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Unanswered Questions - one step in a journey of a thousand steps
(this post should really be authored by both Aaron and Brian as we worked on it together)
Dr. Hoard's news from the American Astronomical Society meeting was an exciting next step in our investigation of epsilon Aurigae. He originally presented a preliminary version of the results at the 1st Citizen Sky workshop last summer, but the audience was asked not to disclose anything while they finish their analysis. Super huge kudos to Dr. Hoard for his discovery and for sharing it with the audience. He's been a good friend of Citizen Sky, even mentioning CS in his press release and during his press conference. He is one shining example of why astronomy is a unique science through its wonderful relationship between professionals and citizen scientists.
So, what does that mean now? This was one important step in a journey of a thousand steps. Brian Kloppenborg and I just sat down in the library at AAVSO HQ and brainstormed the following open questions about epsilon Aurigae. We spent just 20 minutes on this list, yet it gives you an idea of all that still needs to be done.
- So we now know there is a dusty disk blocking the light. The big pink elephant in the room is: where did it come from? It's not everyday that we see a huge dust disk orbiting a star. In fact, we've yet to find a star that is identical to epsilon Aurigae. A few stars are similar, but none have all the peculiar properties epsilon Aurigae has.
- Why are the dust particles so large? Dr. Hoard announced that the dust in the disk are more like grains of sand than the more fine dust grains usually found in space.
- Dr. Hoard agrees with an earlier proposal that the F-star is instead and Aymptotic Giant Branch (AGB) star, instead of an F-supergiant. Those two stars have different evolutionary paths that could cross. Is it possible that we have caught epsilon Aurigae in this unique stage of its evolutionary history?
- What is the distance to epsilon Aurigae? This fundamental question is surprising difficult to nail down and would answer a lot of geometric questions.
- What is the deal with the out-of-eclipse variations? The star varies by about 0.1 magnitudes all the time. So far, it doesn't seem to be periodic. Why?
- epsilon Aurigae regains its brightness at the end of the eclipse more rapidly than it loses it at the beginning of the eclipse. Why is the egress quicker than the ingress? What does that suggest about the disk?
- Is the mid-eclipse brightning real? What would it mean?
- During minima, there is no carbon monoxide absorption line. Yet, it appears during mid eclipse and then slowly fades after the end of the eclipse. What is the source of the carbon monoxide absorption line and why does it behave like this?
- Is the length of the eclipse getting shorter as reported in Stencel and Hopkin's book? Is it an artifact over how we have analyzed past eclipses or is it the real deal? What astrophysical causes could explain it?
- Finally, the biggest mystery is still the disk itself. What is it's physcial extent? That is, what does it look like? What is its thickness and radius? Is it supported by gas pressure or something else? If it gas pressure, that might imply that the disk is a Young Stellar Object (YSO) disk. How can that be reconciled with an old AGB star being right next door?
Some of these questions are good topics for teams to work on. Remember, we'll be assigning a professional liason to any team interested in these topics. (Brian and I also brain stormed other team ideas and will post them shortly.)
This is NOT an exhaustive list! We spent just a little bit of time to list some of the many unanswered questions about this system. Do you have other questions? Do you have any ideas of theories about any of these questions? Share away!
Caroll et al http://adsabs.harvard.edu/abs/1991ApJ...367..278C and others in the past put the case for the F star being a supergiant which up until this announcement was the majority accepted view. This seems to raise questions about how a post AGB star and a Supergiant can be distinguished. (Were it not for these observations involving the secondary, [edit Oops! it is not the secondary any more - better call it "the eclipsing object"] it seems that we would still today catalogue eps Aur as an F0 supergiant) What observations would be needed to confirm, independent of the eclipsing object that the F star is indeed post AGB?
Robin,
I agree, how can we distinguish between the two objects? There are often very good spectroscopic markers, but I guess nobody has looked at these lately (part of my thesis for sure). The last detailed spectroscopic comparison was quite a while ago, possibly before the phrase "AGB" was coined.
As for the nomenclature, I've taken to calling them the "F-star" and "Disk" or "Disk + B-Star" in my writing.
Is there connection between nature of eps Aur and dark cloud LDN 1475? See yesterday's APOD: http://antwrp.gsfc.nasa.gov/apod/ap100108.html
The two are spatially coincident, and they are both "unusual" objects. That always raises a flag in my mind. As a scientist, the proper way to answer the question is to estimate the distances to both objects and not discard the possibility out of hand. If the distances are nearly equal, then there may be some association. A dark cloud often implies star formation, for example, and eps Aur would then perhaps be better modeled with a young object rather than an AGB object. To the best of my knowledge, no one has checked the possibility to see if it should be rejected.
Arne
SIMBAD is giving for eps Aur
PI= 1.60+/-1.16 mas d= 625+/-453 pc => M-m= 9.0.
For comparison we take BD+44 1080- Star in Nebula, which is in LDN 1475, and it is approx. 26.5 minutes from eps Aur.
