What does it mean? Part 1

The set of images published in Nature on April 8th 2010 represent only a few nights of observing, mainly during ingress phases of this eclipse.  Brian and I will, in tag team form, blog about a number of facets about the observation and its implication, and provide a sense of what's next in this process.

First, this direct detection of the disk is a wonderful demonstration of the scientific method: long theorized to be there, and at long last it is observationally confirmed.

You might ask: The disk must be large, but how large? And how massive? How far is the companion from Epsilon Aurigae?

How large?  It's big - nearly reaching the orbit of Jupiter around the Sun if we moved it into our solar system, and as thick as earth's orbit in the vertical dimension.   This dimensional estimate is dependent on distance assumptions, but we'll come back to that.

How massive?  Surprisingly lightweight!  In the Nature paper we estimated that only an earth mass of gravel would be required to optically darken a volume that size!  Some of you may remember what dark spots cometary fragments were able to produce on Jupiter a few years ago.  Even the Zodial dust cloud occupies a huge volume in our solar system but is a tiny amount of mass compared to Earth.  Finally, debris disks like those around beta Pictoris and Vega are detected but similarly lightweight.  Photons are easy to stop - even your thin eyelids are sufficient.

How far from the companion star that is being eclipsed now?  The mass ratio and estimated masses suggest that the orbital separation is 19 AU, about the orbit of Uranus.  For completeness, the bright companion star itself is about as wide as the orbit of Venus, making this a "close binary" even if the distances are huge.

What else have we learned so far?  Over to Brian for Part 2...

What does it mean? Part 2

Dr. Bob and I are taking turns explaining the implications of the recent Nature paper.  In the first post, Dr. Bob discussed two very important questions: "How big?" and How massive?"  In this post, I'm going to cover another big-picture topic: the orientation of the disk.

I'll start with what we thought happened.  Most of the literature drew the disk as something that more-or-less bisected the F-star, following Kemp's 1986 drawing (note, a similar drawing was also published in the 1985 epsilon Aurigae conference proceedings):

We now know that this picture was very, very close to being correct.  Considering that the parameters in the paper (see pg. L13 of Kemp's work above) all scale to the radius of the F-star, Kemp et. al. did a good job of estimating the parameters for the disk just from polarimetric data and modelling!  The only large change is the point of first contact which was assumed to be in the northern hemisphere.  Instead, it is clearly in the southern hemisphere.

A big question, and a good team project, might be to investigate whether or not Kemp's polarization data is consistent with the current findings.  If it isn't, this could mean the disk or orbit has precessed significantly in the 27 year since the last eclipse! (if you are interested in doing this, please contact me).

For those of you who do not have access to Nature, I have extracted a few parameters in the table below (these are rounded, the parameters in the paper are more precise):

Disk Radius*	3.8 AU
Disk Thickness**	0.76 AU
Central Opening (?)	0.5 AU?
Disk Inclination**	85 +/- 5 or 95 +/- 5 degrees
Disk Tilt**	< 20 degrees
Orbital Inclination (?)	88 +/- 2

* From "Infrared images of the transiting disk in the ε Aurigae system" by Kloppenborg et. al. 2010 April, Nature
** From "Taming the Invisible Monster: System Parameter Constraints for Epsilon Aurigae from the Far-Ultraviolet to the Mid-Infrared" by Hoard et. al. 2010 ApJ.
(?) Unknown parameter, this is a best-guess.

Try out these values in the new light curve generator and see how close you can get the light curve to match with previously observed values!  Do the parameters work?  If not, what do you think is wrong?  Do we have the entire picture or is there more to discover?

What does it mean? Part 3: Eyjafjallajokull

Eyjafjallajokull may sound like a word lifted from Finnegans Wake (a novel by James Joyce), but the Icelandic volcano has grabbed worldwide attention by producing a dense ash cloud that threatens jet aviation over much of northern Europe again this week.  In this third blog exploration of the implication of direct detection of the disk in epsilon Aurigae, via interferometric imaging, I want to explore with you how terrestrial volcanic ash provides some analogies with the dusty material that scientists believe make up "debris disks" seen around a surprising percentage of normal stars. 

Debris disk is the term describing an infrared excess detected over and above the stellar flux distribution, signalling the presence of an extended, presumed flat distribution of particles that resemble our Zodiacal Cloud [see Paul Kalas' webpage on this, http://www.disksite.com/ for details].  The best known examples include the disk around beta Pictoris - see http://seds.org/hst/BetaPicB.jpg .  The surprising fact about debris disk is how little total mass is involved - Earth's Moon is more than enough, if pulverized and volcanically dispersed, to create a solar system sized cloud of sufficient opacity.  Fun fact about the beta Pic disk - it seems to have ring structure - see http://hubblesite.org/newscenter/archive/releases/2000/02/image/a/ .  Robin Leadbeater and I have already announced evidence for similar in epsilon Aurigae - see http://arxiv.org/pdf/1003.3617 - more about this in my next blog.

Astronomers surmise that the material in these debris disks resembles some of the volcanic ash our terrestrial friend, Eyjafjallajokull has been spewing lately - micron sized fine particles of mainly silicates.  In the cold of space, any number of gas atoms may end up freezing to said particles, providing an icy coating.  This size and composition makes detection tricky - we see a broadband infrared emission characteristic of their temperature, but few spectroscopic details because of their large size compared to single atoms and molecules.

In our own solar system, the protosolar nebula condensed into fine particles in just a couple of million year timescale approximately 4.6 billion years ago.  These particles went on to form large planetessimals that ultimately formed the surviving planets, asteroids and moons.  If similar physics applies in epsilon Aurigae, the condensation into dust particles may have "just happened" in its recent past.  Aspects of the disk suggest it is dynamically young and quickly evolving.  In our Nature paper, we deduced the disk mass to be about one Earth mass in total, distributed over the nearly 8AU long x 1AU tall disk structure.  Next challenge: determine the composition.  This is where finding out more about what is known about the dust in beta Pictoris AND in terrestrial ash cloud physics might provide insights.  Perhaps we will see infrared spectra of the Eyjafjallajokull ash in the sky over La Palma observatories - http://www.ing.iac.es/ - sometime soon!