Hubert Klahr - Academia.edu (original) (raw)

Papers by Hubert Klahr

We study axisymmetric models of layered protoplanetary discs taking radiative transfer effects in... more We study axisymmetric models of layered protoplanetary discs taking radiative transfer effects into account, and allowing for a residual viscosity in the dead zone. We also explore the effect of different viscosity prescriptions. In addition to the ring instability reported in the first paper of the series we find an oscillatory instability of the dead zone, accompanied by variations of the accretion rate onto the central star. We provide a simplified analytical description explaining the mechanism of the oscillations. Finally, we find that the residual viscosity enables stationary accretion in large regions of layered discs. Based on results obtained with the help of a simple 1-D hydrocode we identify these regions, and discuss conditions in which layered discs can give rise to FU~Orionis phenomena.

The planetary mass-radius diagram is an observational result of central importance to understand ... more The planetary mass-radius diagram is an observational result of central importance to understand planet formation. We present an updated version of our planet formation model based on the core accretion paradigm which allows us to calculate planetary radii and luminosities during the entire formation and evolution of the planets. We first study with it the formation of Jupiter, and compare with previous works. Then we conduct planetary population synthesis calculations to obtain a synthetic mass-radius diagram which we compare with the observed one. Except for bloated Hot Jupiters which can be explained only with additional mechanisms related to their proximity to the star, we find a good agreement of the general shape of the observed and the synthetic M - R diagram. This shape can be understood with basic concepts of the core accretion model.

The Astrophysical Journal, 2015

The Astrophysical Journal

ABSTRACT

Proceedings of the International Astronomical Union, 2008

... in massive star formation and accretion disks Rolf Kuiper, Mario Flock and Hubert Klahr Max .... more ... in massive star formation and accretion disks Rolf Kuiper, Mario Flock and Hubert Klahr Max ... Right: Evolution of Maxwell alpha at different resolution, converging against each other ... A&amp;A 457, 343 Mignone, A., Bodo, G.. Massaglia, S. Matsakos, T., Tesileanu, O., Zanni, C., &amp; Ferrari ...

Astronomische Nachrichten

Context: During their evolution, massive stars quickly become so luminous that their radiation pr... more Context: During their evolution, massive stars quickly become so luminous that their radiation pressure onto the environment exceeds their gravitational attraction. Hence, radiation pressure plays a major role in shaping the circumstellar environment. Methods: Numerical highlights of our 1D, 2D, and 3D self-gravity radiation hydrodynamics simulations of various collapsing pre-stellar cores of gas and dust include a grid in spherical coordinates with non-uniform resolution down to 1 AU (Kuiper et al. 2010, ApJ 722), a highly accurate frequency-dependent ray-tracing approach of the stellar irradiation (Kuiper et al. 2010, A&A 511), and the consideration of temperature- and density-dependent gas opacities (Kuiper & Yorke 2013, ApJ). Results: We determine the impact of the radiation pressure on the forming accretion disk and the bipolar outflow cavities: a) The well-known radiation pressure problem in the formation of massive stars can be circumvented via classical disk accretion: The f...

This document contains refereed supplementary information for the paper ``Rapid planetesimal form... more This document contains refereed supplementary information for the paper ``Rapid planetesimal formation in turbulent circumstellar discs''. It contains 15 sections (\S1.1 -- \S1.15) that address a number of subjects related to the main paper. We describe in detail the Poisson solver used to find the self-potential of the solid particles, including a linear and a non-linear test problem (\S1.3). Dissipative collisions remove energy from the motion of the particles by collisional cooling (\S1.4), an effect that allows gravitational collapse to occur in somewhat less massive discs (\S1.7). A resolution study of the gravitational collapse of the boulders is presented in \S1.6. We find that gravitational collapse can occur in progressively less massive discs as the grid resolution is increased, likely due to the decreased smoothing of the particle-mesh self-gravity solver with increasing resolution. In \S1.10 we show that it is in good agreement with the Goldreich & Ward (1973) st...

