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 (a<aJ) but collisions may be too rare in the trans-Neptunian region (a>aN) 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.