Binaries in the Kuiper Belt

Binaries in the Kuiper Belt

Keith NollHotSci@STScI

July 7, 2010

illustration credit: G. Bacon

Several things (we think) we know about the Kuiper Belt, some of which we know (in part) because of binaries

Keith NollHotSci@STScI

July 7, 2010

illustration credit: G. Bacon

Why do we care?

1. There are many more binaries in the Kuiper Belt than might have been expected!

• More than 400 KBOs have been searched for companions with HST

• > 60 new binaries discovered by HST

- Total known > 70

• What is the fraction of transneptunian binaries (TNBs)?

-> binaries/total ~15%

BUT... this number is subject to observational and sample biases

• TNBs are more loosely bound than would have been predicted (from analogy with Main Belt)

• TNBs are much larger relative to their primaries than previously known populations


2. Binary formation took place in a denser, dynamically colder disk than currently exists.

• TNBs formation mechanisms

• Collision - produces predominately wide binaries- requires ~103 higher density at all sizes

• Dynamical Capture- requires ~103 higher density at smaller sizes (<100 km)- relative velocities must be on the order of the Hill velocity

• Direct Collapse- requires significantly higher surface density for instabilities to form- produces nearly 100% binaries (Nesvorny 2010)

• The separation distribution of TNBs is strongly peaked at small separations

- rules out collisional origin- consistent with capture or collapse

• The angular momentum of TNBs is high- incompatible with fission- inconsistent with known or suspected collisional systems

• The eccentricity distribution of TNBs rules out exchange reactions



3. The colors of Kuiper Belt objects are primordial.

• TNO colors span a very wide range in the optical from slightly blue to ultra red (relative to solar colors)

•TNB colors span the same range

• Benecchi et al. (2009) found that the colors of TNBs are correlated

-significance of 99.983% (Spearman rank correlation)

• This correlation cannot be attributed to any known environmental effects

• implies that colors are primordial• indicates existence of one or more “snow lines” in the region of TNO formation• color gradients should be present in protostellar disks

• TNO colors span a very wide range in the optical from slightly blue to ultra red (relative to solar colors)

•TNB colors span the same range

• Benecchi et al. (2009) found that the colors of TNBs are correlated

-significance of 99.983% (Spearman rank correlation)

• This correlation cannot be attributed to any known environmental effects

• implies that colors are primordial• indicates existence of one or more “snow lines” in the region of TNO formation• color gradients should be present in protostellar disks• Schaller et al. (2010) propose CH3OH as a possible candidate

CH3OH



3. The colors of Kuiper Belt objects are primordial.4. The rotation of binaries constrains formation

models.

Figure 4 from Schlichting et al. 2008 predict spins as a function of relative velocity.

Figure 1d from Johansen and Lacerda 2010 predict uniformly prograde spin.

• 6 of 8 binaries with known pole positions are prograde (Grundy et al 2010)

• Favors capture at moderately high relative velocity: v > RH

- effect of collisions remains tbd- Kozai resonance mechanism seems not to affect inclinations




models.5. TNOs have wide range of densities.

• Orbits yield system mass: M ~ a3/P2

• Diameter from thermal emission or direct measurement yields density

• Measured densities span 0.5-4.2 g/cm3

• High density objects are hydrostatically or collisionally compressed and provide evidence of differentiation

• Low density objects require very high fraction of void space => “bubble” piles




models.5. TNOs have a wide range of densities.6. The current Kuiper Belt is a composite of multiple

populations.

• The inclination distribution of binaries in the non-resonant, non-scattering population (Classical TNOs) departs strongly from random (99.2% probability)

• Binaries are concentrated in the low inclination portion of this population (Cold Classicals)

• Cold Classicals show a high proportion of red objects (Tegler and Romanishin 2003, Doressoundiram et al. 2005; ~2 sigma) and higher albedo (Brucker et al. 2009)





populations.7. One of these populations may be an undisturbed

remnant of the protoplanetary disk.

• Seven known ultra-wide binaries (a/RH >0.08) (Parker et al. 2010)- all are Cold Classical- can be unbound by single collision in 2-6 km size range- dynamical stability depends on size distribution - existence of binaries constrains number of small KBOs

• Simulations show that none of these binaries can survive scattering with Neptune as it migrated outward

• Suggests the Cold Classicals have never interacted with Neptune





populations.7. One of these populations may be an undisturbed

remnant of the protoplanetary disk.8. Binary statistics may help constrain models of

Neptune migration.

• The capture of TNOs into resonances depends on the nature of Neptune’s migration

• Smooth migration results in a detectable difference in the binary fraction in the 2:1 resonance relative to the 3:2 (Murray-Clay and Schlichting 2010)

• Current statistics

3:2 0.05 (+0.05/-0.02)2:1 0.23 (+0.16/-0.08)

Summary



3. The colors of Kuiper Belt objects are primordial.4. The rotation of binaries constrains formation models.5. TNOs have a wide range of densities.6. The current Kuiper Belt is a composite of multiple populations.7. One of these populations may be an undisturbed remnant of

the protoplanetary disk. 8. Binary statistics may help constrain models of Neptune

migration.

Binaries in the Kuiper Belt

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