Discs around Young and Evolved Stars
In many key stages of their formation and evolution, stars are surrounded by circumstellar material, often in the shape of a disc. It is via such discs that stars accrete the material in order to reach their final mass, and in its remnant that planets are formed. Eventually such discs evaporate, but later in their evolution stars stars regain a circumstellar disc. These are assumed to be responsible for the variety of shapes of the nebulae ionized by hot evolved stars.
The edge on dusty disc surrounding Beta Pictoris, a young star which is just completing its formation. An infant solar system may eventually be created.
Classical theories of star formation assume that a rotating, collapsing cloud will form a disc, through which the star will accrete matter. This has been confirmed observationally for low mass, T Tau objects. However, for the more massive stars, this has so far remained fairly elusive. Even the best data can only indicate flattened, possibly rotating structures around the stars. Such studies are important, as discs are a crucial part in forming a massive star in the first place - if it weren't there the young star could exert radiation pressure on the infalling material, and halt the accretion, creating a very low upper mass limit. We have therefore started a program in Leeds to find and parametrise such discs.
This is being tackled from different angles; we are conducting a program of direct imaging, including Speckle interferometry, adaptive optics imaging at mid-IR wavelengths, and radio studies. Indirectly, we study these discs spectroscopically, looking for spectral signatures that indicate rotation, spectropolarimetrically, and most recently via spectro-astrometry. Ultimately, this work also serves as preparation for follow-on optical, near-infrared and (sub)millimeter interferometric studies.
Further in their evolution, we are interested in the remnant (debris) discs that are still visible around some Main Sequence stars, the limited lifetimes of these discs give stringent upper limits to the period in which planets can be formed. It is therefore very important to know the number of these so-called Vega-type stars. Following on earlier successful work by Leeds personnel, we now have selected objects with infrared excess from the MSX space experiment, a mission in the infrared at highger spatial resolution than IRAS, tripling the number of main sequence stars with debris discs over previous studies.
Lastly, when both low and high mass stars evolve off the main sequence, they lose vast quantities of mass, often resulting in disc like structures. By studying the circumstellar structures in detail, we learn much more about the evolution of such stars than stellar studies alone (work at Leeds on mass-loss from massive stars is noted on the Stellar Winds and Galactic Superwinds research page). In Leeds we study such discs in order to learn about the mass loss process itself, and the effects rotation has on it. In particular, we are now studying Galactic and Magellanic Cloud B[e] supergiants and Luminous Blue Variables using high resolution spectropolarimetry. A report on this appeared in the AAO Newsletter in April 2004.