Meanwhile, on Centauri B…

by Paul Gilster on December 28, 2006

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 α Cen A is much greater than that of α Cen B according to their masses. Because α Cen A evolves faster than α 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 α 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.

andy December 28, 2006 at 17:00

Then again, bear in mind this is an ESA mission. That means that it’s a long time before the data gets out (nevermind that the European taxpayers are funding this, and still don’t get access to any of the data). Venus Express (another ESA mission) seems to release about two pictures a year. Policy at ESA seems to be to release the data only after the mission is complete (which is why the Hipparcos stuff is now available). We probably won’t be hearing any of the results of CoRoT for a while.

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