Young Stellar Objects and their Environments
(Chrysostomou, Lucas, Riaz)
To understand how stars form is one of the major goals of modern astrophysics research. As a discipline, star formation underpins the study of a range of phenomena, all varied in size and scale, such as the formation and evolution of galaxies (including our own), the formation of the first stars in the Universe through to our own Solar System and others like it, and perhaps of life itself.
Our research programme on young stellar objects (YSOs) aims to understand not only their formation process, but also how the YSO itself interacts with its environment. The main research goals can be summarised by the following key scientific questions:
- What physical processes are associated with the outflow and inflow of material onto YSOs?
- What role does the circumstellar magnetic field play in the star formation process?
- How do binaries form and what is the binary fraction for YSOs in comparison to the field population?
Recent Highlights:
Rotating jets from YSOs
A long-standing problem in astrophysics is how material from a giant molecular
cloud is able to collapse and contract onto a (perhaps) solitary stellar body. In physics, angular momentum is a conserved quantity which means that a rotating body will speed up
as it becomes more compact (consider a figure skater performing a spin). The size scales involved in star formation only compound the problem with the inevitable conclusion being
that centrifugal forces would disrupt any collapsing body before it had a chance of forming a star. Of course, we know that stars do form, so for decades astronomers have searched
for a solution to this problem.
A prevailing theory is that the outflows which are observed to be ubiquitously associated with YSOs must somehow play a role, by removing material with high angular momentum from the central protostar, leaving low angular momentum material to collapse onto it and increase its mass. However, till now, instrumentation has not been sufficiently accurate to provide the necessary proof. With the advent of high-precision instrumentation on new facilities (such as the Hubble Space Telescope and the new generation of 8-m class telescopes) researchers at the University of Hertfordshire, in a broad collaboration with astronomers in Italy, Ireland and Hawaii, have discovered evidence for rotation in jets which emanate from YSOs. It now remains to establish just how prevalent and dominant this mechanism is. We stand tantalisingly close to understanding how and why we see these enigmatic and beautiful jets emerging from YSOs.
The role of magnetic fields in star formation
There are many theories in astrophysics which appeal to magnetic fields, however,
observational confirmation of their presence and influence can be challenging and/or limited. In star formation, magnetic fields seem to play a dual role. They are held responsible
for the regulation of material onto the central protostar by providing added support against the gravitational inflow of material. Also, once material is accreting onto the protostar,
magnetic forces within the accretion disk lifts material off it and drives it away in the form of a jet (taking an amount of angular momentum with it - see above).
For many years, the University of Hertfordshire has pioneered the use of polarimeters as unique probes of the morphology and properties of the environments of YSOs. Using circular polarimetry, in conjunction with sophisticated Monte Carlo modelling techniques, we have determined, for the first time, the presence of a helical magnetic field in a YSO outflow. This not only lends support to our work on rotating jets, but indicates that helical fields may be a necessary component in order to keep the outflow collimated over the large distance scales they are observed to be.
Our research into magnetic fields has in the main concentrated on the relation between the structure and (estimated) strength of the magnetic field and the star formation process. We have made the first measurements of the magnetic field structure threading through high-mass protostellar objects and showed that the magnetic field in these massive and young objects is not sufficient to support it from collapse (hence we predict that they will start to form stars in the near future), but the uniform and regular field threading the objects may have had a role to play in channelling material into its present state. Our work has shown that on larger scales magnetic fields play a passive role and may only take up a more influential and active role on smaller scales. We are now involved in large-scale surveys for star formation in the Galaxy which will allow us to better investigate the role magnetic fields play in molecular clouds and star formation.
Spectroastrometry
This is a simple yet powerful technique which allows one to probe at scales within the optical limitations of telescopes. No specialist instrumentation is needed (just a good spectrograph and a CCD camera with relatively small pixels), but by careful measurement of the position of peak emission as a function of wavelength one is able to establish differences in position at milliarcsecond scales. For the nearest star forming regions (approximately 500 light-years), this translates to similar scales as the Earth-Sun distance.
Using this technique we have been able to discover new binaries which had previously remained undetected due to their close proximity. We are also able to establish the presence of outflows from individual binary components and also, perhaps most intriguingly, the presence of gaps within circumstellar disks which could only have been caused by the presence of planetary bodies within them.