Effects of radiation feedback on Star and Planet Formation

Star formation plays a major role in the evolution of our Universe and understanding the circumstellar (CS) environment of protostars and young stars can also provide crucial information on how planets form. It is believed that low mass stars may form by piling up (accretion) of material onto a circumstellar disc, the material finally spirals into the protostar, increasing its mass. However the formation process of high mass stars is still not well understood. When the central engine of the newly born star turns on, the radiation produced impinges on the circumstellar dust and gas. The higher the mass of the star, the stronger the radiation emitted by it. When the photons hit the dust and gas two important effects may occur: (1) At the first encounter with the dust, the radiation pressure generated pushes the material away from the star. If the momentum transferred from the photons to the dust/gas mixture is high enough the star may lose its ability to accrete material and become a more massive star. Some studies predict that the limit for the formation of stars by accretion is in fact 10 solar masses. However stars more massive than 10 solar masses exist and have been observed! If the theory that radiation pressure on dust grain inhibits accretion is true, it means that higher mass stars must form in a different way (maybe by merging of multiple, lower mass stars). It is therefore crucial to establish a more robust limit for stars that can form by accretion through a disk. Although some studies have been carried out in the past, the calculations are very complex and so far unrealistic approximations have had to be adopted, meaning that the final word on this subject is yet to be said. I have developed new tools and the expertise to study this process in detail, by constructing realistic models for many different environments in which stars form. The models will simulate the photon interactions with the dust grains, allowing a definite answer to this fundamental question to be finally obtained. (2) High mass stars are very hot and emit high energy radiation. The high energy photons heat the cooler gas in the circumstellar region of hot stars. The temperature increase makes the gas expand creating a compression at the boundary of the heated region, which may encourage the formation of new stars there. However another effect that must be considered is that stars form from the collapse of very large dust and gas clouds, which fragment as they collapse and may therefore form multiple stars at the same time. However if massive stars form first and start sweeping away all the gas from the centre of the cloud, this could prevent the formation of other stars. The question of whether high energy radiation has a net positive or net negative effect on the rate of star formation is still open. The models I will construct will allows us for the first time to peek into the interaction regions and determine if the formation of stars is being helped or harmed by the radiation from massive stars. Radiation pressure in low mass stars is too weak to impede accretion or to significantly affect cluster environments and the formation process in this case is better understood. Nevertheless, special attention is due to the circumstellar environment of young low-mass stars because of their potential of hosting a planetary system like our own. Protoplanetary discs are deeply affected by the radiation from the newly born star. For example, emission from young solar-mass stars can heat the outer layers of their discs, making them disperse (the timescale of dispersion is crucial to planet formation). X-ray photons can penetrate to larger depths, heating the gas there and providing the mechanism thought to be responsible for disc accretion. I will study the structure of irradiated protoplanetary disks and create templates that may be used to decipher the wealth of observational data already available to us and in the future with new facilities.