Star-Formation and Radio-Astronomy

Star formation can be considered as one of the most basic processes in the Universe. It is hard to imagine a Universe without stars. Perhaps the most amazing aspect of the star formation process is that it takes matter which is at a very low density, of the order of hundred thousand molecules per cubic centimetre and which is very cold, about -240 degrees Celsius, and ends up with a star with densities in the core of about 10^{26} particles per cubic centimetre and with a temperature of more than 10 million degrees Celsius! The process of converting very cold low density material into high density very hot material is very complex and involves a large number of physical processes.

Studies of star formation are basically concerned with the very early phases of the above described process. The initial steps of the star formation process takes place deep inside of what is known as molecular clouds. These are literally very large clouds of very low density gas in the Galaxy and is found mostly in the spiral arms.

Molecular clouds consist primarily of molecular hydrogen although other molecules such as, for example, CO, and other more complex molecules are also present. Molecular clouds can have masses that range from a few hundred times the mass of the Sun to hundreds of thousands times the mass of the Sun. The very large and massive molecular clouds are called Giant Molecular Clouds (GMCs). The matter in a molecular cloud is not distributed in a uniform way throughout the cloud. Instead it has denser and less dense parts. The denser parts can be divided into what is called clumps and cores. This means that the gravitational field inside the cloud is also not uniform. A small perturbation in the density of a core region may result in that part of the cloud becoming gravitationally unstable and start to contract. This is the start of the process of star formation.

The very early phase of star formation takes place very "deep" inside molecular clouds. This means there is sufficient material between the observer and the region where the star forms that it is not possible to observe the process in the visible part of the electromagnetic spectrum. Astronomers and astrophysicists rather have to unravel what is going on by using infrared and radio emission that originates in the star forming region. The infrared and radio emission carries specific signatures of the physical processes that takes place during star formation and it also depends on the mass and evolutionary state of the star that is in the process of formation.

A very interesting phenomenon that is associated with the formation of massive stars, i.e., stars with masses greater than ten solar masses, is the presence of so-called MASERS. A maser is the microwave equivalent of a laser and the acronym stands for Microwave Amplification through Stimulated Emission of Radiation. Maser emission associated with a number of molecules, such as, OH, CH3OH, H2O has been found toward many high mass star forming regions. Specific types of CH3OH masers have been found to be exclusively associated with high mass star forming regions, that is, they are not found associated with very young lower mass stars.

Of particular interest is the observation that currently nine out of about a total of 1000 CH3OH masers shows periodic flaring behaviour. This behaviour has been discovered in 2003 by Sharmila Goedhart, then a PhD student at the NWU. This periodic behaviour is at present the only known periodic phenomenon associated with the very early phases of massive star formation. Understanding what gives rise to the periodic behaviour is currently the main focus of the star formation research within the CSR.

There are basically two reasons for the periodic behaviour of the CH3OH masers. The first is that the process that excites (pumps) the CH3OH molecule to higher states may be time dependent. The second is that the background source of photons that the maser amplifies has some periodic behaviour. Our current focus is to investigate both possibilities. A possible explanation for changes in the background flux of seed photons is that the young stellar system is a binary system rather than a single star. Although the stars are still very young they already have stellar winds. The interaction of the winds gives rise to very hot shocked gas that emits ionizing photons is modulated by the orbital motion of the stars. The changing flux of ionizing photons leads small changes in the radio emission from the outer parts of the HII region that surrounds the two stars. It is postulated that some of the periodic masers amplifies the radio emission from the outer parts of the HII region and that changes in the maser flux follows the changes in the radio emission of parts of the HII region.