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!