The Dynamical Evolution of Classical Be Stars
Abstract
This thesis focuses on the evolution of the disks of two classical B-emission (Be) stars, 66 Ophiuchi and Pleione, and on the thermal structure for disks tilted out of the star's equatorial plane.
We used a hydrodynamic code to model the disk of the Be star 66 Ophiuchi. Observations from 1957 to 2020 were compiled to follow the growth and subsequent dissipation of the disk. Our models are constrained by new and archival photometry, spectroscopy and polarization observations. Using Markov chain Monte Carlo methods, we confirm that 66 Oph is a B2Ve star. We constrain the density profile of the disk before dissipation using a grid of disk models. At the onset of dissipation, the disk has a equatorial density of $\rho(R) = 2.5\times 10^{-11} (R/R_{\star})^{-2.6}~\rm{g~cm^{-3}}$. After $21$ years of disk dissipation, our work shows that 66 Oph's outer disk remains bright in the radio. We find an isothermal disk with constant viscosity with an $\alpha = 0.4$ and an outer disk radius of $\sim115$ stellar radii best reproduces the dissipation. We determined the interstellar polarization in the direction of the star in the V-band is $p=0.63 \pm 0.02\%$ with a polarization position angle of $\theta_{IS}\approx85.7 \pm 0.7^\circ$. Using the Stokes QU diagram, we find the intrinsic polarization position angle of 66 Oph's disk is $\theta_{int}\approx98 \pm 3^\circ$.
We acquired H$\alpha$ spectroscopy from 2005 to 2019 that shows Pleione has transitioned from a Be phase to a Be-shell phase. We created disk models which successfully reproduce the transition from Be to Be-shell with a disk model that varies in inclination while maintaining a constant, equatorial density of $\rho(R) = 3\times 10^{-11} (R/R_{\star})^{-2.7}~\rm{g~cm^{-3}}$, and an H$\alpha$ emitting region extending to $R_{\rm out}=15~\rm{R_{eq}}$. We use a precessing disk model to follow variability in disk inclination over $120$ years. The best-fit disk model precesses with an inclination between $\sim25\rm{^{\circ}}$ and $\sim144\rm{^{\circ}}$ with a period of $\sim80.5$ years. Our precessing models match some of the observed variability but fail to reproduce all of the historical data available. Therefore, we propose an ad-hoc model based on our precessing model and recent disk tearing simulations of similar systems. In this model, a single disk is slowly tilted to an angle of $30^{\circ}$ from the stellar equator over $34$ years. Then, the disk is torn by the companion's tidal torque, with the outer region separating from the innermost disk. The inner disk returns to the stellar equator as mass injection remains constant. The outer disk precesses for $\sim15$ years before gradually dissipating. This model reproduces all the variability trends, repeating every $34$ years.
Our research on Pleione led to a detailed investigation of the thermal structure of tilted disks. For this research, we modelled the radiative transfer in tilted disks self-consistently. We constructed disk models for a range of spectral types, rotation rates and disk densities. We find as the tilt angle increases to $60^{\circ}$, the minimum disk temperature of our B0 V star model, with $W=0.95$ and $\rho_0=10^{-11}~\rm{g \, cm^{-3}}$, can increase up to $\sim114\%$, while the maximum disk temperature decreases by up to $\sim8\%$. When $W=0.7$, the changes in disk temperature for the same model are smaller, and at lower density the disk temperature increases globally. In the B2 V model, both the disk temperature and ionization fraction globally increase. In the B5 V and B8 V models, the disk temperature globally decreases, but increases around $\sim10~\rm{R_{eq}}$. The ionization fraction increases as modest changes to the disk temperature allow it to exceed the hydrogen ionization temperature. Overall, we find that the trends in the disk temperature and ionization fraction with the disk tilt angle greatly depend upon the stellar spectral type.