Collisional history of the Kuiper belt and binary Kuiper belt objects

(an observationally biased talk)

Csaba Kiss, Konkoly Observatory

Missing links from disks to planets, 10-13 October 2016

1 Binaries in the main belt and near-Earth populations

• Important observational bias — typical size range of asteroids at different heliocentric distances: • Main belt (<3AU): 1-50km • Jovian Trojans (~5AU): 10-100km • Centaurs and trans-Neptunians: 100-1000km + the largest dwarf planets (>1000km)

• First system discovered: 243 Ida and its moon Dactyl (Galileo flyby, 1993) • Large systems form by collisions • Smaller systems form by rotational fission in most cases (rubble pile interior), caused e.g. by the YORP-effect • The total binary fraction is ~15%, but it includes all small satellites (we cannot see this for JTs and TNOs)

2 The first Kuiper belt binary discovery: the Pluto-Charon system

• Charon’s discovery: Christy and Harrington, 1978 • System mass: high albedo and small size, previously it was very uncertain • Additional moons were detected by HST

Charon Naval Observatory, Flagstaff Station

3 Presently known Kuiper belt binaries (by W. Grundy)

4 Binary TNOs

• At the beginning, it was considered to be an exception (not Noll et al. (2008) very common in the main belt) • Wide pairs have been discovered by 4m-class telescopes • Most binaries were discovered by HST programmes between 2000-2010 • Some are also images by adatptive optics systems of large telescopes (Keck) • In many cases binarity is deduced from marginally resolved PSFs (many possible with HST due to its extreme PSF stability) • Kuiper belt binaries: 100-1000km

5 A season a mutual events for Sila-Nunam

• Mutual events can constrain binary asteroid mutual orbits, shapes, and densities (e.g., Descamps et al. 2007) and to distinguish surface compositions and map albedo patterns on Pluto and Charon (e.g., Binzel and Hubbard 1997). • Sila-Nunam (Grundy et al., 2012) is a Cold Classical binary system • Most likely synchronized rotation • Mutual events help to resolve component sizes, colors, shapes, and albedo patterns 18 +0.37 -3 • System mass of (10.84±0.22)x10 kg, average bulk density: 0.72−0.23 g cm

Grundy et al. (2012)

6 Densities of Kuiper belt objects

2002 UX25

• Binaries are the only systems for which reliable density estimates could be given • System mass is derived from Kepler’s law (orbit and orbital period) • S i z e i s e s t i m a t e d f ro m i n d e p e n d e n t measurements: thermal emission or occultation Brown (2013) • How can large (and dense) Kuiper belt objects form from smaller ones? • Very large porosity for 100-1000km bodies (>0.5) • Less dense objects preferentially acquire satellites and large objects form from high density small objects • Objects of the dwarf planet size evolve to their high densities through the effects of Brown (2013) giant impacts — a sufficient amount of ice should be lost that impact modeling cannot Most of the size estimates here are produce at the moment derived by our “TNOs are Cool!” Herschel Open Time Key Program 7 How can binaries form? • Capture: • three-body interactions to remove angular momentum and produce a bound pair • (Goldreich et al., 2002): two formation channels: • L3 models: three discrete bodies • L2s models: the third body is replaced by a dynamical drag coefficient due to a “sea” of weakly interacting smaller bodies • Astakhov et al. (2005): Extension of the capture model: a weakly and 1/3 temporarily bound (a ~ RH, RH = a⊙(M1/3M⊙) the Hill radius) pair of big bodies “hardens” when a third small body (“intruder”) passes within the Hill radius 3 2 • The L and L s channels require that binaries form before v > vH (vH ≡ ΩKRH) — the primordial classical belt may have enjoyed dynamically cold (sub-Hill or marginally Hill) conditions for a longer duration than the primordial scattered population. • Dynamical capture is the only viable formation scenario for many TNBs with high angular momentum • Can explain ALL presently known systems with some assumptions 8 How can binaries form?

• Collisions: • First proposed for the Pluto-Charon system (McKinnon, 1984, 1989, also Canup 2005) • Dynamical interactions of the smaller moons with Charon are needed to explain the currently observable satellite system (Ward and Canup, 2006). • Some collisional simulations can produce low mass ratios (Durda et al., 2004; Canup, 2005) and explain the ~1% mass of the satellites (e.g. for Haumea and Eris) • Additional mechanisms (tidal interactions) are needed to explain the observed orbital characteristics of the moons (Brown et al., 2006) • To occur with reasonable frequency, collisions between TNOs must be gravitationally focused — may only work for large (R~1000km) bodies. • Relative velocities must be less than the Hill velocity ⇒ Impacts must have taken place while the disk was dynamically cold 9 How can binaries form?

