Stars, Binaries and Planets: Their Properties and System Architecture

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandling

The dominant formation mechanism of close binary stars and short-period planets is unknown. Disc migration says that stars and planets form and migrate within circumstellar discs. High-eccentricity migration says that dynamical interactions between multiple bodies generate highly eccentric orbits that are subsequently tidally circularised. The latter class of theories predict that the orbital plane is often misaligned relative to the stellar spin-axis -- a prediction seemingly confirmed by observations of misaligned short-period planets. However, an incomplete knowledge of star formation and stellar tides prevent a clear interpretation of current observations. Disc migration is usually assumed to produce aligned orbits, an assumption that hinges on protoplanetary discs being well-aligned. Unfortunately, this assumption has no strong basis in theory or observations.

In this thesis, I expand our knowledge of spin-orbit alignment beyond short-period planets. I analyse the spin-orbit alignment of visual binaries on wide orbits and find that a previously reported trend in the spin-orbit alignment is spurious. I present an analysis of the bright planet-hosting binary star $\tau$ Boötis. By combining decades of astrometric data and high-precision radial velocities, I constrain the projected spin-orbit alignment of the system and find it to be well-aligned and coplanar. I argue that the eccentric stellar binary and close-in planet formed and migrated in well-aligned discs. This system offers strong evidence that disc migration is a viable mechanism for producing short-period planets.

Poorly understood tidal forces are a major challenge for the interpretation of spin-orbit angles. I investigate the role of tidal forces in shaping the orbits of close binaries. To do this, I discover and characterise hundreds of close eclipsing binaries and combine these with known systems to measure the orbital eccentricity distribution over a wider range of stellar temperatures and orbital distances than previously explored. I find that a large fraction of hot binary stars are circularised much more efficiently than predicted by tidal theory.

Finally, I present preliminary results of a study of the alignment of highly eccentric binaries. Here, we find that three out of five systems with reliable spin-orbit measurements are consistent with alignment. However, the two misaligned systems are the longest-period systems in our study, suggesting possible tidal influence. These findings tentatively suggest that disc migration may be a common mechanism for forming eccentric close binaries or that tidal theory needs a major revision.
ForlagAarhus Universitet
Antal sider211
StatusUdgivet - 2020

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