Generating Planet-Induced Vortices with Slowly-Growing Gap-Opening Planets
Author
Hammer, MichaelIssue Date
2021Keywords
Astronomical observationsHydrodynamics
Numerical simulations
Planets
Protoplanetary discs
Vortices
Advisor
Kratter, Kaitlin M.
Metadata
Show full item recordPublisher
The University of Arizona.Rights
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction, presentation (such as public display or performance) of protected items is prohibited except with permission of the author.Abstract
In the past decade, the Atacama Large Millimeter/sub-millimeter Array (ALMA) became the first telescope capable of collecting a myriad of highly-resolved protoplanetary disc observations, confirming the long-thought belief that protoplanetary discs are not featureless and instead are rich in structure. Among the structures ALMA has observed are large-scale crescent-shaped features often at the edges of gaps or cavities in these discs. Such features could potentially be explained by gap-opening planets generating dust-trapping vortices through the Rossby Wave instability (RWI) at one or both of their gap edges where a maximum in the disc's radial inverse vortensity profile develops. It remains an open question as to whether such vortices can survive for a long enough period of time to expect to observe even the relatively small number of crescent-shaped features found in observations. Developing strong constraints on expected vortex lifetimes could potentially also constrain disc conditions and planet masses, properties that if known precisely would make it possible to study which disc conditions connect to which types of planets and to be able to connect the population of newly-formed planets in discs to the older population of planets detected more directly around main sequence stars. Previous studies have demonstrated that various effects such as dust feedback, disc self-gravity, layered viscosity profiles, and sub-optimal disc aspect ratios can all drastically shorten vortex lifetimes. In this work, we explore yet another effect that can alter both the lifetimes and appearances of planet-induced vortices that has been largely neglected in previous studies, namely the growth of the planet. In the core accretion model, the preferred method for forming most gap-opening planets, the growth of the planet is not instantaneous, even in the runaway gas accretion phase when it accretes the bulk of its final mass. In spite of these potentially slow planetary growth timescales, previous computational studies of planet-induced vortices by others merely introduce a fully-grown planet into the disc on an unrealistically quick timescale of 10 to 100 planetary orbits. In our work, we study the effects of introducing the planet into our simulations on a slower, more realistic timescale with two different growth methods, namely prescribed growth and the more realistic process of a planetary core accreting its gaseous atmosphere directly from the disc. With realistic planet growth timescales, we find the planets highly preferentially induce vortices that are elongated, a stark contrast from the compact vortices that typically form with unrealistically quick planet growth. The underlying difference in the structure of these elongated vortices is that they do not develop a minimum Rossby number $< -0.15$ that is needed for them to become compact. With a more elongated extent in the gas and a flatter pressure bump through the bulk of the vortex, we find the dust trapped in these elongated vortices circulates around nearly the entire azimuthal extent of the vortex instead of spiraling inwards to the center like in compact vortices. As result, elongated gas vortices should typically have elongated azimuthal extents and off-center peaks in ALMA observations, two features that distinguish them from compact vortices. Double peaks are also possible while the dust is circulating through the middle of the vortex. While higher viscosities ($\alpha \sim 10^{-4}$) are strong enough to cause these vortices to decay into rings, we find that shocks from the planet's spiral waves are more responsible for breaking up elongated vortices in lower-viscosity discs ($\alpha \sim 10^{-5}$). As a result, elongated planet-induced vortices are much longer-lived in regions of a low-viscosity disc with larger aspect ratios due to the weaker shocks associated with the planet in these conditions. With lower aspect ratios, low-mass planets can still induce long-lived by asymmetries by causing the vortex to re-form. With higher-mass planets, however, the gap typically becomes too wide too quickly to maintain the prospect of the vortex re-forming beyond a relatively short amount of time. As a consequence of these effects and somewhat counterintuitively, lower-mass planets tend to produce longer-lived asymmetries than planets near Jupiter's mass in discs with low to intermediate aspect ratios. Overall, the long lifetimes we expect in the lower-mass planet cases do not seem to be consistent with the paucity of crescent-shaped features in protoplanetary disc observations, in particular those with two-sided gaps that are conventionally expected to arise from planets in a protoplanetary disc. This discrepancy adds support for the proposed mechanisms for shortening vortex lifetimes. Nonetheless, the dust signatures we find with vortices induced by slowly-grown planets are a natural explanation for the elongated extent and off-center peak in HD 135344 B, one of the few discs with a crescent-shaped feature at the edge of a conventional two-sided gap. They can also potentially explain more bizarre signatures such as the two clumps in MWC 758, the separation between the dust and gas cavities in Oph IRS 48, the double peaks in HD 142527 or V1247 Orionis. Our work lays the foundation for understanding how planet-induced vortices could potentially constrain disc conditions and planet masses with better-resolved multi-wavelength observations in the future.Type
textElectronic Dissertation
Degree Name
Ph.D.Degree Level
doctoralDegree Program
Graduate CollegeAstronomy