• Hybrid models: • Weidenschilling (2002): • A third big body collides with one member of the scattering pair • Since physical collisions have smaller cross-sections than gravitational interactions, this mechanism requires ~100x more big (R ~ 100 km) bodies than currently observed • It also predicts an unobserved prevalence of widely separated binaries • Funato et al. (2004): • Exchange reaction Ls + L → L2 + s — a small body of mass Msm, originally orbiting a big body of mass Mbig, is ejected by a second big body • In the majority of ejections, the small body’s energy increases by its orbital binding energy, leaving the big bodies bound • This model predicts a prevalence of very-high-eccentricity binaries that is not observed • Other mechanisms like volatile driven splitting (observed in comets) are unlikely in this size range • Both capture and collisional formation models share the requirement that the number of objects in the primordial Kuiper belt had to be at least a couple of orders of magnitude higher than currently found ⇒ all TNBs observed today are primordial 10 Binary fraction in the trans-Neptunian populations

• Binary fraction always have to be considered for a specific separation and magnitude limit!!! Hot Classicals

• Cold classical TNOs (i<4.6º): ~22% of all TNOs in this population are binaries (~5% in all others) • Cold classicals are believed to be the less disturbed population in the Kuiper belt that kept

the original physical characteristics of the objects Cold Classicals • Is the size of the primary correlated with binary status? Noll et al. (2008) • Brown et al. (2006): apparently higher fraction among the largest TNOs: satellites of Pluto, Eris and Haumea were known, but no moon was identified for Makemake at that time • We will return to this question at the end…

• Clustering at Δmag<1: intrinsic characteristic of TNBs? This in not the case in the main belt and near-Earth populations • Near-equal binaries are natural outcome of dynamical capture models (e.g. Astakhov et al., Noll et al. (2008) 2005) 11 Formation of Charon

• Canup (2005): Formation of Charon from a large impact is a plausible explanation • Charon may have formed from the debris ring material • Bromley and Kenyon (2015): formation of circumbinary moons from the debris ring — smaller satellites from a disk that spread well beyond the orbit of Hydra were not identifed by New Horizons • Smullen and Kratter (2016): 10-30km remnants of the debris ring might have survived as Plutinos (3:2 resonance), forming the Pluto-Charon collisional family • Any remnants of the Charon-forming ring? — It was a serious concern before New Horizons’ flyby.

12 Dust ring in the Pluto-Charon system?

• The complex satellite system — dust ring (risk for New Horizons)? • Extensive study resulted in the discovery of Kerberos and Styx (Showalter et al. 2011, 2012) • Total dust mass and impactor mass estimates for New Horizons (10-4 g, Steffl & Stern, 2015 ) • Search for signature of dust on Herschel 70um images (Marton et al., 2015): • Presence of a dust ring should cause a widening of the PSF compared with a point source (using a quasar PSF due to its more appropriate SED) • No sign of extended emission was found (<8.7x10-5 g impactor mass) • Indeed, New Horizons survived the encounter (and did not find any ring…)

13 Dust rings around TNOs?

• Dust ring around the large Centaur Chariklo (Braga-Ribas et al., 2014) using occultation data • A similar feature is reported around Chiron (Ortiz et al., 2015) • To date, no similar feature was observed for more distant objects… • Chariklo and Chiron are ~200km — this size range is hardly available for TNO occultations • Origin: most likely collision, but could be material from an outburst

• Why among Centaurs? — Impact velocities maybe too high in the main belt (aaN)

Braga-Ribas et al. (2014)

Braga-Ribas et al. (2014)

14 Haumea and its collisional family

Brown et al. (2005)

• Haumea: one of the largest Kuiper belt objects, Hi’iaka officially a dwarf planet • Two moons, Hi’iaka and Namaka (Brown et al., 2005) • Hiʻiaka has strong absorption in the NIR ⇒ Haumea crystalline water ice, similar to the surface of Haumea Namaka • The moons are likely a collisional fragments of Haumea • Origin of the moon system: sub-catastrophic collision that removed ~20% of the progenitor’s mass (reason for large density ~2.6 g cm-3) • This imact also created the “Haumeas collisional family” — set of objects with similar orbits and small velocity dispersion (Brown et al., 2007) • Dynamically identified members of the family also have similar spectra and high albedo, much higher than that of objects on nearby orbits

15 The latest discoveries: Makemake

Discovery images WCS-rotated stack of visit 1 images with co-registered visit 2 images subtracted

Co-registered images at two epochs

Parker et al. (2016) 16 The latest discoveries: Makemake

Dark moon hypothesis: •excess emission at “short” FIR wavelengths (70um) •can be explained by the presence of a very dark moon (pV=2-4%, D= ~300km)

Parker et al. (2016) 17 Thermal emission of the Eris-Dysnomia system

Hubble Space Telescope Keck AO, K’

Brown et al. (2007)

F606W: brightness ratio of ~1:500 K’: brightness ratio of ~1:60

F606W and K’ band a l b e d o s c a n b e estimated from the reflectance spectrum Dysnomia is probably quite (Alvarez-Candal et al., dark (pV<10%), even if 2011) highly reflective surface (e.g. olivin) is assumed 18 Thermal emission of the Eris-Dysnomia system

Eris + Dysnomia • Kiss et al. (2016, in prep): The thermal emission is very difficult to explain with a single-terrain, very reflective Eris (excess at Eris only shorter wavelengths) • The presence of a dark (and warm) moon (D>500km, pv<5%) can explain the total thermal emission of the system • Dysnomia itself is likely a dwarf planet (Vesta is ~600km in the main belt) • Dysnomia could have slowed d o w n t h e ro t a t i o n o f E r i s considerably

19 K2 — a new binary discovery machine? (NASA Kepler and K2 Science Center) • K2 mission: after the death of the third reaction wheel in the Kepler spacecraft the telescope can only be stabilized by solar radiation pressure, if it looks along the Ecliptic.

