The imprint of the disc dispersal phase on the demographics of giant planets

Jüngste Beobachtungsprogramme von Exoplaneten haben die Existenz einer beeindruckenden Vielfalt von Planetensystemen aufgezeigt. Dies wirft daher die Frage auf, wie Planetensysteme wie das Unsere entstehen und sich entwickeln können. Der Schlüssel zur Erklärung dieser Vielfalt liegt im Verständnis der statistischen Trends, die sich aus der jüngsten Fülle von Exoplanetendaten abzeichnen. Einer davon ist ein Peak in der Halbwertsachsenverteilung von Gasriesen, die sich bevorzugt bei Bahnradien von etwa 1–2 astronomischen Einheiten anhäufen. Es wurde kürzlich die Hypothese aufgestellt, dass dieses charakteristische Merkmal während der Zeit der Planetenmigration in der gasreichen protoplanetaren Scheibe entsteht, die durch die Auflösung der Scheibe mittels röntgengetriebener Photoevaporation gestoppt wird. In dieser Dissertation untersuche ich den Einfluss der Dispersionsphase der Scheibe auf den Migrationsprozess von Gasriesen, was zu einem besseren Verständnis ihrer beobachteten Demografie führt. Zu diesem Zweck habe ich mehrdimensionale, numerische Simulationen der Scheiben-Planeten-Wechselwirkung und eine als Planetenpopulationssynthese bekannte Methode verwendet. Anschließend untersuche ich, ob diese Wechselwirkung zwischen der Scheibendispersion und Planetenentwicklung einen möglichen Abdruck in der beobachteten Demografie von Gas- riesen hinterlassen kann. Indem wir die beobachteten Röntgenleuchtkräfte von Sternen mit der Halbwertsachsenverteilung ihrer Gasriesen korrelieren, identifizieren wir ein auffälliges Merkmal, das auch qualitativ von unseren Simulationen vorhergesagt wird. Dies festigt daher unsere Anfangshypothese, dass Röntgen-Photoevaporation tatsächlich die Architektur von Planetensystemen prägt. Die Ergebnisse dieser umfangreichen Studie stellen wichtige Bedingungen für aktuelle Modelle der Planetenentstehung und -entwicklung dar und geben Orientierung für zukünftige Modelle, die eine genaue Behandlung der Dispersionsphase der protoplanetaren Scheibe berücksichtigen müssen.Recent exoplanet surveys have highlighted the existence of an impressive diversity of planetary systems, raising the question of how systems like our own can form and develop. The key to explaining their diversity lies in the understanding of the statistical trends that are now emerging from the recent wealth of exoplanet data. One of these trends is a peak in the semi-major axis distribution of gas giants that preferentially clump up at orbital radii of 1–2 astronomical units. It has recently been suggested that this characteristic feature may be established during the time of planetary migration. The migration of giant planets in the gas-rich protoplanetary disc is halted by disc dispersal via X-ray driven photoevaporation. In this thesis I aim at studying the impact of the disc dispersal phase on the migration process of gas giants, leading to a better understanding of their observed demographics. For this purpose, I have used multi-dimensional numerical simulations of disc-planet interactions and a method known as planet population synthesis. I am then investigating if this interaction between disc dispersal and planet evolution can leave any potential diagnostics in the observed demographics of giant planets. By correlating the observed X-ray luminosities of giant planet host stars with the semi-major axis distribution of their giant planets, we find a prominent feature that is also predicted qualitatively by our simulations, further strengthening the conclusion that X-ray-driven photoevaporation is indeed shaping the architecture of planetary systems. The results obtained from this extensive study pose important limitations on current models of planet formation and evolution and provide guidance for future models that need to take an accurate treatment of the disc dispersal phase into account

Lowest accreting protoplanetary discs consistent with X-ray photoevaporation driving their final dispersal