Turbulence generated by the magneto-rotational instability (MRI) is a strong candidate to drive a... more Turbulence generated by the magneto-rotational instability (MRI) is a strong candidate to drive accretion flows in disks, including sufficiently ionized regions of protoplanetary disks. The MRI is often studied in local shearing boxes, which model a small section of the disk at high resolution. I will present simulations of large, stratified shearing boxes which extend up to 10 gas scale-heights across. These simulations are a useful bridge to fully global disk simulations. We find that MRI turbulence produces large-scale, axisymmetric density perturbations . These structures are part of a zonal flow --- analogous to the banded flow in Jupiter's atmosphere --- which survives in near geostrophic balance for tens of orbits. The launching mechanism is large-scale magnetic tension generated by an inverse cascade. We demonstrate the robustness of these results by careful study of various box sizes, grid resolutions, and microscopic diffusion parameterizations. These gas structures ca...

models of turbulence in protoplanetary disks I. A cylindrical potential on a Cartesian grid and t... more models of turbulence in protoplanetary disks I. A cylindrical potential on a Cartesian grid and transport of solids

formation bursts at the borders of the dead zone in 2D numerical simulations of circumstellar disks

Astrophysics and Space Science Proceedings, 2014

ABSTRACT We review our recent studies demonstrating that the radiation pressure problem in the fo... more ABSTRACT We review our recent studies demonstrating that the radiation pressure problem in the formation of massive stars can be circumvented via an anisotropy of the thermal radiation field. Such an anisotropy naturally establishes with the formation of a circumstellar disk. The required angular momentum transport within the disk can be provided by developing gravitational torques. Radiative Rayleigh-Taylor instabilities in the cavity regions - as previously suggested in the literature - are not required and are shown to be not occurring in the context of massive star formation.

We review the current theoretical understanding how growth from micro-meter sized dust to massive... more We review the current theoretical understanding how growth from micro-meter sized dust to massive giant planets occurs in disks around young stars. After introducing a number of observational constraints from the solar system, from observed protoplanetary disks, and from the extrasolar planets, we simplify the problem by dividing it into a number of discrete stages which are assumed to occur in a sequential way. In the first stage - the growth from dust to kilometer sized planetesimals - the aerodynamics of the bodies are of central importance. We discuss both a purely coagulative growth mode, as well as a gravoturbulent mode involving a gravitational instability of the dust. In the next stage, planetesimals grow to protoplanets of roughly 1000 km in size. Gravity is now the dominant force. The mass accretion can be strongly non-linear, leading to the detachment of a few big bodies from the remaining planetesimals. In the outer planetary system (outside a few AU), some of these bodi...

EPJ Web of Conferences, 2011

Astrophysical Journal, 2006

We perform numerical simulations of the Kelvin-Helmholtz instability in the midplane of a protopl... more We perform numerical simulations of the Kelvin-Helmholtz instability in the midplane of a protoplanetary disk. A two-dimensional corotating slice in the azimuthal-vertical plane of the disk is considered, where we include the Coriolis force and the radial advection of the Keplerian rotation flow. Dust grains, treated as individual particles, move under the influence of friction with the gas, while the

We explore the effect of magnetorotational turbulence on the dynamics and concentrations of bould... more We explore the effect of magnetorotational turbulence on the dynamics and concentrations of boulders in local box simulations of a sub-Keplerian protoplanetary disc. The solids are treated as particles each with an independent space coordinate and velocity. We find that the turbulence has two effects on the solids. 1) Meter and decameter bodies are strongly concentrated, locally up to a

Astronomy and Astrophysics

A first characterization of many exoplanets has recently been achieved by the observational deter... more A first characterization of many exoplanets has recently been achieved by the observational determination of their radius. For some planets, a measurement of the luminosity has also been possible, with many more directly imaged planets expected in the future. The statistical characterization of exoplanets through their mass-radius and mass-luminosity diagram is thus becoming possible. This is for planet formation and evolution theory of similar importance as the mass-distance diagram. Our aim in this and a companion paper is to extend our formation model into a coupled formation and evolution model. We want to calculate in a self-consistent way all basic characteristics (M,a,R,L) of a planet and use the model for population synthesis calculations. Here we show how we solve the structure equations describing the gaseous envelope not only during the early formation phase, but also during gas runaway accretion, and during the evolutionary phase at constant mass on Gyr timescales. We th...