Theory of Disks and Outflows around Massive Young Stellar Objects

Abstract

The inner most regions around massive young stellar objects (YSO) are associated with complex interactions between numerous physical processes. Since the inner few Astronomical Units (AU) are tough to resolve observationally, a theoretical approach is important to create a qualitative picture for these regions around young high-mass stars. This thesis investigates the interplay between important physical processes with respect to the dynamics of jets and inner accretion disks. This thesis provides a bridge between the physical structures of the inner and the outer disk, where the later is observationally easier to access. Above all, the importance of the radiative force in altering the dynamics of a magnetically launched jet is outlined in this thesis. A thin accretion disk model with proper gas and dust opacities is applied for a luminous young high-mass star. This study has furnished estimates of various physical quantities in the inner few AU of the accretion disk. In particular, I have found that the mid-plane temperature around 0.1 AU could be as high as 10^5 K for a 10 Msun star. Such high temperatures in the disk destroy most of the dust grains already at large radii from the central star. This in turn reduces the opacity of the accreted matter thereby overcoming the central radiation-pressure from the young massive star. In addition, such disks are stable to gravitational fragmentation inside of 100 AU from the central star. Thus they form an ideal launching base for long-lasting outflows. Outflows and jets are an ubiquitous phenomenon in young massive star forming regions. Obser- vational surveys have suggested that the outflows become wider as the star grows in luminosity (thus mass) with time. I have performed magneto-hydrodynamical simulations of jet launching in presence of radiative forces from the luminous star and the inner hot accretion disk. The major outcome of this work, is that the radiative force from the central star plays a dominating role in accelerating and de-collimating the magnetically launched jet, while the influence of the disk ra- diative force is rather small. In addition, conducting an extensive parameter study, I have found that the outflows become wider as the mass (or luminosity) of the central star increases. The degree of collimation is also affected by the magnetic field strength and optical thickness of the line. This interplay of radiative and magnetic forces provides a physical insight to the trend in degree of collimation suggested by observations. Finally, a fully three-dimensional simulation is conducted to understand the manner in which the inner accretion disk transports material onto the central massive star. The hydrodynamic flow in the disk is simulated in the presence of radiative transfer and/or self-gravity. The transport of angular momentum is solely due to gravitational torques. My first results indicate that a locally isothermal disk becomes gravitationally instable and fragments in the inner parts as it is fed with matter from the outer massive core with a steady accretion of 10^-3 Msun yr^-1. About 10% of the mass added onto the disk is accreted onto the central star in form of clumps. On the other hand, no fragmentation is seen in an adiabatic disk whose initial temperature profile is consistently derived from radiative transfer calculations. This investigation complements the above semi-analytical study of the inner disk to single out the physics of angular momentum transport in massive accretion disks.