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A Problem of Distance
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.
Hi Brian, Iam intrigued to knowif your back of envelope diameter for the disc and the implied mass of the F starassuming a Keplerian orbit for the disc material compares with the HSSpaper figures for the Fstar. Robin
Hi Robin, With a lot of assumptions, the data imply that the ring is actually inside the F-star. If the F-star were a little closer, say 580 pc instead of the 625 pc that the HHS paper(and my Nature paper for that matter) the ring will again be on the outside of the F-star at a fairly reasonable distance. Note, moving the star 45 pc closer is actually in better agreement with the non-Hipparcos measurements, but as I mention they too have their own problems that need to be discussed. With that said, this is still very much a back-of-the envelope calculation and the periodicity of the H alpha emissions hasn't been verified. If anyone is interested in helping me do this, please let me know. This will likely be too much work for me to do all on my own. Brian
Fascinating stuff and pleased to see both van de Kamp and Heintz included (note the correct spellings!. I knew them many years back and have a couple of observations about what you've written. You should double-check various statements in what you've written because, from what I know from first-hand, personal contact they are in error! Heintz was not a student a van de Kamp's! Heintz did his doctoral work at the University of Munich (I just confirmed this by digging up his NY Times obituary-- http://tinyurl.com/39g7bq5) van de Kamp taught at the University of Virgina before coming to Swarthmore (you can confirm via the NYT and his obit). It was John Hershey, not Heintz, who discovered the embarrassing error in van de Kamp's work (how cool it would have been, though, had Barnard's Star actually had a planet-- or two!). He found that the removal of the Sproul lens for cleaning had led to its being repositioned slightly on reinstallation--and, thus, producing the slight perturbations in the star images. Among other sources, the book The Lives of the Planets discusses this. See page 244 in it http://tinyurl.com/2a9jteq (Even Wikipedia has this right!) See also http://www.american-buddha.com/barnardstarvandekamp.htm As to Heintz's old reductions, you should contact the department of Physics and Astronomy at Swarthmore directly (perhaps Jensen, the chair, or Cohen). I'll wager they still have his material or could give you clues as to where his materials might be. There's a new observatory, named after van de Kamp, so the astronomers should know what happened to the plates and dates when the old one was converted to other uses. They may still be in the old Sproul Observatory basement! Another minor aside-- you've mis-spelled its as it's in a couple of places! By the way, all three astronomers (vdk, wh, and jh) were wonderful individuals, each in their own way (very different personalities). Hershey's still going strong with the Space Telescope Science Institute--he's continued to do fine astrometric work relating to the Hubble Space Telescope, including the effects of guide stars being double. Incidentally, my early morning constellation photographs today showed little different in epsilon Aurigae's brightness compared to 3 years ago. Perhaps fainter by a tad... are we in the middle of the doughnut hole? :-)
astroman2007, Thank you for pointing out my errors. I have made corrections to my above post... I hope I caught all of them! I don't know how I confused Hershey and Heintz. I have had several conversations with an astronomer associated with DU about that very situation. It's amazing that they were able to track the apparent motion of the stars down to something as simple as the lens getting cleaned and reinserted in a different position. Perhaps now this fact will be cemented in my memory. I sent an email to Dr. Jensen right around the time I wrote this article. He was going to look through their archives or at least contact Heintz's family to see if they might have the record. As an aside, it is quite interesting to do work on epsilon Auriage because much of my work has been reading literature that is two to three years older than I am. I wish I could spend more time learning about who these people were, rather than just using their data or discussing their ideas, but this dissertation thing keeps looming above my head. Thank you again, Brian