THE ORIGIN AND EVOLUTION OF FREE-FLOATING PLANETS IN STAR CLUSTERS M.B.N. (Thijs) Kouwenhoven Kavli Institute for Astronomy and Astrophysics, Peking University Hao Wei (MPIA), Li Yun (KIAA), Wang Long (KIAA), Zheng Xiaochen (KIAA), Maxwell Xu Cai (NAOC), Liu Beibei (KIAA), Liu Chuanwu (Melbourne), Hagai Perets (Technion), Rainer Spurzem (NAOC), Simon Goodwin (Sheffield), Melvyn Davies (Lund), Dimitris Stamatellos (UCLAN),... 2014 12 17
PLANETS IN STAR CLUSTERS Most stars from in clustered environments Planet formation process Dynamical evolution environment? environment?
OUTLINE Planetary systems formation/dynamics Planetary systems in star clusters Li Yun, Liu Beibei, Zheng Xiaochen Hao Wei, Wang Long, Xu Cai, Zheng Xiaochen Free-floating planets in star clusters Wang Long, Zheng Xiaochen Re-capture Summary of free-floating planets
Part 1 Planet formation and dynamics
FORMATION OF BINARY STARS AND PLANETS Core accretion Disk fragmentation Core fission Dynamical capture mass-dependent companion fractions, mass ratio distributions, etc. (Kouwenhoven et al. 2005, 2007, 2009)
Core accretion Disk fragmentation M < 5 MJupiter a < 40 AU quite stable M > 5 MJupiter a = 50-150 AU very unstable Zheng, Lin & Kouwenhoven (in prep.) Li, Kouwenhoven, Stamatellos & Goodwin (in prep.)
THE FEW-BODY PROBLEM Newtonian/relativistic Rotational gravity flattening, tidal distortion Tidal circularization Tidal synchronization (spin, obliquity, precession) Inflation, evaporation, mass Stellar evolution Physical Escape collisions transfer
eccentricity (1) Precession semi-major axis CHANGES UNDER INFLUENCE OF PERTURBATIONS (2) Angular momentum exchange time time (3) Energy exchange
Jupiter Saturn Secular perturbations Courtesy Melvyn Davies
EARTH: ECCENTRICITY AND SPIN Loutre & Berger (2000), adapted
IRRADIATION BY THE SUN (EARTH) mid-june, 65 North Loutre & Berger (2000), adapted
THE PAST AND FUTURE OF OUR SOLAR SYSTEM High computational accuracy required Initial conditions are crucial. Simulations beyond ~60 Myr (CeresVesta encounter) are only possible futures Laskar & Gastineau (2009) simulated 2500 possible futures over 5 Gyr Mercury has the least stable orbit
Part 2 Planetary systems in star clusters
CLUSTERED STAR FORMATION time Hydrodynamical simulations IMF / Primordial binary population N-body simulations Kouwenhoven et al. (2005, 2007)
Did our Solar system form in a star cluster?
THE SUN S BIRTH CLUSTER Supernova explosion nearby the early Solar System (Hester et al. 2004; Looney et al. 2006, Adams 2010) M SNE 15 25 M sun DSNE 0.02 1.6 pc What does this tell us about the birth environment of the Solar system?
