The field of stellar astrophysics, broadly, includes the formation, structure, composition and evolution of stars. Stars are an underlying fundamental astrophysical object, and contain many windows in which to study them and their processes. Stars serve as as information to study further way parts of the universe, and therefore are important to studying.
Faculty and students at CCRG are specifically working on various topics in low-mass and high-mass evolved stars including binary interactions on the post-main sequence, the origin of strongly magnetized compact objects, and the physics of common envelopes. Some of these research topics are described below.
Common Envelopes, Tides, Magnetic Fields and Orbital Dynamic
The post-main sequence is accompanied by significant expansion of the stellar radius and strong mass loss. When one evolving star expands then it may be large enough to interact with it's binary companion. Mass transfer will occur if the star fills its Roche lobe, which will cause stars to evolve differently than if they were an isolated single star system. 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 various theoretical and computational projects to understand this important phase of stellar evolution.
Common Envelope Evolution is believed to be the cause of many close binary systems, energetic events, and bizarre objects. This includes close degenerate binaries responsible for type 1a supernovae, Cepheid Variables, and High Field Magnetic White Dwarfs. At the CCRG, We currently have a group that focuses their research on the latter. Using 3D MagnetoHydroDynamical codes, to simulate how the magnetic field strengthens in the degenerate core of an Asymptotic Giant Branch star during the late stages of Common Envelope 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.