Photoevaporation from high energy stellar radiation has been thought to drive the dispersal of protoplanetary discs. Different theoretical models have been proposed, but their predictions diverge in terms of the rate and modality at which discs lose their mass, with significant implications for the formation and evolution of planets. In this paper we use disc population synthesis models to interpret recent observations of the lowest accreting protoplanetary discs, comparing predictions from EUV-driven, FUV-driven and X-ray driven photoevaporation models. We show that the recent observational data of stars with low accretion rates (low accretors) point to X-ray photoevaporation as the preferred mechanism driving the final stages of protoplanetary disc dispersal. We also show that the distribution of accretion rates predicted by the X-ray photoevaporation model is consistent with observations, while other dispersal models tested here are clearly ruled out.Comment: Corrected typo in Eq 19 alpha -> log10(alpha) 6 Pages, 4 Figures, accepted for publication in MNRAS Letter

Giant planet migration during the disc dispersal phase

Transition discs are expected to be a natural outcome of the interplay between photoevaporation (PE) and giant planet formation. Massive planets reduce the inflow of material from the outer to the inner disc, therefore triggering an earlier onset of disc dispersal due to PE through a process known as Planet-Induced PhotoEvaporation (PIPE). In this case, a cavity is formed as material inside the planetary orbit is removed by PE, leaving only the outer disc to drive the migration of the giant planet. We investigate the impact of PE on giant planet migration and focus specifically on the case of transition discs with an evacuated cavity inside the planet location. This is important for determining under what circumstances PE is efficient at halting the migration of giant planets, thus affecting the final orbital distribution of a population of planets. For this purpose, we use 2D FARGO simulations to model the migration of giant planets in a range of primordial and transition discs subject to PE. The results are then compared to the standard prescriptions used to calculate the migration tracks of planets in 1D planet population synthesis models. The FARGO simulations show that once the disc inside the planet location is depleted of gas, planet migration ceases. This contradicts the results obtained by the impulse approximation, which predicts the accelerated inward migration of planets in discs that have been cleared inside the planetary orbit. These results suggest that the impulse approximation may not be suitable for planets embedded in transition discs. A better approximation that could be used in 1D models would involve halting planet migration once the material inside the planetary orbit is depleted of gas and the surface density at the 3:2 mean motion resonance location in the outer disc reaches a threshold value of $0.01\,\mathrm{g\,cm^{-2}}$.Comment: 16 pages, 11 figures; accepted for publication in A&

Linking circumstellar disk lifetimes to the rotational evolution of low-mass stars

The high-energy radiation emitted by young stars can have a strong influence on their rotational evolution at later stages. This is because internal photoevaporation is one of the major drivers of the dispersal of circumstellar disks, which surround all newly born low-mass stars during the first few million years of their evolution. Employing an internal EUV/X-ray photoevaporation model, we have derived a simple recipe for calculating realistic inner disk lifetimes of protoplanetary disks. This prescription was implemented into a magnetic morphology-driven rotational evolution model and is used to investigate the impact of disk-locking on the spin evolution of low-mass stars. We find that the length of the disk-locking phase has a profound impact on the subsequent rotational evolution of a young star, and the implementation of realistic disk lifetimes leads to an improved agreement of model outcomes with observed rotation period distributions for open clusters of various ages. However, for both young star-forming regions tested in our model, the strong bimodality in rotation periods that is observed in hPer could not be recovered. hPer is only successfully recovered, if the model is started from a double-peaked distribution with an initial disk fraction of $65\,\%$. However, at an age of only $\sim 1\,\mathrm{Myr}$, such a low disk fraction can only be achieved if an additional disk dispersal process, such as external photoevaporation, is invoked. These results therefore highlight the importance of including realistic disk dispersal mechanisms in rotational evolution models of young stars.Comment: accepted for publication in Ap

Towards a population synthesis of discs and planets. II. Confronting disc models and observations at the population level

