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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Astronomy

Supervisor

Jones, Carol E.

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.

Summary for Lay Audience

There are many different types of astrophysical disks observed in the Universe. For example, there are disks of material swirling around black holes, our own Milky Way Galaxy is in the shape of a flattened disk, even our Solar System formed from within a disk of material leftover when the Sun formed, and there are many more examples. As such, it is important to understand the physical conditions in astrophysical disks and the processes that operate in them.

My work involves a particular type of massive star that is surrounded by a disk of material sometimes extending to hundreds of the star's radius from it's surface. They are called classical B-emission stars, Be stars for short. These stars are bright and numerous making them ideal candidates to help understand disks. The starlight interacts with disk material, and when a telescope or orbiting satellite observes it, signatures of both the disk and star are collected together which provides lots of information. Despite decades of study, we still do not know for sure what triggers the formation of a disk in these stars. Certainly, rapid rotation of the star helps to launch material into orbit and form a disk but something else must also be operating since we think they rotate below their break-up speed.

I construct detailed computer models that follow disk evolution over time so that I can try to match all of the observed signatures in the light to understand how these systems work. My thesis research focuses on two particular Be stars. One star, called 66 Oph, completely lost its disk during my study. With the other star, called Pleione, the disk seems to tilt, become unstable, then tears into two pieces and eventually the cycle repeats. Finally, I investigate the temperature distribution within these tilted disks. As the disk tilts, it interacts with light coming from the star's surface which has different temperatures. Getting the disk temperatures correct is essential because it directly affects the state of the gas in the disk, which in turn, allows us to predict observables. If the gas temperature is wrong, then the interpretation of observations will be wrong too.

My research is making significant advances to understand these systems and it has the potential to help understand other types of astrophysical disks too.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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