Research

Protoplanetary disks

The dusty gas disks surrounding many young stars are the left-overs of the star formation process. These disks are often called "protoplanetary disks" because it is believed that they are the birthplaces of planets and planetary systems. Our own Solar System was once formed from such a disk surrounding the young sun, 4.567 billion years ago. With powerful telescopes such disks can be observed and studied around young stars, and we can thus get a glimpse of what our own pre-planetary solar system looked like at its birth.

In my research I study the structure and evolution of protoplanetary disks using numerical modeling, and the comparison of these models to observations. This research involves the study of their viscous evolution and the transport processes happening inside of these disks (e.g. Dullemond, Natta & Testi 2006, Visser & Dullemond 2010). It also involves studying their detailed structure, in particular the temperature structure (the warm surface layer, cool interior, internal heat production, etc, see e.g. Dullemond & Dominik 2004a, Kamp & Dullemond 2004, Min, Dullemond, Kama & Dominik Icarus, submitted as of Nov 2010), which is computed using radiative transfer calculations (see my software web site for more information about my radiative transfer tools). Another important aspect that I am interested in is the photoevaporation of these disks by the EUV, FUV and X-ray radiation from the star itself (Gorti, Dullemond & Hollenbach 2009).

Using my radiative transfer codes I have also been involved in many observational campaigns: To interpret observations of protoplanetary disks and possibly their envelopes in terms of disk models. Most of this work is in fact carried out by my collaborators, who use my codes (RADMC, RADMC-3D) to interpret their data (e.g. Pontoppidan et al. 2007a and 2007b, Andrews et al. 2009 and 2010 and many more).

Dust coagulation in disks

What originally started as part of my research on disk structure gradually became my new main field of research: the growth of dust grains in protoplanetary disks due to the process of aggregation (also often called coagulation). The original goal was to understand how the opacities change over time in protoplanetary disks, and to be able to understand the wealth of solid state features and feature shapes seen in emission in infrared spectra of these disks (ISO, Spitzer, VLT, Keck, etc). We developed models of dust coagulation which we then insert into our radiative transfer tools to make synthetic spectra and images, which can then be compared to observations (early work: Dullemond & Dominik 2005, 2004 2008, Dominik & Dullemond 2008)

However, dust growth is also the very first step in the process of planet formation. By 2006 this became the main focus of this research, and my students basically took charge of this. Firstly, Frithjof Brauer managed to design a full-disk-scale dust coagulation modeling code that is able to evolve the dust throughout the disk including the fragmentation (Brauer, Dullemond & Henning 2008), something which I did not manage to do efficiently in my 2005 paper. He also found a way to overcome the dreaded "meter size barrier" by trapping particles in a pressure trap (Brauer, Henning & Dullemond 2008).

Subsequently, Til Birnstiel improved Frithjof's model and merged it with a disk evolution model (Birnstiel, Dullemond & Brauer 2009). Now the model is (almost) complete: it is very efficient (using a fully implicit integration method), it includes radial drift and mixing as well as growth and fragmentation. In addition to this, he developed semi-analytic model fits to his results, which can be useful for other modelers ( Birnstiel, Ormel & Dullemond 2010), and applied his model to millimeter wave data of disk (Birnstiel, Ricci, Trotta, et al. 2010).

At the same time, Andras Zsom worked with the Braunschweig laboratory group to develop a new experiment-based dust coagulation kernel (Güttler, Blum, Zsom et al. 2010). It is the first time that a dust coagulation kernel has been constructed that is based on the last 10 years of laboratory collision experiments, and this is thus a major step forward. However, as a result of this work they found a new barrier to growth: the "bouncing barrier" (Zsom, Güttler et al. 2010). At the moment we do not yet understand how Nature is apparently able to overcome this barrier.

From planetesimals to planets

The work on dust coagulation has led us to ask the question how the next step proceeds: How are rocky planets formed from swarms of planetesimals? Chris Ormel, a Humboldt postdoctoral Fellow linked to our group, has developed an ingenious new method for modeling this problem. It is a Monte Carlo method that carefully deals with the transition from runaway growth to oligarchic growth (Ormel, Dullemond & Spaans 2010). This method is particularly powerful in that it can span a huge range in planetesimal size, by use of a clever grouping method.

We are currently moving more and more in this (for us new) direction, and we have the ultimate goal of modeling rocky planet formation all the way from dust to planets. In the next few years I hope to be able to report on many new results in this area.

Radiative transfer

I spend some amount of my time on developing radiative transfer tools. This is not really my "research" field. It is meant as a crucial tool to link our models to direct observations of the objects we study. So, to ensure also in the future a smooth linkeage between our models of disks and dust on the one hand and direct observations of our objects on the other hand, I spend time on updating my radiative transfer tool set.

It has turned out, however, that many other scientists also have interest in using these tools. I have therefore decided that I spend considerable effort in making these tools easy-to-use and well-documented, and publically available. I see this as a service to the community.

Collaboration with others at the ZAH

Planet formation is strongly linked to the process of star formation. At the ITA there is a strong expertise on this topic in the star formation group of Ralf Klessen. For this reason, and because of my expertise in 3-D radiative transfer, I collaborate strongly with his star formation group.

I am also setting up a collaboration with Rainer Spurzem on N-body modeling of planetesimals and planets during planet formation.

I hope to be setting up more collaborations with people at the ZAH in the months/years to come.