Aims. We want to find the distribution of initial conditions that best reproduces disc observations at the population level. Methods. We first ran a parameter study using a 1D model that includes the viscous evolution of a gas disc, dust, and pebbles, coupled with an emission model to compute the millimetre flux observable with ALMA. This was used to train a machine learning surrogate model that can compute the relevant quantity for comparison with observations in seconds. This surrogate model was used to perform parameter studies and synthetic disc populations. Results. Performing a parameter study, we find that internal photoevaporation leads to a lower dependency of disc lifetime on stellar mass than external photoevaporation. This dependence should be investigated in the future. Performing population synthesis, we find that under the combined losses of internal and external photoevaporation, discs are too short lived. Conclusions. To match observational constraints, future models of disc evolution need to include one or a combination of the following processes: infall of material to replenish the discs, shielding of the disc from internal photoevaporation due to magnetically driven disc winds, and extinction of external high-energy radiation. Nevertheless, disc properties in low-external-photoevaporation regions can be reproduced by having more massive and compact discs. Here, the optimum values of the $\alpha$ viscosity parameter lie between $3\times10^{-4}$ and $10^{-3}$ and with internal photoevaporation being the main mode of disc dispersal.Comment: Accepted for publication in A&A; minor changes in the reference lis

Three-dimensional, Time-dependent MHD Simulation of Disk-Magnetosphere-Stellar Wind Interaction in a T Tauri, Protoplanetary System

We present a three-dimensional, time-dependent, MHD simulation of the short-term interaction between a protoplanetary disk and the stellar corona in a T Tauri system. The simulation includes the stellar magnetic field, self-consistent coronal heating and stellar wind acceleration, and a disk rotating at sub-Keplerian velocity to induce accretion. We find that initially, as the system relaxes from the assumed initial conditions, the inner part of the disk winds around and moves inward and close to the star as expected. However, the self-consistent coronal heating and stellar wind acceleration build up the original state after some time, significantly pushing the disk out beyond $10R_\star$. After this initial relaxation period, we do not find clear evidence of a strong, steady accretion flow funneled along coronal field lines, but only weak, sporadic accretion. We produce synthetic coronal X-ray line emission light curves which show flare-like increases that are not correlated with accretion events nor with heating events. These variations in the line emission flux are the result of compression and expansion due to disk-corona pressure variations. Vertical disk evaporation evolves above and below the disk. However, the disk - stellar wind boundary stays quite stable, and any disk material that reaches the stellar wind region is advected out by the stellar wind.Comment: Accepted to ApJ, 12 pages, 11 figure

Addressing Outstanding Problems in the Physics of Massive Stars with the Line Emission Mapper X-ray Probe

We present some of the salient aspects of the scientific motivation for high resolution soft X-ray spectroscopy of early-type stars with the Line Emission Mapper X-ray Probe. The major strength of {\it LEM} for hot star physics is its large effective area, aided by the inherent energy resolution of its microcalorimeter that readily achieves resolving powers of 1000 and obviates the need for relatively inefficient dispersive optical elements. This increased sensitivity enables much fainter and more distant high mass stars to be observed than are accessible with present-day facilities, greatly increasing the pool of potential targets. For brighter sources, the sensitivity opens up time domain studies, wherein sufficient signal can be garnered in short order and exposure times, probing source variations on ks timescales. We argue that these capabilities of {\it LEM} will yield breakthroughs in all types of hot star systems, from understanding single OB and WR star winds and how they vary with metallicity, to probing the shocks of colliding wind systems and the magnetically channeled winds of magnetic OB stars. {\it LEM} will also study the energetics of WR star bubbles and feedback from their powerful pre-SN stellar winds.Comment: A Line Emission Mapper White Pape

Breakthroughs in Cool Star Physics with the Line Emission Mapper X-ray Probe

We outline some of the highlights of the scientific case for the advancement of stellar high energy physics using the Line Emission Mapper X-ray Probe ({\it LEM}). The key to advancements with LEM lie in its large effective area -- up to 100 times that of the {\it Chandra} MEG -- and 1~eV spectral resolution. The large effective area opens up for the first time the ability to study time-dependent phenomena on their natural timescales at high resolution, such as flares and coronal mass ejections, and also opens the sky to much fainter targets than available to {\it Chandra} or {\it XMM-Newton}.Comment: A Line Emission Mapper X-ray Probe White Pape