Coregrowth: a python toy model to follow planet formation

Protoplanet growth is complex.  There are a multitude of physical processes -- for instance, planetesimal fragmentation, radial drift, turbulent diffusion, gas drag -- that determine its efficiency.  Catching all these mechanisms in one self-consistent model is virtually impossible.  Let alone to perform a statistically viable parameter study.

Here, a toy model serves as a useful tool to quickly explore the parameter space. We construct a toy model for the protoplanet growth, emphasizing simplicity and versatilty....

[More...] [ArXiv]

Opacity calculation for evolving dust aggregates in dense molecular clouds

Coagulation will affect he dust size distribution in dense molecular clouds.  This, in turn, affects the dust opacity -- a critical quantity required for any interpretation of observational data sets. Following previous work previous work where we computed the dust size distribution as function of time, we now present the corresponding mass-weighted opacities for infra-red wavelengths.

The figure shows that the opacities change on timescales of ~Myr (or less if the cloud's density is higher than the assumed n=10⁵ cm^-3).  At visible and near-IR wavelengths the opacitiy decreases, but at longer wavelengths it will increase with time.  We have quantified this evolutionary trend in terms of the strength in the 9.7μm silicate feature vs near-IRcolor excess and in terms of the sub-mm slope β.

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

The effect of gas drag on the growth of protoplanets

Protoplanets can sweep-up particles effectively due to their gravitational focusing effect which allows particles to be accreted with a collision cross section much larger than the geometrical cross section of the protoplanets.  The amount of focusing depends to a large degree on the velocity at which the bodies approach, with the largest focusing being achieved at low relative velocities.  This effects is well described in the literature; however the effects of gas drag --a force that becomes especially important for small particles-- on this process are less clear.  In this project we determined how gas drag affects the gravitational focusing of particles.

The figure shows some examples of particle trajectories under the influence of varying levels of gas drag.  The protoplanet is in the center of the coordinate system.

[More...] [ADS] [ArXiv] [A&A Highlights]

Distinguishing between runaway and oligarchic growth of protoplanets

When bodies are large enough to attract each other gravitationally  the effective cross section for collisions becomes larger than the geometrical cross section.  This enhancement factor is called the gravitational focusing factor (GFF) and is approximately given by the square of the ratio of the escape velocity of the protoplanet and the relative velocity at wich the bodies approach.  The nature of this phenomenon is such that the largest bodies grow faster than other bodies, a situation referred as runaway growth (RG). RG indicates a positive feedback effect: due to the growth, the gravitational focusing increases.  RG or, generally, a large GFF is required in order to grow protoplanets in a sufficiently short time span. 

However, the positive feedback of RG is counteracted by viscous stirring, the (long-range) deflection of bodies' trajectories that causes the mean relative motion to increase.  The growth of the protoplanet then slows down, switching to the much slower oligarch growth stage. [More...] [ADS] [arXiv]

Monte Carlo simulations of planetesimal accretion

When bodies reach km-sizes (planetesimals), their collisional and dynamical evolution becomes dominated by gravity. Ideally, one would model the collisional evolution by an N-body methods; hower the shear number of planetesimals limit these attempts in practise.  However, to model the (runaway and oligarchic) growth correctly, its discrete nature must be taken into account.  For this reason we have extended our Monte Carlo /superparticle code to treat dynamical interactions. [More...] [ADS] [ArXiv] [Movie]