Planet Formation

May 29, 2015

This talk session is all about planet formation, and is co-chaired by Rachel Worth and Kimberly Cartier. This blog post has been written by Ben Nelson.

Planet Formation in Binary Systems - how Solid is the 1 km Barrier? (Kedron Silsbee, Princeton)

There are two types of binary gemoetries: S-type (circumprimary) and P-type (circumbinary). Binary planet formation is difficult. Planetesimal eccentricity gets excited to high levels and leads to high collision velocities (~1km/s). This problem isn’t as bad when the binary and planetary semi-makor axes are very different. Maybe planet migration helps? They want to know where the planet disks form in the binary.

Stellar binaries can induce secular perturbations in a non-Keplerian potential and an eccentric disk can arise. Disk gravity dominates eccentricity excitation and precession of the free eccentricity throughout much of the disk. Small planetesimals get aligned with the disk. Large planetesimals end up on orbits dictated by the gravity of the disk.

A secular resonance (same time averaged precession rate between two objects) due to the combination of disk gravity and binary gravity at a few AU makes planetesimal coagulation very difficult at those separatons. In situ planet formation in Kepler-16 like systems is difficult even under the most favorable disk assumptions without invoking several km.sized initial planetesimals.

Effects of Disk Photoevaporation on Planet Migration (Alexander W. Wise, University of Delaware)

In type-II migration, material entering a gap formed by a giant planet interacts with the outermost resonances. The asymmetrical strength of the inner and outer resonances drives the giant planet inward. Alexander is looking at how photoevaporation erodes the surface density of the disk and how that affects migration of the planet. Depending on what part of the disk gets eroded, the planet can stop migrating (outer resonant locations) or migrate even quicker (inner resonant locations get eroded).

Vortices in Dead Zones of Protoplanetary Disks (Ryan Miranda, Cornell)

Deadzones in protoplanetary disks (areas with suppressed angular momentum transport) exist in the midplane from ~0.1 to at least several AU. Rossby wave instabilities occur at axisymmetric “bumps in inverse vortensity. Dead zones are a convenient way of producing this bumps and therefore an instability since they are MRI-inactive.

Simulations start off with a smooth disk then mass piles up in the dead zone. “Modes” form and move at slightly different frequencies, causing them to catch up and merge with each other. Visocity parameter “alpha” > 0.04 - 0.07 suppresses these instabilities. Anticyclonic vortices trap dust grains which can result in rapid planetesimal formation.

Dynamical stability of imaged planetary systems in formation: Application to HL Tau (Daniel Tamayo, University of Toronto)

ALMA got this spectacular image of concentric gaps in the HL Tau disk. Are we really seeing giant planets forming in each of these gaps? Probably can’t stick one in ALL of them. An instability occurs on rapid timescales (~10^5 years)

Neptune mass objects won’t create an instability but then again aren’t massive enough to carve these gaps. It turns out, the outer three gaps are in locations associated with resonances with the inner gaps. But the gaps are eccentric! This can arise when planets’ eccentricities are caught between being excited from a MMR and being damped from the disk. Perhaps the planets got caught in resonance and grew around the same time.

After the disk dissipates, these giant planets can scatter off one another to ejection/high eccentricities. So HL Tau system as we know it might not be long lived. This could explain all those RV planets with high eccentricities.

Formation of Misaligned Hot Jupiters in Stellar Binaries (Kassandra Anderson, Cornell)

There are a lot of Hot Jupiters with significant star spin - orbit misalignment. One way of forming this is through a hierarchical triple system (planet at ~few AU and binary at ~100s of AU). If their mutual inclination is >40 degrees, the perturbations of the binary cause the planet’s inclination and eccentricity to oscillate with the conserved quantity sqrt(1-e^2)*cos(i). This is the Lidov-Kozai mechanism.

During these Kozai cycles, what’s the stellar spin doing? It’s definitely not sitting there. The spin angle chaotically evolves but eventually settles to a constant value as the planet’s orbit decays. From population synthesis models, the fraction of systems resulting in a Hot Jupiter depends on the planet mass. Distribution in spin-orbit angle is always bimodal for Jupiter mass planets. Higher mass planets are typically more misaligned. It’s difficult to produce Hot Saturns by Lidov-Kozai.

Chaotic Dynamics of Stellar Spin Driven by Planets Undergoing Lidov-Kozai Oscillations (Natalia I. Storch, Cornell) Conveniently continuing from where Anderson left off! Can the stellar spin axis keep up with the planet orbital axis? If planet’s orbital axis precesses rapidly, star sees this as a “time-averaged” spin axis, so spin axis precesses slowly (“non-adiabatic”). If planet’s orbital axis is slow, the opposite effect happens (spin axis precessing rapidly, “adiabatic”).

So how is the chaotically evolving explained axis explained? Chaos is often a consequence of overlapping resonances. We can use Hamiltonian perturbation theory and get a resonance condition: averaged stellar spin precession = integer multiple of eccentricity oscillation rate.

So what about the bimodality in the spin - orbit axis distribution? As planet’s semi-major axis decays, you go from the “non-adiabatic” case to “adiabatic” case described above. Bimodality is the result of interaction with the N=0 resonance during this transition.

Eccentricity Excitation of Giant Planets: Shedding Light on the Eccentricity Valley (David Tsang, McGill University)

How do disks affect eccentricity? You can think of it as changing the energy - angular momentum ratio. Outer Lindblad resonances like to pump eccentricity, inner Lindblad resonances like to damp eccentricity. If a gap is cleared, inner Lindblad resonances go away, but non-co-orbital corotation resonances damp eccentricity. Gap heating can cause eccentricity excitation.

Let’s jump to a planetary census. There seems to be a lack of giant eccentric planets around low metallicity stars. Disk self-shadowing can prevent gap heating. No gap heating, no eccentric planets, an explanation for this paucity of eccentric giant planets.

Note: you can’t just assume eccentricity is zero for giant planets when starting post-disk evolution.

The Habitable Zones of Pre-Main-Sequence Stars (Ramses Ramirez, Cornell)

Pre-main-sequence M-stars are many times more luminous than main-sequence and this part of their lifetime is very long (up to 1 Gyr). For the pre-MS Sun, the outer edge of the habitable zone is around 2.5 AU. HZs for these larger luminosity stars are put further out, which would make it easier to directly resolve these planets.

Some complications. Long-lasting runaway conditions are triggered during pre MS-on Venus, Earth, and Mars. M dwarfs will have smaller disks so planets won’t be able to accrete more volatiles. But late heavy bombardment can replenish water on planet’s surfaces.

And that’s all folks! The only things left to do for this are clean up (boo!) and dinner (yay!)

For those of you who tuned in to the live blogging, thanks for faithfully reading my words. I hope that you took as much away from this that I got writing it, and hopefully I will see you in the future at the 2016 ERES at Cornell! Good night!

-Kimberly Cartier, AstroLady

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