About three weeks ago I went through the second-to-last administrative hurtle in the process to getting a Ph.D.: a dissertation proposal defense. During this meeting I presented what I propose to do for my dissertation and defended the topic and methods to my committee. Now all that remains is actually doing the work, writing the dissertation, and defending it in front of my committee. (On a side note, Aaron just defended his dissertation this week. He did an excellent job and passed. Congratulations to Dr. Aaron Price!)
Over the next few weeks I'm going to discuss what I have identified are the fundamental problems with studying epsilon Aurigae. As you could guess, these are the beginnings of a few chapters in my dissertation, but they will also serve quite well as literature reviews.
The first fundamental problem with studying epsilon Aurigae is that the distance to the F-star is not well defined. Without an accurate distance several things cannot be properly calibrated: the absolute magnitude of the F-star (and hence it's luminosity) and it's size (from interferometric measurements).
The most recent parallax measurement comes from a re-reduction of data from the HIPPARCOS satellite. This information puts eps Aur at 654 +- 551 pc (pc = parsec, between 336 and 3930 light years away)! In galactic terms that's either in our own back yard or 4% of the way across the galaxy! The reason for this high degree of uncertainty is twofold: 1) the distance to epsilon Auriage is near the formal limit of what Hipparcos can measure and 2) most of the observed motion over the 3.5 year mission was orbital, therefore the parallax was calculated from an orbital solution.
There are, however, other distance estimates for the epsilon Auriage system. One of the oldest references is by Strand. In his paper he measured the parallax from 124 photographic plates taken over 56 nights between 1926 and 1958. I have not fully read into how these measurements were conducted, but our local astrometry expert suggests these were most certainly visual estimates and that the uncertainty in the measurement could be reduced if the plates were re-measured using modern techniques.
A slightly more modern estimate of distance comes from van de Kamp in his 1978 paper. The distance estimate was derived from 1090 photographic plates taken at the Sproul Observatory between 1939 and 1977. The distance estimate from this data is 581 +- 0.27 pc which clearly agrees with the Hipparcos estimate, but one thing is troubling. The orbital parameters used in the fit are likely incorrect. van de Kamp lists the relative positions of the F-star as a function of time and also provides an image of the orbit on the sky (see figure below). !
Caption: From van de Kamp 1978. Notice the angle of the orbit in the inset.
The motion in declination does not agree with the interferometric images we published earlier this year (the angle the in declination is incorrect). Using the first two data points in our series of observations I created an orbital solution for the motion of the F-star that shows much higher inclination:
Now, even though I could only use two data points (which I caution makes this result VERY preliminary, and unconfirmed... so be skeptical about it) I can say that the angle the eclipsing object has made with the F-star has remained nearly constant so the angle in declination is more like 20 degrees rather than zero as implied by van de Kamp.
A more recent work by Wulff Heintz provides another distance estimate.
An interesting historic note is that Heintz was a student of van de Kamp, but after Heintz showed that van de Kamp's claim of finding a planet around Bernard's Star was due to flawed data, their friendship was in disarray. (These statements are in error, thank you to astroman2007 for pointing this out. astroman2007 provides three links below that explain the situation that I mistakenly misconstrued.) Heintz re-analyzed the data from van de Kamp using slightly improved methods and additional data and came up with a slightly further, but still consistent 606 +- 55.1 pc for eps Aur. Unfortunately very little additional information is available about this measurement. Dr. Heintz passed away in 2006. and I have not been successful in reaching anyone at the Sproul Observatory to see if his observing / reduction notes are still available. [I have recently contacted the chair of the Physics and Astronomy department at Swarthmore College to see if additional information can be retrieved. Keep your fingers crossed!]
As part of my dissertation I'm attempting to reconcile this distance problem. First I'll see if I can get any additional information from the aforementioned measurements. If that fails, I have a recourse using a very “synergistic” approach: spectroscopy and interferometry. Back in 1994 Cha et. al. published a paper discussing a possible ring of material around the F-star which had been first proposed by Margherita Hack in one of her earliest works on epsilon Auriage.
Caption: A proposed ring around the F-star in Cha et. al. 1994.
In Cha's paper they notice a semi-repetitive behavior in the blue and red-wing emissions around the Hydrogen alpha line. Using this data and an angular diameters from interferometry I came up with a back-of-the envelope distance estimate of about 580 pc! I haven't finished analysis here nor have I confirmed that the H-alpha variations are indeed periodic. This work will take several more months to complete, but with uncertainties propagated I think it could yield one of the better estimates for the distance to the system.