Forums / The Science / Data Analysis / A Delay in First Contact?
A Delay in First Contact?
As noted by Jeff Hopkins in the Epsilon Aurigae Campaign newsletter and a recent Astronomer's Telegram, the Eps Aur eclipse seems to have started later than was predicted from previous eclipses. It has been noted (in Hopkins and Stencel's book) that the overall length of the eclipse has been shortening from year to year. Sky and Telescope's article on Eps Aur (see page 3) has a figure that shows a hint of the shortening eclipse.
Past explanations to this problem have been:
1) The Disk is shrinking in width.
2) The primary star is shrinking.
3) The out of eclipse light variations are causing misinterpretations about the start-time of the eclipse.
Can you think of any other explanations for the eclipse shortening? What things might you consider as possibilities? What sort of observations would you need to confirm your theory?
HelloIMO one should look more into cases like e.g. EE Cep or KH 15D than the zet Aur EB systems to find some other ideas.So I've a few other ideas:4) The disk is symetric, constant in radius, but the orbit of the secondary around the primary is a bit eliptical and preceding.5) The disk is symetric, constant in radius, but a bit inclined against the orbital plane of the primary and secondary and the disk is preceding.6) The disk is a bit asymetric and preceding.7) The period changed or is not sufficiently determined yet (point 3 might have influenced the 1st to 4th contact times and therefore the E0, P, D, and d determination in the past too).CSWolfgang
If the primary star were shrinking, wouldn't the depth of the eclipse also change as proportionately more of the star would be covered by any given disk thickness ? Unless of course the primary's luminosity increased as it shrank but wouldn't we then see a temperature / colour change too ?
Aaron, Are we certain that the eclipse length is wavelength dependent? I have been following Richard Miles' high precison V, Ic photometry this eclipse and so far V-Ic has been very constant and similar to the out of eclpse value.Robin
Hi Robin!There is reddening during eclipse. Attached is CMD of ingress and egress of Eps Aur from Jeff Hopkins's 1982- 88 data (see http://www.hposoft.com/Campaign09.html). As you can see reddening is quite similar to interstellar reddening law R(V)= 3.3.Where A(V)= R(V)*E(B-V). We have for Eps Aur R(V)= 3.28.
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Hi Nikki, I cannot access Jeff's website currently but from what I have seen, while there is certainly a relationship between V and B-V, this also exists with the out of eclipse variations (Cepheid type pulsations of the primary?) I am not convinced that there are any B-V colour changes directly related to the eclipse. See Schmidtke's analysis from the 1985 NASA conference for example, particularly fig 4 on page 74 (attached) There is no obvious correlation between the colour and the brightness changes at ingress and egress. Robin edit: V curve from same paper added for comparison
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Hi Robin!I know that normally we make time dependent magnitude or CI graph of variable stars, but sometimes is proper to do CMD of them. As you can see on CMD of all Jeff Hopkins's 1982- 88 data (http://www.hposoft.com/EAur09/Data/eaur8288.zip), the pulsation is well defined in maximum and in minimum as two clouds in CMD! Also scatter of above CMD (just ingress/egress) is probably from pulsation of primary.
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I was thinking about this and wondered if we would expect to see an increase in the rotation speed of the disk as it contracted ?
Yes, if the primary star were to considerably change in radius, we would expect to see some change in temperature or color, but if the star were to shrink by a small fraction of it's radius, we might not notice much of a change. I suppose the only method to determine if the star has considerably changed is to look at archival spectra of the star and analyse the data for changes in spectral features while the star is out-of-eclipse.
One thing to note: I think some work needs to be done in characterizing the reported long term trend of a shrinking eclipse. I don't doubt it is real, but I think it isn't well characterized. In a post I made recently in another thread, I described how it seems like the trend was determined based on data from a mix of band passes. And since the eclipse length varies by bandpass, that needs to be taken into account to truly describe this trend - which you need to do before determining the delay of first contact. I think there is enough V-band data in the literature to link the last 3 eclipses, maybe 4. But if you use visual data you can go back to the 1874 eclipse and have a nice long baseline.