For a long time now, my day job has been “theoretical physicist,” as a quick glance at my papers will confirm. But it was not always thus! Very few people are actually born as theoretical physicists. When I was an undergraduate astronomy major at Villanova, I wasn’t thinking about quantum field theory or differential geometry; I was working on photometric studies of variable stars. My personal favorite star was Epsilon Aurigae, a mysterious eclipsing binary. One of the very few stars out there that has both a Facebook page and a Twitter feed. And now Epsilon is in the news again!
Among this star’s claims to fame is that it has the longest period of any known eclipsing binary: over 27 years. But it’s not just about facile record-holding; this system is truly puzzling, especially the nature of the secondary (the thing that eclipses the primary star). The basic problem is that the eclipse has a fairly flat bottom, as seen in this light curve from the previous eclipse in 1982-84.
A flat-bottomed light curve is usually associated with a total eclipse; the secondary completely blocks the light from the primary for a while. But in this case, the spectrum of the system seemed to remain unchanged, indicating that most of the light was still coming from the primary star, even in the middle of the eclipse. This led Huang in 1965 to propose a clever model, in which the secondary is actually a disk seen edge-on; the eclipse is therefore not total, but the disk blocks out part of the light without emitting much of its own. And indeed, with modern infrared telescopes we can discern the light from the secondary — it does look like a relatively cold disk, about four astronomical units in radius, with a hot central star.
The 1982-84 eclipse raised a problem with Huang’s model, however. If you look closely at that light curve above, you’ll notice that it gets brighter right near the middle. (The gap in data is from when the star was behind the Sun and unobservable.) Your first guess is that this is probably just a fluctuation in the in brightness of the primary star; but it turns out that this can’t be right. The primary is indeed variable, but its color changes in lockstep with its brightness, an effect that can be measured by observing with different filters. And the mid-eclipse brightening shows no variation in color. It’s not due to variability in the primary; somehow the disk is letting more light past, right during mid-eclipse.
This is where I come in, as an undergrad doing research under Ed Guinan. For my undergraduate thesis, we tried to put together the most sensible picture we could manage of Epsilon Aurigae; our picture has now become the consensus model. (One difference is that we thought the primary was a supergiant, but now it appears that it’s likely to be a smaller star.) As far as the disk is concerned, we built on a variation of Huang’s disk model, due to Wilson in 1971. Wilson suggested that the disk should be thin and tilted, rather than thick and edge-on, with a semi-transparent inner region. In that case, we could imagine that the central star (presumably holding the disk together) could clear out a region near the center, and light passing through that hole could account for the brightening. I wrote a simple computer program (QuickBASIC on an IBM PC!) to calculate the light curve in this model, and we were able to get an extremely good fit to the data. Here’s the killer plot from our paper, with three different models: edge-on thick disk, opaque tilted disk with a central hole, and tilted disk with a central hole surrounded by a larger semi-transparent region.
And now, it’s eclipse time again! (I’m getting old.) Epsilon started going into eclipse in August 2009, right on schedule, and the eclipse is predicted to last until May 2011, so we’re just a bit before mid-eclipse right now. Here’s the current light curve. But technology has advanced quite a bit since my student years. Nowadays, we don’t need to puzzle out the meaning of a light curve and come up with an elaborate story involving tilted disks; we can just take pictures of the thing.
And that’s just what we did. Under the leadership of Brian Kloppenborg and Robert Stencel at the University of Denver, we put together a proposal to observe the eclipse using CHARA, the Center for High Angular Resolution Astronomy. CHARA is an array of optical telescopes on Mt. Wilson that act as an interferometer, enabling extremely high-resolution imaging of astronomical objects. And by “we,” I do mean that I was included in the proposing team — a little return to my roots. My contributions to the final results were not zero, but they were small; the lion’s share of the credit certainly goes to Brian and Bob and the rest of the team.
And our results just appeared in the form of a paper in Nature. Unfortunately behind a paywall, but there is an extensive NSF press release. But who cares about the words? The stunning things are these pictures — you can actually see the disk begin to move across the surface of the star. There’s even a video where you can compare a model to the actual data. Click to embiggen.
I have to admit that, while these images are unambiguously amazing, the result is somewhat bittersweet for me. John Monnier, another one of the team leaders, admits that he was skeptical about all this tilted-disk business; it just seemed like a house of cards. But I wasn’t skeptical for a moment, having gone through the work of trying to fit the data with various different possibilities. The extraordinary thing to me about observational astronomy was always how you could put together an apparently baroque model of some complicated system, just on the basis of a precious few data points, and yet have some degree of confidence that you were on the right track. Reality is very constraining. So in some perverse sense, it almost seems like cheating to actually take pictures of the thing. Where’s the fun in that? (Of course it’s a great deal of fun.)
And now we need to see what happens at mid-eclipse! I predict we’ll be able to image that small central hole we posited many years ago. Not at all certain about that — I suspect the star at the center of the disk is pretty active itself, and there are probably significant variations in the opacity of the central disk region. That’s what data are for. I will always love being a crazy-eyed theoretical physicist, but there’s something uniquely rewarding about digging into the data and coming to an understanding of far-flung pieces of our universe.