Dr. Robert Stencil of the University of Denver will undertake a detailed study of the enigmatic star epsilon Aurigae. In the year 2010, the star will undergo an eclipse by a massive, dusty circumstellar disk. These eclipses take place only about every 27 years. In this effort, Dr. Stencel and his collaborators will use interferometric observations to make high-resolution images of the stellar surface and the disk, and to examine the properties of the disk compared to those in other systems.
The project will have broader impacts in education and workforce development through the training of graduate and undergraduate students. Results from this study will be widely disseminated at scientific meetings. This effort is part of a larger international collaboration which will study this star with many different observational techniques.
Project Outcomes Summary, NSF AST 10-16678. For many decades, the bright star, epsilon Aurigae, has confounded astronomers, because the third magnitude object dips to nearly 4th magnitude - every 27 years - but with no visible companion . The latest dimming, 2009-2011, enabled professional and amateur astronomers to collect an unprecedented array of measurements that seem to finally provide a consistent description for this most troubling of the stars. What has been long known is that the obvious star is a high luminosity F type star that outshines the sun by nearly 50,000 times, is a bit hotter than the sun (7500K versus solar 5750K), and has solar abundances (except for enhanced sodium – more about that later). Even now, the mass remains uncertain: the F star mass could be as low as 3 to 4 solar masses, but possibly as high as 10 to 15 solar. The evidence today favors the low mass interpretation, which indicates that the F star is in a highly evolved state, possibly on its way to becoming a planetary nebula (or possibly has a supernova fate in store). In this scenario, the F star was initially more massive than 3-4 solar masses, but has transferred fair fraction of its mass to the companion object. During the latest eclipse, 2009-2011, Brian Kloppenborg and I employed the Michigan Infrared Combiner [MIRC] at the Georgia State University CHARA Array atop Mount Wilson, California, to interferometrically image the eclipse. We decisively proved the existence of an opaque, elongated structure as the cause of the eclipse . Additionally, these observations argue for a mass ratio that makes the disk and its contents about 50 percent more massive than the F star. Other infrared spectroscopy suggests the disk is made of a variety of solids, but dominated by particles generally larger than those submicron scale bits seen in the interstellar medium or in circumstellar shells. Several other lines of evidence indicate sub-structure in the disk, possibly due to interactions among asteroidal objects, and/or tidal 'kicks' during periastron passage. Thus, we have the current system model: a lighter but more evolved star orbited by a heavier but disk-shrouded star. This begins to resemble a classical Algol type interacting binary, with an evolutionary history once described as the "Algol paradox" - how did the lighter star get to be more evolved? Answer: mass transfer as a result of Roche Lobe overflow. The unique thing about epsilon Aurigae is that the eclipses provide a tomographic or CAT scan of an astrophysical disk, and the disk itself is in so-called transitional or debris disk phase, like the famous case, beta Pictoris. Using interferometric imaging – we detect a disk-shaped object, long and thin, causing the eclipse by covering very nearly 50 percent of the F star [Nature 2010 April]. Rarely in astrophysics can we be so certain, but the pictures do not lie. More importantly, we can estimate dimensions of the disk from the transit, relative to the previously mentioned 2.3 milli-arcsecond diameter of the F star. The disk is estimated to be at least 12 milli-arcsec long by 1.1 milli-arcsec thick. We can convert those angular sizes to Astronomical Units (AU – the earth orbit radius), if the distance is known. As previously mentioned, the distance is large enough (about 2,000 light years) to be difficult to measure by any modern method, including HIPPARCOS. Adopting the result by astrometry expert, Peter van de Kamp*, the distance is 580 +/- 30 pc (1,840 LY). This suggests that the F star spans nearly 1 AU, while the disk spans about 8AU, and is half an AU thick. Equally important, with a given set of orbital parameters, relative velocities can be deduced from our measurements, which provides a key to the mass ratio and individual masses. The work of iterating consistent solutions for an orbit as constrained by photometry, spectra and imaging is an important part of the Ph.D. thesis work being done by Brian Kloppenborg, at the University of Denver. Although the 2010 eclipse is fading into memory, the bonanza of data is providing researchers both ample constraints for checking the current model, and inspiration for how to design observations that can confirm ideas without waiting another 27 years for an infrequent eclipse. Forthcoming facilities like JWST (we hope) and ALMA may be able to pursue some interesting measurements of the disk, for example. Key among the goals in these studies is pinpointing the age of the disk and its evolutionary state, whether there might be YSO-like magnetic accretion at disk core (as suggested by the polarimetry and occasion spectrum flare lines of helium). The F star itself is an important part of the study: does it have an active atmosphere or giant convective cells, flares or even a strong stellar wind?
