Astronomers from the Centre for Astrophysics Research at the University of Hertfordshire have provided the first evidence as to why stars are able to continue to accrete matter and grow, and how quasars can continue to fuel themselves preventing them from switching off. The results will be reported in two papers in the 1st November issue of Nature.
Although the papers deal with two entirely different types of objects, newly forming stars in our Galaxy and distant quasars, they solve very similar problems that have lacked direct observational evidence in tackling them.
Stars form from molecular gas clouds which collapse under gravity. However, as the cloud collapses it spins up, gaining angular momentum - which depends on the mass and rotational speed, and the clouds should fly apart preventing the star forming. Yet we know stars do form! It has been known for sometime that newly forming stars produce jets and outflows of material but their exact role and how the jets are constrained and don't simply dissipate, has not been clear. The University of Hertfordshire team, using the Anglo-Australian Observatory, NSW, Australia, studied the jets associated with a young star, and showed that helical magnetic fields, rather like the coils of a spring, are able to keep the jets collimated and that this aids the removal of angular momentum, thereby allowing the star to increase its mass.
Quasars are the very bright cores of galaxies and are believed to be powered by supermassive black holes. These have masses billions of times that of the Sun, and material is fed into the black hole through a disk of material around it, known as an accretion disk. As material is accreted onto the disk, it starts to spin very quickly and this prevents further material being added, and the quasar would then run out of new fuel and switch off. Observations at the William Herschel Telescope La Palma, made by teams from the University of Hertfordshire and the Rochester Institute of Technology, New York, have shown that powerful rotating winds are launched from the accretion disk, and this reduces the angular momentum of the disk and allows the black hole to be fed and the quasar to continue shining brightly.
Observations for both discoveries used optical and infrared instruments sensitive to the polarisation of light (as are Polaroid sun glasses). 'The University of Hertfordshire has a world reputation for astronomical polarimetry,' said Professor James Hough, Director of Astronomy Research, and added that 'these results showed that polarimetry is a key technique in many areas of astrophysics ranging from stars to distant quasars.'