsewhere.

Yildiz believes the Centauri stars are useful as a testbed whose seismic data can be measured against other systems, and he analyzes the methods that should direct such studies. The paper is Yildiz, “Models of ? Centauri A and B with and without seismic constraints: time dependence of the mixing-length parameter,” slated for publication in Monthly Notices of the Royal Astronomical Society and available online as a preprint.

The Darkening of Centauri A

by Paul Gilster | Dec 27, 2006 | Exoplanetary Science | 11 comments

Take a look at the image of Alpha Centauri in the Centauri Dreams logo. It’s the bright object at far left, not the single star that it appears but a triple system whose glare masks its two major components. Centauri A is a G2 star much like our Sun, while Centauri B is a K1. The two are separated by an average of 23 AU, with an orbital period of some eighty years. Indiscernible in the image is Proxima Centauri, an M-class dwarf which is actually the closest star to Earth.

Bright, nearby and highly studied, the Centauri stars would seemingly be well characterized. But new results from the European Space Agency’s XMM-Newton x-ray satellite show anomalies. Unlike optical wavelengths, where the larger Centauri A dominates, Centauri B is the brighter object in x-ray emissions. What’s odd is that repeated monitoring of the two by XMM-Newton shows that Centauri A faded by no less than an order of magnitude in x-rays during the two-year observing range, a behavior out of keeping with all prior observations of the star at these wavelengths.

Below are side by side views of Centauri A and B in x-ray wavelengths. Note in the image on the left that Centauri A shows up just above and left of the brighter Centauri B in these wavelengths. The later image, on the right, shows a dramatic drop in x-ray brightness.

Image (click to enlarge): Two EPIC MOS images of Alpha Centauri A+B, taken in March 2003 (left) and Feb. 2005 (right). The separation of Alpha Centauri A+B is in March 2003 around 12″, in Feb. 2005 somewhat above 10″. Alpha Cen B is the X-ray brighter object down right and exhibits a comparable X-ray luminosity in both exposures. In contrast Alpha Cen A, a star very similar to our Sun, is only visible in the left image. It has fainted in X-rays by at least an order of magnitude, a behaviour never observed before despite several observations of the Alpha Centauri system over the last 25 years. Image and caption credit: Robrade, Jan and ESA.

From a recent paper on this work:

…a strong decrease of the total emission measure is necessary to explain our ?ndings. While smaller differences in the long term evolution of X-ray luminosity may be explained by the use of the various instruments, the decline seen over the XMM-Newton campaign can only be explained by a X-ray activity cycle or an irregular event. While no de?nite statement can be made about an irregular event, the scenario of an activity cycle would require, that all previous X-ray measurements were made when ? Cen A was near the ’high state’ of its cycle. Putting all the observation dates together, this would require a cycle with a duration of ~ 3.4 years from maximum to maximum.

A definitive statement on the cause of this x-ray ‘fainting’ is impossible without further data. We do know that our own Sun has a coronal activity cycle with a period of some eleven years. Centauri A’s similarity to the Sun makes further study of Solar x-ray emissions a possible key to the mystery, but it seems remarkable that such a cycle would have gone unnoticed until now. It will be interesting to see what this ongoing observation program comes up with next. Also interesting: the XMM-Newton work has observed a flare on Centauri B, confirming its nature as a flare star.

The paper is Robrade et al., “X-rays from Alpha Centauri – The darkening of the solar twin,” accepted by Astronomy & Astrophysics, with preprint available online. Thanks to Ian Jordan (Space Telescope Science Institute) for the heads-up on this work.