THE SUN S BIRTH CLUSTER Portegies Zwart (2009)
PLANETS IN STAR CLUSTERS Exoplanet surveys in star clusters have been largely unsuccessful Difference between the planet frequency in star clusters and in the field? Does planet formation depend on environment? Olczak, Pfalzner & Spurzem (2006)
DISRUPTION OF SINGLE-PLANET SYSTEMS IN STAR CLUSTERS Zheng, Kouwenhoven & Wang (submitted)
DISRUPTION OF PLANETARY SYSTEMS IN STAR CLUSTERS Survival of planetary systems depends (mostly) on Star cluster environment Semi-major Planetary axis multiplicity
Planetary system in star cluster ionization evaporation Escaped Planetary system re-capture ejection Free-floating planet Escaped evaporation Free-floating planet in star cluster Zheng, Kouwenhoven & Wang (submitted)
Planetary system in star cluster ionization (1-x) % evaporation x% Escaped Planetary system re-capture 4% 1% ejection Free-floating planet Escaped evaporation Free-floating planet in star cluster 95% Zheng, Kouwenhoven & Wang (submitted)
single-planet systems a = 100 AU Surviving planetary systems star cluster N = 1000 R = 1 pc homogeneous substructured expanding equilibrium very substructured Zheng, Wang & Kouwenhoven (submitted) Time (N-body units) collapsing
DISRUPTION OF SINGLE-PLANET SYSTEMS IN STAR CLUSTERS (N = 1000, R = 1 pc) Frequency Planetary systems in the cluster in the field Free-floating planets in the cluster in the field semi-major axis (AU) Zheng, Kouwenhoven & Wang (submitted)
THE BIG PROBLEM WITH MULTI-PLANET SYSTEMS IN CLUSTERS Large dynamical ranges in time, mass, position Round-off errors tend to cancel out in star cluster simulations, but are cumulative in planetary systems Energy checks difficult Cai et al. (in prep) Wang et al. (in prep)
A Monte Carlo scattering approach
ENCOUNTER DISTRIBUTIONS The formation of very wide b Relative velocity (~Maxwellian) vy (km s-1) 6 Impact 2 2 parameter 0 f ( b ) -2 b Initial -4 Normalized distribution v v f ( v )4= exp 2 4σ 2σ π 2 2 (or present-day) mass function -6-6 f ( M ) = IMF -4-2 0 2 vx (km s-1) 4 6 Time intervals: Poissonian Figure 3. The distribution of velocities and 0.20 0.15 0.10 0.05 0.00 0 2 4 6 8 10 12 Relative speed (km s-1) Kouwenhoven et al. (2010) Hao, Kouwenhoven Spurzem (2013) relative speeds &for Plummer
DECAYING PLANETARY SYSTEMS Orion Nebula Cluster Black = Jupiter, Saturn, Uranus, Neptune Blue = Five Jupiters at 1.0, 2.6, 6.5, 16.6, 42.3 AU Hao, Kouwenhoven & Spurzem (2013)
PLANET SURVIVAL RATES Orion Nebula Cluster Multiplicity affects survival chances! Black = Jupiter, Saturn, Uranus, Neptune Blue = Five Jupiters at 1.0, 2.6, 6.5, 16.6, 42.3 AU Hao, Kouwenhoven & Spurzem (2013)
SHORT-PERIOD PLANETS IN CLUSTERS ARE NOT SAFE Planet 1 Planet 2 Planet 3 Planet 4 Planet 5 Planet 1 Planet 2 Planet 3 Planet 4 Planet 5 Jupiter Saturn Uranus Neptune Jupiter Saturn Uranus Neptune Multi-planet systems in star clusters Single-planet systems in star clusters Hao, Kouwenhoven & Spurzem (2013)
Part 3 Free-floating planets in star clusters
FREE-FLOATING PLANETS - ORIGIN Sub-brown dwarfs (?) Gravitational fragmentation Ejected embryo Photo-eroded star Rogue planets (!) Ejected systems from planetary
POSSIBLE ORIGINS OF (ROGUE) FREE-FLOATING PLANETS Planet-planet scattering External forces (stellar encounters,, molecular clouds, Galactic tides, etc) Stellar evolution ~2 free-floating planets per main sequence star in the Galactic neighborhood (Sumi et al. 2011) 2GM v ffp > vescape = r
FREE-FLOATING PLANETS IN STAR CLUSTERS Free-floating planets start out with a higher average velocity than stars, since they are ejected super-virial population σ Some 2 ffp σ 2 stars +σ 2 ejection planets may immediately escape from the star cluster 2GM cluster (< r) v > vescape = r
PLANET EJECTION VELOCITIES 1 Prompt Delayed ejection after a close encounter Cumulative frequency ejection following a close encounter MI = 0.6 M (delayed) Malmberg et al. (2011) Hao, Kouwenhoven & Spurzem (2013) Wang, Kouwenhoven, et al. (in prep.) MI = 0.6 M (prompt) 0.8 MI = 1 M (delayed) MI = 1 M (prompt) 0.6 0.4 0.2 0 0.1 1 veject /km s 1 10
ESCAPE TIMESCALES High-velocity planets escape within a dynamical time Low-velocity stars escape beyond a relaxation time Wang, Kouwenhoven, Zheng, Davies & Church (in prep.)