• 13 campaign fields (each ~80 day long) have been observed so far

• Next proposal cycle deadline: November 3 (Fields 14-16)

• Possibility to obtain long, uninterrupted photometry of Solar systems targets, down to R~23mag

20 K2 — Jovian Trojans

• Szabo et al. (2016): K2 photometry and rotation periods for 56 Jovian Trojans

• Jovian Trojans: Minor planets orbiting the Sun at the L4 and L5 points of the Sun-Jupiter system

• They were likely NOT formed at their current location, but were captured during their inward migration (~LHB?)

21 K2 — Jovian Trojans

• Known JTs are in the 10-100km size range

• Maximum critical densitites calculated from spin rates are ~0.5 g cm-3 — “cometary like” bodies

• Notable excess of slow rotators (>50h) compared with main belt asteroids

• Based on the longest rotation periods the fraction of binaries is >20±5% in the JT population

22 K2 — Jovian Trojans

• Known JTs are in the 10-100km size range

• Maximum critical densitites calculated from spin rates are ~0.5 g cm-3 — “cometary like” bodies

• Notable excess of slow rotators (>50h) compared with main belt asteroids

• Based on the longest rotation periods the fraction of binaries is >20±5% in the JT population

23 K2 — 2007 OR10 • The4th largestand unnamed 2014 body UZ224 in the Solar system, the —3rd most⩗ distant currently after Eris and V774104 (rh=85AU) • Latest analsys of K2 and Herschel data indicate a size of D=1575km (third largest dwarf planet after Pluto and Eris) and a slow rotation (P=45h). • Slow rotation: tidally locked binary? No moon has been reported so far… • The moon/primary must have a mass ratio of at least 1:10 to reach synchronized rotation (~half the size of the primary) • But smaller moons can slow down the rotation to the observed value, too… • So have a look at some archival HST images…

24 K2 — Two new, slow rotating TNOs

• 2003 QW90 (a Cold Classical): • P = 3.55d • D ~ 440km from thermal emission (Herschel) • a = 1500-1800km, depending on the density (0.7—1.2 g cm-3) • one of the most compact pairs known, hard to resolve 0.05” • 2005 RS43 (resonant): • PBinary = 10.0d fraction always have to be considered for a • D ~ 240kmspecific from thermal separation emission and (Herschel) magnitude limit!!! • a = … • Given the size range (200-500km) both system are expected to be equally sized, if their binarity is true • Challenging, but can be resolved (marginally) with HST and/or with AO systems • Number of close pairs could be notably larger than previously thought

25 Do all large Kuiper belt bodies have moons?

• “Large” body: ~1000km or larger — Pluto, Eris, 2007 OR10, Makemake, Haumea, Quaoar, Orcus • According to the latest results, all of them have at least one moon

• Binary fraction of large TNOs is ~1 — OR10 any collisional model of the early Kuiper belt should reproduce this Makemake

• Non-distructive collisions (for the primaries) in the early Kuiper belt must have been very frequent — further strengthening the evidence for a very massive early Kuiper belt

• A search for satellites of large TNOs is ongoing (Sedna, etc…)

26 Backup slides: Binary orbit calculation Example of orbit determination: Sila-Nunam

(from the binary KBO page by W. Grundy) 28 Equations for orbit determination (e.g. Herstroffer et al., 2007)

Using the

This can be solved for C Equations for orbit determination (e.g. Herstroffer et al., 2007)

“Innes” constants Equations for orbit determination (e.g. Herstroffer et al., 2007)

• 1. Assume an observational noise (e.g. Gaussian with ~10 mas.); • 2. Extract randomly 3 points xo from the observational data, and add a random prior noise x = x0 + dx; • 3. Assume an orbital period P randomly in some given realistic range; • 4. Derive the (elliptic) orbit from Thiele–Innes method for the 3 points x; • if there is no (elliptic) solution, go to step 1; • if there is a solution go to step 5; • 5. Compute the Observed–Calculated vector for the whole observational data-set; • 6. Test whether this solution is acceptable with respect to the affordable observational noise • If not, no solution. • If yes, keep the solution and compute the orbital elements in the plane of the ecliptic. It is stressed that in such frame, this ends with two solutions for the orientation of the orbital plane and the direction of the pericentre, solutions that are symmetric with respect to the field-of-view plane. • 7. Eventually return to step 1. Equations for orbit determination (e.g. Herstroffer et al., 2007)