Planetary formation around other stars

Planetary formation around other stars mid-term review Grant Kennedy (RSAA) principal supervisor Scott Kenyon (Harvard-CfA)

introduction My thesis involves the study of planet formation around stars other than the sun. This largely means formation around solar to low-mass stars in the case of observable planets, but extends to A-stars in the case of debris disks. My work is largely theoretical, the next year will contain more observational links.

outline Background Research so far Current work Future work Summary

background

planets form in disks 40AU

resolved images Padgett et al 1999

unresolved disks star plain photosphere star + disk infra-red excess star + disk + gap disk structure

disk removal fraction of stars with excess (different PMS tracks) Haisch, Lada, & Lada 2001

grain growth grains provide most of the optical depth in circumstellar disks, so growth can remove the infra-red excess without removing material from the disk

other mechanisms viscous accretion onto star photoevaporation central star or local environment standard models include both

minimum mass solar nebula σ f ice a 3/2 snow line surface density M V E J S U N M cores radial distance (a, AU) Weidenschilling 1977

growth beyond grains When objects are no longer gas affected: growth rate = n v σ coll where the collision cross section σ coll = πd 2 F g = πd 2 ( 1 + v2 esc v 2 ) rate is also dependent on orbital period

oligarchic growth low eccentricity large bodies embedded in a disk of stirred small objects eccentricity Kokubo & Ida 1998

isolation mass oligarch becomes isolated from its surroundings, accreting all mass within an annulus of width ~8 Hill radii R H = a (M olig /3M ) 1/3

chaotic growth near 1AU, isolation mass ~ Mars Kenyon & Bromley 2006

core accretion near 5AU, total isolation mass ~10 Earths mass (earth masses) gas core M iso Pollack et al 1996

solar system evolution 50 40 Uranus and Neptune start between Jupiter and Saturn Distance (AU) 30 20 N U 10 S J 0 0 10 6 Time (years) Thommes et al 1999 2 10 6 3 10 6 4 10 6 5 10 6

solar system evolution semi-major axis (AU) or a bit further out... Nice model Tsiganis et al 2005

RESEARCH HIGHLIGHTS research to date Hear, hear Cell 127, 277 289 (2006) Researchers have uncovered a novel mechanism underlying inherited deafness. Christine Petit of the Pasteur Institute in Paris, France, and her colleagues studied the mouse equivalent of a protein known to be defective in some people who are profoundly deaf. They found that the protein, otoferlin, is sited at a key location within the inner hair cells (pictured) of the cochlea. These cells transform sound into signals that trigger auditory nerves to fire. Sacs of neurotransmitters are anchored to the inner side of membranes of these hair cells. They fuse with the membrane to release their contents, activating neighbouring nerve endings. Otoferlin is essential for fusion. PLANETARY SCIENCE Frosted Earths Astrophys. J. 650, L139 L142 (2006) Recent observations have shown that some small stars called M dwarfs host icy planets that are roughly ten times more massive than the Earth. How are these super-earths made? Planet formation around dwarf stars is different to that around Sun-like stars. This is because the dwarfs fade and shrink during the process, pulling in the snow line, which separates regions of icy-planet formation from those of rocky-planet formation. Grant Kennedy of the Australian National University in Weston Creek and his team IL-2, involved in conveying immune signals. This small molecule binds within a crevice of IL-2 (pictured below). They found that it targets many of the same contact points as does IL-2Rα, despite having a different structure. This is possible because IL-2 is very flexible. The finding shows that small molecules do not need to structurally mimic the proteins they displace. STEM CELLS Grown naturally Nature Biotechnol. doi:10.1038/nbt1259 (2006) Researchers have edged a step closer to making cells that might cure diabetes.

