Abstract

While isolated stars evolve in fairly predictable ways, the evolution of a binary star (i.e., a pair of stars) is not as well understood. Many close binary systems undergo a phase of evolution wherein one star engulfs the other such that both stars share a common stellar envelope. During this phase, the orbital distance between the two stars rapidly shrinks by a factor of 100-1000 in approximately 1-10 years. As a result of this so-called “inspiral,” the companion deposits energy into the envelope until the shared envelope itself becomes gravitationally unbound and is ejected from the system. The efficiency with which energy is transferred from the orbiting companion to the common envelope (CE) has been observationally shown to be low in low-mass stars and high in high-mass (>8 solar masses) stars. I investigate the role of the internal structure of the envelope, specifically the convective properties, on modulating the ejection efficiency and final outcomes of CE evolution. Convection, a process which occurs in all stars, is able to mix and transport energy to the surface of the CE where it can be lost via radiation, thereby lowering the efficiency of the interaction. I explore the effects of convection on both low-mass CE outcomes (double white dwarfs) and high-mass CE outcomes (Wolf-Rayet binaries). I find that the lowered CE efficiency due to convection and radiative losses produces double white dwarfs with final separations that match observations. In addition, high-mass binaries which undergo a convective CE phase also match observations despite the limited role convection plays in lowering the ejection efficiency for high-mass envelopes. The inclusion of convection in CEs reconciles the discrepancy in efficiencies observed between low-mass and high-mass post-CE binaries.

Publication Date

3-2022

Document Type

Dissertation

Student Type

Graduate

Degree Name

Astrophysical Sciences and Technology (Ph.D.)

Department, Program, or Center

School of Physics and Astronomy (COS)

Advisor

Mary Lynn Reed

Advisor/Committee Member

Joel Kastner

Advisor/Committee Member

Silvia Toonen

Campus

RIT – Main Campus

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