D+44 1080
B5V, V= 9.29, B= 9.91, B-V= 0.62 (SIMBAD)
M(V)= -1.1, (B-V)0= -0.16 (from Zombeck's Handbook of Space Astronomy and Astrophysics for B5V star)
E(B-V)= 0.78 A(V)=3.2*E(B-V)= 2.50 => M-m= 7.9, d= 380 pc
So D+44 1080 is in one (huge) sigma at the same distance as eps Aur, but proper motion and RV are very different! Also some authors are counting eps Aur as member of Aur OB1 association (http://www.crya.unam.mx/rmaa/RMxAA..39-1/PDF/RMxAA..39-1_arellano.pdf).
So no definite answers! ;) Nikki
Here is infrared picture of eps Aur 5 deg. field. It is false LRGB, where L is at lambda 100 mum, R is 60 mum, G 25 mum and B 12 mum IRAS data from IRIS (see http://www.cita.utoronto.ca/~mamd/IRIS). Eps Aur is blue dot (12 mum) in the center of the picture. Eps Aur is siting at the top of the pillar (finger, horn ...?) of above mentioned dark cloud LDN 1475, which reminds on famous "Pillars of Creation" in M 16! Note that distance to M 16 is 1749 pc (http://www.univie.ac.at/webda/cgi-bin/ocl_page.cgi?cluster=m+16), while eps Aur is at distance 606 +/-55 pc (Heintz and Cantor 1994, 1994PASP..106..363H). "We acknowledge the use of NASA's SkyView facility (http://skyview.gsfc.nasa.gov) located at NASA Goddard Space Flight Center."
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Nikki,
Agreed, the uncertainties make things a little difficult. There are other estimates for the distance to eps Aur other than Hipparos, from which the SIMBAD distance comes. Some of these sources even have lower uncertainties, but they may not be completely reliable. Based upon the series of estimates I've seen so far, I'd call it 625 +- a few tens of parsecs.
Brian
I wonder whether there are also some open questions concerning a secondary eclipse (more precisely, the B component being eclipsed by the F component). I haven't heard a thing about secondary eclipse for eps Aur.
Using the data that was attached to Dr. Bob's blog entry, if my math is correct the ca 89 deg inclination could be "enough edge-on" to cause an eclipse of the B star by the F star even in the new model, and the eclipse would last for a couple of months (comparable to the ingress duration during the current eclipse).
Looking at the composite of the model and measured spectra of the components from the Spitzer results:
I understand there is a faint but distinctive UV excess caused by the B star, so the "secondary" eclipse should be visible in UV < 200 nm (too bad, I think this is blocked by the Earth's atmosphere pretty effectively).
CS
Heinz
Heinz, In preparation for the eps Aur CHARA paper I put together a nice spectroscopic solution for the system (potentially having solved all of the orbital parameters!).
I might be able to figure out when the secondary eclipse should have happened. I'd guess in the mid-90s for which we have a little UV data and some other unpublished data... we'll see if we can find anything!
Brian
Hi Heinz,
Carroll et al suggested in 1991 using the secondary eclipse and various other observations outside eclipse to determine the evolutionary state of the system and the mass ratio of the components (extract from the paper attached) but a literature search does not reveal any follow up of these ideas. Perhaps it could all have been sorted out over 10 years ago ;-)
Robin
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Thanks for the extract!
Ah well...looking for a reason to ask for a 14 year NSF grant extension ? Here you are :-)
CS
Heinz
The real problem with the secondary eclipse is that nobody knows exactly when it is. For starters, the orbital inclination is poorly constrained and the diameters of the B-star + Disk isn't known so predicting whether or not there is a secondary eclipse is difficult, let alone when it will occur.
Brian
I would have thought that the diameter of the F star would be even more important for this?
CS
HBE
Well, they are both important. If the "secondary" object moves across the face of the star during primary eclipse, but isn't large enough to be eclipsed during the secondary eclipse ("large enough" is a factor of the secondary's size and the orbital parameters), there may not be a secondary eclipse at all.
Brian
I see.
But let's assume the data that came along with the latest Spitzer et al. observations:
My understanding is that if
R_F >= a cos i
we will have at least a ~50% partial eclipse (assuming a constant orbital separation of a, inclination i, and radius of F star R_F), no matter what R_B is.
For the data above, we have a*cos(i) =~ 215 R_sun in the worst case , and in the best case a*cos(i) =~72 R_sun, so indeed no clear case.
CS
HB
You actually left a major one out: what is at the center of the dust cloud? If it really is just a normal B star, then it has more mass than the AGB star and the picture actually is that the AGB star is circling the dust enshrouded object rather than the other way around. The AGB star scenario also only works if the current distance estimate is correct. If it is a normal B star, then you need to make sure that the evolutionary time scale of the two stars is correct. Finally, some mechanism has to be in place to (a) remove the small grains and (b) replenish the dust cloud, as collisions in such a dense region should clear it out pretty quickly. If it is an AGB star, where is the rest of the mass that it has lost - surely there must be a shell or two that should be visible. I look at the Spitzer results as adding a few more (important!) pieces of information to the puzzle, but a long way from solving the mystery.
Arne