Star Formation
How interstellar molecular clouds collapse and form new stars is a key question in astrophysics. At Leeds, we are particularly interested in the birth of the most massive stars. These play an important role in the evolution of galaxies by injecting large amounts of enriched material and energy back into the interstellar medium and powering spectacular phenomena such as H II regions, stellar winds and supernovae. Observational investigations are led by Drs Hoare and Oudmaijer and cover wavebands from optical, infrared, millimetre through to the radio. High angular resolution information is sought both directly through the use of interferometry and adaptive optics, and indirectly via velocity-resolved spectroscopy and spectropolarimetry. Research is currently focussed on determining the nature of accretion discs around massive young stars and their role in the winds and outflows that are an integral part of the star formation process. We also study the effect of these hot stars on their environments once their strong ultraviolet radiation field starts to ionize the surrounding gas and form compact H II regions, which may in turn trigger further star formation.
The star forming region S106.
Theoretical research led by Profs Dyson and Hartquist concerns the important physical processes involved in star formation. These include the role played by magnetic fields threading molecular clouds, which can slow the collapse and channel the flow along field lines. Molecular ions provide the main coupling between the gas and the magnetic field and so an understanding of the chemistry in dense clouds is very important. Leeds is leading the way in combining astrochemical studies with state-of-the-art numerical simulations, in association with Prof Falle in the Department of Applied Mathematics. Other aspects of star formation that are being studied include the effects of fast stellar winds and radiation fields on the surrounding material.
The physical processes and the techniques used to study star formation have much in common with those in the field of evolved star research. Since much can be learned from such parallel investigations there are also ongoing projects involving asymptotic giant branch stars, supergiants, Wolf- Rayet stars and planetary nebulae. The mass-loss from these objects is invariably very inhomogeneous which has important implications for stellar wind dynamics.
Planet Formation
The relative importance of mechanisms in the formation of planets in discs around solar-type stars remains a subject of considerable debate. One theory concerns the agglomeration of grains to create large solid bodies, which in turn grow through further collisions. A major problem is that the results of simple models imply that the growth of bodies in a critical size range takes more time that the protostellar disc survives.
Representations of midplane density for simulations
of 4 different solar nebula models (with permission
from
Astronomy & Geophysics).
The rate of growth would increase if fragments of a dusty medium were to form due the the fragments' self-generated gravitational fields. The question of whether or not such self-gravitating fragments form in discs, like that in which our Solar System was born, has been addressed with hydrodynamic simulations. Many simulations show the development of spiral structures, similar to those seen in some galaxies including the Milky Way. So far, different groups have reached conflicting conclusions about the role of the gravitational instability. Some results suggest that cooling in discs is efficient enough to allow giant protoplanets, precursors to Jupiter-like planets, to form due to the gravitational instability in the spiral structures. Other results lead to the opposite conclusion.
While in Leeds, one of our postdoctoral research assistants, Andy Lim, began the development of a code which will be used to produce simulations designed to resolve the uncertainity. Recently, Tom Hartquist has collaborated with USA-based colleagues to use analytic arguments and simulations to show that if gravitational bound protoplanets do form, they are most likely in some cases to do so at orbits where the sprial waves co-rotate with the discs.
For an introduction to the possible role of the gravitational instability in giant planet formation, see the article The Race Is Not to the Swift by M. K. Pickett and A. J. Lim in Astronomy & Geophysics, 45, 1.12 (2004).