Eps Aurigae’s dark secret (interferometry rules!)

Since a few weeks some PhD students and postdocs have been organising astro-ph coffee meetings three times a week, where the youngsters in the department can sit together and chat about recent papers. The advantage of having these meetings for only students and postdocs is that we can admit to our utter ignorance about stuff we should really know about – like gamma ray burst light curves or cosmic strings – without fearing the judgment of our supervisors. An additional benefit for me is that I now have a small army of minions scoping out interesting new research for me to blog about here. Ha!

Anyway, this morning we talked about a letter in Nature by Kloppenborg et al, published today, showing some fabulous new observations of the space oddity that is ε Aurigae. This star forms part of a binary system and is unusually  eclipsed by its invisible companion every 27 years for a lengthy 18-month period. Astronomers have known about this for almost 200 years, and had hoped that the current eclipse, which began last August, would finally provide some definitive answers. The AAVSO has even enlisted citizen scientists worldwide to gather data for the star’s light curve with its Citizen Sky project, and I spotted today that ε Aurigae also has a twitter feed.

The essence of the paper is shown in the above video. These images look like a clever simulation – but these are actually real images (well, apart from the white lines, obviously). They are resolved images of the stellar disk of  ε Aurigae! They were taken last November and December, in the early phases of the eclipse, and they clearly show that something dark is moving in front of the star. I’m an instrumentalist, I’m supposed to know this stuff, what we can and can’t observe, but I really didn’t.

The images were taken in the near-infrared H band (around 1.5 micron), using Georgia State’s Center for High Angular Resolution Astronomy (CHARA) interferometer in California – so basically by combining the light from several small telescopes to get the resolution of a much bigger one. The CHARA array has 6 1-m telescopes that can be combined over 15 possible baselines. This allows it to produce images at these wavelengths with a resolution of 0.5 milli-arcseconds, while the resolution of a single 1-m telescope would be around 300 milli-arcseconds, orders of magnitude worse. This amazingly fine resolution allows it to resolve the disk, which measures a couple of milli-arcseconds across.

From these images, scientists deduce that the eclipse of ε Aurigae is caused by a disk of dust moving in front of the star. The simulation video above shows the outline of the disk that best matches the observations so far.

So does this solve the mystery? Not quite. Scientifically these images are not giving a whole lot of new info. The scenario of the obscuring dusty disk actually dates back to 1965 (Huang, 1965), and these data give further confirmation of that model. More light was shed on the matter in another recent paper by Hoard et al (2010), who brought together observational data of the binary system spanning wavelengths from the ultraviolet to the mid-infrared, which is what we need to get a complete picture of what’s going on. They conclude that the system is most likely to be composed of (1) the main star, which is a regular F-type star that’s nearing the end of its lifetime (a post-AGB star), (2) a companion B-type star, and (3) a thick dust disk around the B star that completely buries the star in its interior.

Even if this scenario is confirmed by the recent observations and those likely to follow over the course of the current eclipse, this still leaves us with questions. Lots of stars have disks around them, but we’ve never seen any that are big enough to swallow up an entire fully-formed B star. One way of getting that much dust around the B star is to have all of the stuff produced in ε Aurigae’s AGB phase pile onto the companion’s disk. Can we catch a glimpse of the central B star through a central opening in the disk? These are fascinating questions to address.

But for now, go and watch that video again. Isn’t it cool?! (I think it needs music! Someone add music?)

Spectral energy distribution (SED) of the eps Aur system. The solid line shows the combined model from all three components (F star, B star and disk). Data point symbols represent observations from AAVSO observers, 2MASS, Spitzer, MSX and ground-based telescopes. Spectra come from Spitzer, HST, IUE, FUSE and ground-based telescopes (from Hoard et al, 2010; click for better resolution).

References

Kloppenborg, B., Stencel, R., Monnier, J., Schaefer, G., Zhao, M., Baron, F., McAlister, H., ten Brummelaar, T., Che, X., Farrington, C., Pedretti, E., Sallave-Goldfinger, P., Sturmann, J., Sturmann, L., Thureau, N., Turner, N., & Carroll, S. (2010). Infrared images of the transiting disk in the ε Aurigae system Nature, 464 (7290), 870-872 DOI: 10.1038/nature08968

D. W. Hoard, S. B. Howell, & R. E. Stencel (2010). Taming the Invisible Monster: System Parameter Constraints for Epsilon
Aurigae from the Far-Ultraviolet to the Mid-Infrared ApJ accepted arXiv: 1003.3694v1

Huang, S. (1965). An Interpretation of ∊ Aurigae. The Astrophysical Journal, 141 DOI: 10.1086/148191