Chris and the Middle Kingdom Chris Ormel

Research Group Leader
Anton Pannekoek Institute for Astronomy
University of Amsterdam (UvA)
The Netherlands

c.w.ormel AT uva DOT nl

News Flashes

March 2018

The efficiency of pebble accretion

What is the likelihood of a drifting pebble to be accreted by a planet? This number — ε — depends on the ratio of two quantities: the pebble accretion rate by the planet and the drift speed of the pebble. These quantities in turn depend on many other parameters (planet mass, planet eccentricity and inclination, stellar mass, turbulence, disk headwind, etc.). In two works we systematically calculate &epsilon. ...

[More...] [ADS 1] [ArXiv 1] [ADS 2] [ArXiv 2] [Download]

January 2018

Pebble accretion in realistic flows

Because of settling, planets embedded in disks can accrete pebbles at a large cross section, under the condition that pebbles can actually reach the planet. Large planets may open a (shallow) gap in the disk, resulting in pebbles piling up at pressure maxima away from the planet. Small planets, likewise, affect the gas disk in their vicinity, resulting in streamlines that curve away from the planet, such as the well-known horseshoes. Because of their strong coupling to the gas, small particles are particularly affected.

This is illustrated in the animation, which shows pebble trajectories and gas streamlines obtained with the new hydrodynamical code DISPATCH. ...

[More...] [ADS] [ArXiv]

accretion of pebbles in realistic gas flows

December 2017

How the core affects the radius of sub-Neptune Planets

Sub-Neptune planets are special. While most of their mass resides in the "rocky" core, their radius is determined primarily by the state of the gaseous envelope. The standard assumption is that the radius is set by a combination of the composition and mass of the envelope. But in this Letter we investigated another possibility: the (remnant) heat of the core that is slow to "escape". ...

[More...] [ADS] [ArXiv]

November 2017

When pebbles become too hot for cores to eat

Pebbles don't just get accreted when they enter a pre-planetary atmosphere — they also get heated! Since the pre-planetary atmosphere (the concentration of gas around a gravitating body in a gas-rich disks) can become very hot, the small pebble may even not reach the core of the planet! Instead, it would end up in vaporized form in the atmosphere. We conducted a 1D calculation to find the mass where this occurs of a pebble of silicate composition and found complete vaporization for a planet a fraction of Earth's. Clearly, growth of a planets accreting pebbles proceeds very differently than accretion of planetesimals!

See this astrobite article by Michael Hammer for a great summary!

[More...] [ADS] [ArXiv] [Astrobite]

September 2017

Learning about pebbles from FU-Ori outbursts

FU Ori outburst are events where the stars becomes very luminous and the disk very hot, presumably for a short amount of time. This makes these systems ideal to study phenomena related to the H2O iceline. One such star is V883 and here we interpret recent ALMA observations, constraining key properties of the pebbles interior and exterior to the H2O iceline.

[More...] [ADS] [ArXiv]