microlensing planet 5.5 Earth masses, orbiting at 2.6AU from 0.22 solar mass Beaulieu et al 2006

pre-main-sequence tracks Siess et al 2000

snow line evolution Palla & Stahler 1999

PMS contraction

simple disk evolution model σ(a, t) f ice M R a δ added Mstar and Rstar dependence to MMSN inner disk radius locked to central star star contracts inner disk moves in disk surface density decreases observations consistent with linear stellar mass dependence

surface density evolution σ f ice R

isolation masses M iso 0.5 M

ejection or coalescence? simulations find ~10 Mars sized objects coalesce to form Earth perhaps expect the same for super-earth formation ~5 Earth mass icy planets but, larger planets orbiting low-mass stars may result in ejections instead investigate with n-body simulations

simple n-body simulation using MERCURY, Chambers 1999

media Releases by CfA, ANU, Utah News in Canberra Times, ABC PM, Nature... When was the last time you did a press release for your students work? RESEARCH Hear, hear Cell 127, 277 289 (2006) Researchers have uncovered a novel mechanism underlying inherited dea Christine Petit of the Pasteur Instit Paris, France, and her colleagues stu mouse equivalent of a protein known defective in some people who are pr deaf. They found that the protein, oto is sited at a key location within the in cells (pictured) of the cochlea. These cells transform sound into s that trigger auditory nerves to fire. S neurotransmitters are anchored to t side of membranes of these hair cell fuse with the membrane to release t contents, activating neighbouring ne endings. Otoferlin is essential for fus PLANETARY SCIENCE Frosted Earths Astrophys. J. 650, L139 L142 (2006) Recent observations have shown tha small stars called M dwarfs host icy p that are roughly ten times more mas the Earth. How are these super-eart Planet formation around dwarf st different to that around Sun-like sta is because the dwarfs fade and shrin the process, pulling in the snow line separates regions of icy-planet form from those of rocky-planet formatio Grant Kennedy of the Australian N University in Weston Creek and his

current work n-body simulations gas giant cores

high resolution simulations

issues... 1 2 3 eccentricity 4 5 6 7 8 9 semi-major axis

gas giant cores seems likely that the Solar System s outer planets originated as cores in a relatively narrow region 5-10AU outside the snow line (e.g. Thommes/Nice models) how does the size of this region, and the number of planets, change with spectral type?

core forming region

a range of stellar masses

main prediction we assume some fixed fraction of cores become gas giants need increased sensitivity (~5-10AU) if planets form like Solar System models predict ~25% more gas giants around early F-type stars than late K-type stars

future work

debris disks are diverse Paul Kalas Paul Kalas

disks decay over time 24μ excess 1 Rieke et al 2005

debris disk formation particle lifetime shorter than stellar age need reservoir of small and large objects small objects stirred by large ones collisions result in fragmentation need objects ~1000km infer planetary formation from presence of debris disks

proposed study aim: to better understand the links between planetary formation and debris disks method: compare predictions of planetary formation theory and models to resolved observations

a bit more detail theory: how does planet size affect debris creation (e.g. Nice/Thommes models) simulations: as above, but numerical simulations: compare grid of models with observed surface brightness (e.g. are there rings vs. disks?)

timeline ~now Mar 07-May 07 June 07-Aug 07 Sep 07-Nov 07 Dec 07-Feb 08 Mar 08-mid 08 RSAA submit gas giant core paper Harvard-CfA Continue simulations Start debris disk-coagulation work Finish gas giant core paper Begin analysis of simulations Debris disk work Continue... Start putting papers together Write and submit papers RSAA Finish papers Complete/write thesis

summary

summary icy super-earth mass planets can form around M Dwarfs (Kennedy et al 2006)

summary icy super-earth mass planets can form around M Dwarfs (Kennedy et al 2006) simulations will explore details

summary icy super-earth mass planets can form around M Dwarfs (Kennedy et al 2006) simulations will explore details the number of gas giants that form is a function of stellar mass (Kennedy & Kenyon 2007, almost submitted) debris disks can be used to test planet formation models

extras

disks are removed quickly accretion rate few transition disks infra-red excess Skrutskie et al 1990

evolution of accretion Calvet et al 2000

inside-out photoevaporation Surface density Radial distance (AU) Alexander et al 2006

external photoevaporation Balog et al 2006