Aaron T. Lee, Ph.D.
Astronomer and Educator
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Research Interests

Star & Planet Formation

 
 

Research Topics

 
A binary system with a third companion and circumbinary disk.

A binary system with a third companion and circumbinary disk.

THE FORMATION OF BINARY SYSTEMS

Almost half of the stars in the Milky Way appear to have a secondary companion. This fraction is even higher in regions were stars are forming. What causes collapsing clouds of gas to form single stars versus binary stars? This likely depends on the birth environment of the stars but the exact role that magnetic fields, protostellar outflows, turbulence, etc., is unknown. Through large simulations, I have studied the formation and evolution of binary star systems in magnetized clouds.

I found that magnetic fields regulate the number of binary systems the form. Without magnetic fields, protostars form in highly clustered environments and gravitational interactions ultimately disrupt the pairs. While weaker magnetic fields ultimately produce more stars, the stars are more often single stars compared to binary or multiple-star systems.

 
A protostar (center) accretes from a magnetized gas.

A protostar (center) accretes from a magnetized gas.

ACCRETION FROM (TURBULENT) MAGNETIZED GAS

Stars form in large clouds of gas called molecular clouds. These clouds of predominately molecular hydrogen may undergo collapse from their own gravity, and this results in the formation of stars. These young stars can roam through the cloud and accrete additional material material. This gas the stars attempts to accrete, however, is threaded by a magnetic field, which affects the dynamics of the gas. Particularly, as gas is brought closer to the star, the gas drags with it the magnetic field, causing the field to build up around the star. This field resists further compression and can ultimately push material away from the star. How then can stars accrete more mass?

We find that magnetic fields resist compression and ultimately reduce the accretion rates onto protostars. However, when the gas is turbulent, sometimes the combination of turbulence and magnetic fields can amplify accretion rates onto protostars. The exact relationship between accretion, magnetic fields, and turbulence, is not yet understood.

 
Cylindrical structure of a star-forming dense core. Magnetic field lines pervade the core and support it against gravitational collapse.

Cylindrical structure of a star-forming dense core. Magnetic field lines pervade the core and support it against gravitational collapse.

THE INTERNAL STRUCTURE OF DENSE CORES

As clouds of gas collapse to form protostars, quiescent dense cores form within the cloud. These cores are the natal grounds for individual or binary stars. How these cores undergo collapse is unclear—do they form out of equilibrium, or do they calmly transition to a point of collapse?

We analytically calculated the critical state of these dense cores and find that a core a few times the mass of the Sun is necessary to undergo collapse.

 
Dynamical friction cartoon. An over-dense wake tugs back on a star, slowing it down.

Dynamical friction cartoon. An over-dense wake tugs back on a star, slowing it down.

DYNAMICAL FRICTION IN A GAS

Whenever a massive object passes through a rarefied medium, it draws surrounding matter toward it. As a result, this material creates an overdense wake behind the object that exerts its own gravitational pull, retarding the original motion. Such dynamical friction arises whether the medium consists of non-interacting point particles, e.g., a stellar cluster, or a continuum fluid, e.g., an interstellar cloud. Material can also fall onto the mass, which imparts both mass and momentum to the particle. The combination of these contributions contribute to the overall gravitational friction force. 

Despite the widespread occurrence of gaseous dynamical friction, there is still no generally accepted derivation of the force, even after 70 years of effort. In the case where the gas is collision-less, the analytic formulation is well known. However, with an isothermal gas, the flow in the vicinity of the gravitating mass is complex both temporally and spatially, as many simulations have shown. We derived an analytical expression for this dynamical friction force in a series of two papers.

We find that the friction force is related to the accretion rate onto stars in a simple manner: the force is the relative velocity multiplied by the mass accretion rate.

 
A gravitationally unstable midplane of an accretion disk goes unstable and throws material away from the central region of the disk.

A gravitationally unstable midplane of an accretion disk goes unstable and throws material away from the central region of the disk.

THE FORMATION OF PLANETESIMALS

In the most venerable of planet-forming scenarios, planetesimals--the km-sized building blocks of rocky planets and gas giant cores--form in the mass-rich midplane of a circumstellar disk through the gravitational collapse of m-sized boulders. However, as mass settles toward the midplane, shear between the dust-rich midplane and the poorer regions above and below produce a shear that can potentially overturn this layer. Additionally, these m-sized objects experience significant drag with the gas, resulting in radial migration time scales of 102 years, which is significantly shorter than the 106 year time scale for planet formation.

If the heavier materials in the disk gently settle toward the midplane, then it is conceivable that the profile of the dust settles into a state with a nearly constant Richardson number. As the dust becomes more centrally condensed, the value of Ri will decrease. Ultimately the shear will become too strong and turnover the midplane, reducing the dust density there. 

But if instead the dust density could continue increasing, it is possible the dust density could exceed a critical value and go gravitationally unstable, where then the midplane would fragment and the dust would collapse, potentially forming planetesimals directly from the mm-sized and smaller dust grains.