From accretion to outflows of massive protostars

Abstract

Massive stars live short but intense lives. While less numerous than low-mass stars, they enormously impact their surroundings by several feedback mechanisms. They form in opaque and far-away regions of the galaxy, such that one of these feedback mechanisms also becomes one of few records of their evolution: their bright large-scale outflows. Their emergence and related phenomena, such as the accretion disk that launches them, comprise the main focus of this thesis. In chapter 2, we present our magneto-hydrodynamic (MHD) simulations that were conducted with non-ideal MHD, self-gravity, and very high resolutions, as they have never been achieved before. In our comprehensive convergence study, we investigate computational conditions necessary to resolve (pseudo-) disk formation, and outflow launching processes and we analyze possible caveats. We explore the magneto-hydrodynamic processes of the collapse of a massive prestellar core, including an analysis of the forces involved and their temporal evolution. We follow the initial 100 M cloud core for up to two free-fall times, during which it collapses under its own self-gravity to self-consistently form a dense disk structure that eventually launches outflows. The setup allows us not only to show a comprehensive evolutionary picture of the collapse, but also enables the resolution of highly collimated magneto-centrifugal jets and magnetic pressure driven tower flows as separate structures. This is only possible in very high resolutions and is, to our knowledge, the first time this has been achieved. Of the two outflow components, the tower flow dominates angular momentum transport, while the mass outflow rate is dominated by the entrained material from the interaction of the jet with the stellar environment and just a part of the ejected medium is directly launched from the accretion disk. Taking into account both the mass launched from the disk’s surface as well as the entrained material from the envelope, we find an ejection-to-accretion efficiency of 10%. Additionally, a tower flow can only develop to its full extent when much of the original envelope has already dispersed as otherwise the ram-pressure of the infalling material inhibits the launching on wider scales. We argue that non-ideal MHD is required to form centrifugally supported accretion disks and that the disk size is strongly dependent on spatial resolution. We find that a converged result for disk and both outflow components requires a spatial resolution of ∆x ≤ 0:17 au at 1 au and sink cell sizes ≤ 3:1 au. Our results indicate that massive stars not only possess slow wide-angle tower flows, but also produce magneto-centrifugal jets, just as their low-mass counterparts. Therefore, the actual difference between low-mass and high-mass star formation lies in the embededness of the high-mass star. This implies that the jet and tower flow interact with the infalling large-scale stellar environment, potentially resulting in entrainment. In chapter 3, we investigate the occurrence of observed asymmetries in position-velocity diagrams and show how even symmetric objects can produce them. To this end, we give a qualitative description of the idea; backed up by direct integration of the line radiation transport equation including the effect of resonances. We show that these asymmetries naturally arise by reabsorption when infall (or outflow) velocities and rotational velocities are of the same magnitude, such that the emission from the warm, central regions of the accetion disk is absorbed in the colder outer regions of the envelope as they reach the same line-of-sight velocities. These multiple resonances of the molecular lines can be considered a special case of self-absorption.