FREE-FLOATING PLANETS IN STAR CLUSTERS Nstar Nplanet The planet-to-star ratio decreases linearly with time average, free-floating planets escape star clusters 40% earlier than stars 5-10% of stars have a close encounter (< 1000 AU) with a free-floating planet Wang, Kouwenhoven et al. (in prep.) Nplanet /Nstar On time time
Part 4 Re-capture of free-floating planets
CAN FREE-FLOATING PLANETS BE RE-CAPTURED BY STARS? Gravitational focusing is less effective for capture of planets than for stars Capture works for stars, and results in wide orbits Kouwenhoven et al. (2010) Detection of free-floating planets and planets in wide orbits is very difficult
CAPTURE OF STARS AND PLANETS Requires fine-tuned 3-body encounters! (Goodman & Hut 1993)! Extremely unlikely in the Galactic field! (a few captures per galaxy)!!! Occurs regularly in star clusters (a few percent of the stars and free-floating planets) The lowest-mass body is usually kicked out
POSSIBLE ORIGINS OF WIDE-ORBIT PLANETS Gravitational fragmentation in disks few 100 AU, large masses (Li, Kouwenhoven & Stamatellos, in prep.) Planet-planet Capture scattering in existing planetary systems of free-floating planets which mechanism is dominant?
5 SEMI-MAJOR AXIS DISTRIBUTION of re-captured free-floating planets in star clusters 0 Stars (H) Planets (H) Stars (F) Planets (F) 1 10 2 10 0.15 Fraction Cumulative Fraction 10 0.1 0.05 3 10 0 2 10 3 10 4 10 5 6 10 10 7 10 Semi Major Axis (AU) 2 10 3 10 4 10 5 10 6 10 Semi Major Axis (AU) 7 10 Perets & Kouwenhoven (2012)
PROPERTIES OF RE-CAPTURED FREE-FLOATING PLANETS 3-6 % of free-floating planets is re-captured by another star Semi-major axes 100-106 AU (highly eccentric) Gravitationally-focused random pairing Thermal eccentricity distribution Random orientation
EXOTIC PLANET HOSTS Black holes: ~5-10% capture a planet (in a star cluster) Neutron stars: Very few, because of high kick velocity White dwarfs: About 1-5% capture a planet (in a star cluster) Brown dwarfs: Relatively few, because of low masses Binary planets: Relatively few, because of low masses Existing planetary systems can also capture planets Perets & Kouwenhoven (2012)
EFFECTS ON EXISTING PLANETARY SYSTEMS Effect of captured planets on existing systems is often small: (i) small masses and (ii) large semimajor axes But can leave strong imprints on Oort clouds Orbital inclination is random w.r.t. existing planets and the stellar rotation retrograde orbits Kouwenhoven & Brasser (in prep.)
TOP STORIES ASTRONOMY There really could be a giant planet hidden far beyond Pluto Pluto is about forty times the distance from the Sun as Earth. But the Solar System is over 50000 times that length across, meaning it could be hiding some huge BY ALASDAIR WILKINS FEB 21, 2012 5:00 PM Share FOLLOW IO9.COM 11,525 26
Part 5 Summary
SUMMARY (I) stars and planets form in star clusters observable imprints on planetary systems Most short-period planets in star clusters Scattering after stellar encounters? Formation? Few Many free-floating planets expected in young star clusters at early times (Nplanet/Nstar > 10%) observable? The planet-to-star ratio in clusters decreases linearly with time (planets escape ~40% earlier)
SUMMARY (2) Single-planet systems in open clusters: Most with a < 100 AU survive Most with a > 1000 AU get disrupted 5-10% of cluster stars have close encounters (< 1000 AU) with a free-floating planet 1%-10% of free-floating planets re-captured by other massive objects in star clusters wideorbit planets Work in progress: N-body evolution of open/ globular clusters with full treatment of multiplanet systems (Cai, Wang, Kouwenhoven, Spurzem, in prep.)
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