Modeling disks and jets around forming massive stars, and a look at a remnant neutron star

Abstract

Massive stars form and evolve in short times compared to their low-mass siblings, but their effects are felt across several scales in the universe. They form from the gravitational collapse of cold and dense material in molecular clouds. A forming massive star is surrounded by an accretion disk, formed thanks to the conservation of angular momentum during the gravitational collapse. The accretion disk can become massive enough to be self-gravitating and form spiral arms and fragments. Magnetic fields present in the molecular cloud are dragged by the gravitational collapse and rotation and launch magnetically-driven outflows. These outflows are sources of mechanical feedback into the natal environment, and remove angular momentum from the protostar.

In this work, I explore the possibility that the fragmentation of the accretion disk produces companion stars to the central forming massive star. I work in solving this question by analyzing radiation-hydrodynamical simulations of massive star formation from the collapse of a cloud core. I find around six companions distributed in the inner and outer disk, with the close companions having the possibility of becoming spectroscopic companions. I study the processes that intervene in the launching, acceleration, propagation and possible termination of the magnetically driven outflows, and the effects of the magnetic fields on the accretion disk. With a catalog of 30 simulations exploring different conditions for the onset of gravitational collapse, I propose a theoretical picture of how the environmental conditions for massive star formation determine the characteristics of the accretion disk and the outflows. Additionally, I compare those results with recent observations of a disk-jet system around a massive young stellar object in the star-forming region IRAS 21078+5211.

At the end of their evolution, massive stars explode as supernovae, capable of producing and delivering heavy elements that enrich the natal environment of future generations of stars and planets. Stellar remnants of massive stars are compact objects: neutron stars and black holes. The structure of neutron stars is an open problem in Astrophysics due to our insufficient knowledge about the equation of state of dense nuclear matter. Neutron stars are commonly observed as pulsars thanks to periodic signals arising from hotspots on their surfaces while they rotate. Rapidly rotating neutron stars are expected to be significantly flattened; such flattening can be described approximately in terms of a mass quadrupole moment. I use spacetime models that aim to describe the exterior spacetime of a rapidly-rotating neutron star in order to study the effects of the quadrupole moment on the pulse profiles and thermal spectrum.