A Thick Cloud of Neptune Trojans and Their Colors

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A Thick Cloud of Neptune Trojans and Their Colors
Scott S. Sheppard Carnegie Institution of
Washington Department of Terrestrial
Magnetism and Chadwick Trujillo Gemini
Observatory
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Why Observe Asteroids?
The stable asteroid reservoirs have a
fossilized imprint from the formation and
evolution of the Solar System.
The effects of the solar nebula, growth of the
planets, planetary migration, collisions,
and resonances all potentially influence asteroid
formation and evolution.
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Trojan asteroids share a planets semi-major axis
but lead (L4) or follow (L5) the planet by about
60 degrees near the two triangular Lagrangian
points of equilibruim
Minor Planet Brightness
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Neptune Trojans
The first Neptune Trojan was serendipitously
discovered in 2001 by Chiang et al. (2003). Our
ongoing Neptune Trojan survey has quadrupled the
known population.
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Numerical dynamical stability simulations show
Neptune may retain up to 50 of its Trojan
population over the age of the solar system after
any significant planetary migration (Nesvorny et
al. 2002, Marzari et al. 2003, Kortenkamp et al.
2004).
No primordial Saturn or Uranus Trojans known or
expected.
Nesvorny and Dones 2002
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Stability Regions for Trojan Asteroids of the
Giant Planets
Saturn
Uranus
Neptune
Nesvorny and Dones 2002
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Neptune Trojans (11) are distinctly different
from other known Neptune resonance populations.
-Kuiper Belt resonances may be from sweeping
resonance capture of the migrating planets (Hahn
and Malhotra 2005). -Trojans would not be
captured and are severely depleted during any
migration (Gomes 1998 Kortenkamp et al. 2004).
Morbidelli et al. 2005
Nesvorny and Dones 2002
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Neptune Trojan a (AU) i (deg) e mag r (km)
2001 QR322 30.14 1.3 0.03 22.5 70
2004 UP10 30.08 1.4 0.03 23.2 50
2005 TO74 30.05 5.3 0.06 23.3 50
2005 TN53 30.05 25.1 0.07 23.6 40

Median Jup 5.2 11.0 0.07
The four known Neptune Trojans appear stable over
the age of the solar system (Sheppard and
Trujillo 2006).
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Neptune Trojan Formation Scenarios
Neptune can not currently efficiently capture
Trojans (Horner et al. 2006). Capture of
Formation of the Neptune Trojans likely occurred
during or just after the planet formation epoch.
Gas Drag not efficient at Neptune. No rapid mass
growth of the planet.
Collisional interactions within the Lagrangian
region (Chiang et al. 2005). -gt
Predicts low inclination Trojans In-situ
accretion from a subdisk of debris formed from
post-migration collisions (Chiang et al. 2005).
-gt Predicts low inclination Trojans Freeze-in
Capture The giant planets orbits become
marginally excited perturbing many minor planets.
Once the planets stabilize any objects in the
Lagrangian regions will also become stable and
thus trapped (Morbidelli et al. 2005). -gt
Predicts low and high inclination Trojans
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The Trojans of the giant planets lie between the
rocky main belt asteroids and volatile-rich
Kuiper Belt.
Parallax
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Neptune Trojan Inclinations
Can test formation theories on the inclination
distribution of Neptune Trojans.
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Magellan-Baade 6.5 meter With the 0.2 square
degree IMACS imager.
41 High i Low i
Sheppard and Trujillo 2006
with radii gt 40 km
Appear 5 to 25 times larger than the Jupiter
Trojans and Main belt asteroid populations
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Freeze-In Capture
Tsigais et al. 2005
Gomes et al. 2005
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Jupiter Trojan Capture Simulation
Morbidelli et al. 2005
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Physical Properties of the Trojans
-Currently the space between the giant planets is
mostly devoid of small stable objects. -Trojans
were likely asteroids in heliocentric orbits
which did not get ejected into the Oort cloud or
incorporated in the planets. -gt The Trojans may
be the key needed to showing us the complex
transition between rocky objects which formed in
the main asteroid belt and the volatile rich
objects which formed in the Kuiper Belt.
Brown 2000
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Comparison of Colors of Outer Solar System Objects
Ultra Red
No ultra red material as seen In the Classical
Kuiper Belt.
Sheppard and Trujillo 2006
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The Dispersed Populations
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Evolution of Asteroids in the Outer Solar System
Morbidelli and Levison 2003
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The End
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KnownStableReservoirs
Main Asteroid Belt 25 gt 200 km
Trojans 5 200 km
Irregular Satellites 5 200 km
Kuiper Belt 10,000 gt 200 km
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Wide-Field CCDs on Small/Medium/Large Telescopes
Power of a Survey A x Omega A Area of
Telescope Omega Solid Angle Observed
CFHT 3.6m/MegaCam
Palomar 1.2m/Quest
Subaru 8.3m/SuprimeCam
Magellan 6.5m/IMACS