Faculty and studnets at CCRG are working on various topics in low-mass and high-mass evolved stars including: the theory of core-collapse supernovae, binary interactions on the post-main sequence, the origin of strongly magnetized compact objects and the physics of common envelopes. Some of these projects are described below.
Common Envelopes, Tides, Magnetic Fields and Orbital Dynamics
The post-main sequence is accompanied by significant expansion of the stellar radius and strong mass loss. Orbiting stellar and substellar companions such as planets, M dwarfs and compact objects, can plunge into their hosts stars either directly or through tidal torques or multi-body dynamical processes. Such common envelope phases lead to rapid orbital shrinkage and involve transfer of substantial energy and angular momentum from the orbit to the envelope. Some companions will survive, emerging in short-period orbits, while others will be destroyed. Faculty and students at CCRG are working on vaious theoretical and computational projects to understand this important phase of stellar evolution.
If the in-spiraling companion is of sufficiently high mass, it survives the interaction. However, lower-mass companions such as planets will be tidally disrupted near the proto-WD. Simulations of accretion disks from disrupted planets in the interior of RGB/AGB stars is shown to the left.
When stars such as our sun reach their end states, their ejected outer layers (initially optically thick and emitting in the IR) ionize as the nascent white dwarf heats. These planetary nebula shine in the optical and show pronounced deviations from spherical symmetry. Magnetized collimated outflows and torii are seen in many systems. The outflows seen in the post-AGB and Planetary Nebula phases are more powerful and energetic than what single stars can produce. The left image shows how companions in common envelopes can shape outflows and produce strong magnetic fields.
Stellar Collapse: Supernovae and their Remnants
Few events match the grandeur of supernovae and none surpass their raw power. Viewed on a cosmic scale, supernovae light up galaxies in spectacular explosions that mix the interstellar and intergalactic media. They make most of the elements in the universe, including those that form our own planet and bodies, and they give birth to the most exotic states of matter known - neutron stars and black holes. However, though supernovae have been at the forefront of astronomical research for the better part of a century, the mechanism by which they explode is not known. Supernovae are inherently multi-dimensional objects in which convection, hydrodynamic instabilities, and neutrino transport play central roles. The image on the is from a 3D core-collapse supernova simulation.
At birth pulsars achieve velocities well above those of their progenitor population. These pulsar kicks typically range from 200 to 400 km/s with the fastest neutron stars achieving velocities near 1,000 km/s. Hydrodynamic recoil during an asymmetric core-collapse supernova provides a natural explanation for neutron star kicks. The left figure shows a supernova explosion in the +Z direction (top panel) while the neutron star is accelerated in the -Z direction (bottom panel). The properties of the remnants after a core-collapse event, and their dependencies on progenitor mass, spin and metallicity, are active areas of research. Furthermore, the delineation between which massive stars form neutron stars and which form black holes is unknown.
Most of the computational work at CCRG requires the use of sophisticated, scalable software deployed on large multi-core machines. I routinely use adaptive-mesh-refinement grid codes where numerical resolution can be placed where needed. The figure to the left shows computational grid cells in a core-collapse supernova simulation. The highest resolution cells occur where the fluid experiences rapid change (the pink region).
The Extraordinary Deaths of Ordinary Stars: 3D Simulations of Common Envelope Phases
Award Number:1102738; Principal Investigator:Jason Nordhaus; Co-Principal Investigator:; Organization:Nordhaus Jason;NSF Organization:AST Start Date:08/15/2011; Award Amount:$226,509.00