14 February 2012

Naked Stellar Core

Title: Discovery of a stripped red giant core in a bright eclipsing binary star
Authors P.F.L. Maxted, D.R. Anderson, M.R. Burleigh, et al.

Before delving into the topic, it should first be pointed out that the discovery paper was first published in the Monthly Notices in September of 2011. The article can be found here.

Figure 1. Phase folded light curve for 1SWASP J024743.37-251549.2.
The title of the article is very effective at describing the system presented in this paper. Maxted et al. announced the discovery of an eclipsing binary system in which the primary star completely occults the secondary star. However, the secondary star is found to have a higher effective temperature than the primary star. Photometric analysis suggests the primary is an A star and that it contributes approximately 90% of the total flux of the system.

Figure 1 neatly elucidates this information. The deeper eclipse (at phase 0) indicates when the cooler star is passing between our line-of-sight and the hotter star. However, the fact that the eclipse profile is flat at the bottom implies the hotter star is being completely occulted, yet the total flux received from the system is hardly affected.

To further complicate things, a rough age estimate can be obtained from the system's kinematics. Space motions indicate the system is a part of the galactic thick disk, meaning the system has undergone significant disk heating and has acquired a larger vertical component to it's motion than would be expected from a young system forming in the galactic thin disk. The characteristic age for the galactic thick disk is > ~7 Gyr meaning the system has an age well older than the lifetime of an A star!

So what is going on here? Numerical modeling has shown that the system is consistent with a red giant that has had its outer layers stripped off, leaving behind a He core with a H envelope. There is some degree of H burning taking place in a shell around the inert He core. How was that mass stripped off and where did it end up?

Since the secondary was plausibly a red giant before it had mass stripped away and since the primary is, as far as we can tell, a normal main sequence star, the secondary star must be more massive to be in a more advanced evolutionary stage. As the more massive star puffed up when it started to become a red giant, it overflowed it's Roche lobe and began funneling material to its lower mass companion. Eventually the mass transfer halted and left the system in the state we find it today.

Stellar evolution dictates that a star will begin it's ascent up the red giant branch once it has exhausted most of the hydrogen in its core. However, the core is not yet hot enough to ignite helium burning (hence the core contracts and the exterior inflates to conserve flux), leaving the star with an inert He core. It will live out the rest of it's life as a He white dwarf once the H shell burning halts (pre-He-WD). As for the star that received the additional mass, it is now living the life of a higher mass star (hence the A spectral type). These particular stars are known as blue stragglers.

The system is rare and exciting, but the WASP team has indicated they have more examples of blue straggler/pre-He-WD systems. Stay tuned and keep an eye out for more of these remarkable systems!