Young Stellar Objects

(Lucas, Chrysostomou, Hough)

Newly born stars, or protostars, are surrounded by an accretion disc and a larger scale envelope, which are composed of a mixture of gas and dust. These appear as beautiful reflection nebulae in star formation regions. Many protostars also power partially ionised jets and molecular outflows which help to remove angular momentum so that the star can form.

The distribution of matter tells us a great deal about the details of the star forming process, while the size distribution of the dust grains informs us about the grain growth that leads to formation of planetesimals and then planets. The structure of the magnetic field threading the matter around the protostar helps to explain the distribution of matter, for example the well collimated jets.

At Hertfordshire we have a long history of infrared and optical polarisation studies. We use the combination of polarimetry (both linear and circular) and three dimensional Monte Carlo models to map the magnetic field structure around individual protostars and across whole clusters. Polarisation data distinguish dense, optically thick regions from low density parts of the reflection nebula and tell us about the dust grain size distribution.

In the past we have tended to focus on detailed studies of individual objects, often using polarimeters designed and built at Hertfordshire by Prof Hough. Now our main work in this subject is to use our Monte Carlo light scattering models to interpret the data supplied by new wide field polarimeters such as the Japanese-owned Infrared Survey Facility in South Africa.

Left: The Young Stellar Object HH135-136 and its bipolar outflow (near infrared 3 colour image). The protostar was the subject of a Nature paper by our group (Chrysostomou, Lucas & Hough 2007, vol.450, p71).