High albedos of low inclination Classical Kuiper belt objects

Abstract

We present observations of thermal emission from fifteen transneptunian objects (TNOs) made using the Spitzer Space Telescope. Thirteen of the targets are members of the Classical population: six dynamically hot Classicals, five dynamically cold Classicals, and two dynamically cold inner Classical Kuiper Belt Objects (KBOs). We fit our observations using thermal models to determine the sizes and albedos of our targets finding that the cold Classical TNOs have distinctly higher visual albedos than the hot Classicals and other TNO dynamical classes. The cold Classicals are known to be distinct from other TNOs in terms of their color distribution, size distribution, and binarity fraction. The Classical objects in our sample all have red colors yet they show a diversity of albedos which suggests that there is not a simple relationship between albedo and color. As a consequence of high albedos, the mass estimate of the cold Classical Kuiper Belt is reduced from approximately 0.01 M ⊕ to approximately 0.001 M . Our results also increase significantly the sample of small Classical KBOs with known albedos and sizes from 21 to 32 such objects. ⊕ Keywords: Infrared Observations, Kuiper Belt, Trans-Neptunian Objects, Trojan Asteroids We include in our analysis those objects which fall into the Classical regime (the main Kuiper Belt) in either of the two major TNO classification systems described by the Deep Ecliptic Survey (DES) team (Elliot et al. 2005) and described by Gladman et al. (2008). Both classification systems rely on numerical integrations over 10 Myr to determine orbital behavior.The main difference between the two classification systems lies in the definition of Scatter(ing/ed) Disk Objects (SDOs). According to Gladman et al. (2008), SDOs are only the objects "currently scattering actively off Neptune" as evidenced by a swiftly changing semimajor axis in the orbital integrations, specifically a change greater than or equal to 1.5 AU. In contrast, the DES system uses a time-averaged Tisserand parameter less than 3 relative to Neptune to separate the Scattered-Near KBOs from the Classical KBOs. The Detached TNOs of Gladman et al. (2008) have eccentricities greater than 0.24 while the DES Scattered-Extended KBOs have Tisserand parameters greater than 3 relative to Neptune and eccentricities greater than 0.2 (Elliot et al. 2005). As a result, some objects that are Scattered-Near and Scattered-Extended in the DES system are Classical KBOs according to Gladman et al. (2008).

Identifying Collisional Families in the Kuiper Belt

“…The apparent visual magnitude was then calculated, assuming observation at opposition, from (Trujillo & Brown 2001) m v = m −2.5 log[pr 2 ]+2.5 log(2.25×10 16 R 2 (R−1) 2 ), (5) where m = −26.75 is the apparent V magnitude of the sun, p the albedo, r the radius of the body, and R the heliocentric distance. The albedo was assumed to be 6% for all bodies, consistent with the CFEPS model, but likely an underestimate by a factor of ∼2-4 for the cold classical KBOs (Brucker et al 2009). …”

“…The first of these is the classical belt, consisting of a dynamically cold population at low inclination and a dynamically excited (hot) population at inclinations larger than ∼5 • . Besides evidence for these two populations in the inclination distribution (Brown 2001), they are also apparently distinct in eccentricity ), color (Peixinho et al 2008), absolute magnitude (Levison & Stern 2001), albedos (Brucker et al 2009), binary fraction (Noll et al 2008), and differential size distribution (Fraser et al 2010). A second population is inherently unstable on ∼Gyr timescales and is known as scattered disk objects, since they are in orbits that are scattering off of Neptune, usually with perihelia below ∼35-40 AU.…”

“TNOs are Cool”: A survey of the trans-Neptunian region

“…Obtaining size/albedo values for a larger sample is best achieved from thermal radiometry, whereby thermal and optical observations are combined within a thermal model. After the pioneering measurements on two objects from ISO (Thomas et al 2000) and ground-based observations on a few others (see Stansberry et al 2008, and references therein), Spitzer provided the first large such dataset, with about 60 Centaurs/KBOs measured at 24 and/or 70 μm (Cruikshank et al 2005;Grundy et al 2005Grundy et al , 2007Stansberry et al 2006Stansberry et al , 2008Brucker et al 2009). This approach (multi-wavelength photometry) also constrains the thermal regime of the object, phenomenologically described by the "beaming factor" (Jones & Morrison 1974;Lebofsky & Spencer 1989;Harris 1998).…”

“…Furthermore, correlations between size, albedo, color, composition and orbital parameters may provide diagnostics on the dynamical, collisional and physical history of KBOs. The size distribution of TNOs is also a diagnostic relating to that history, but in the absence of measured sizes the distribution has typically been estimated from the luminosity function and an assumed albedo, an uncertain step given that the average albedos seem to be different for the different dynamical populations (Grundy et al 2005;Brucker et al 2009). Instead, and to constrain Kuiper belt formation/evolution models it is desirable to directly determine the size distribution of the different populations, and infer their individual total masses.…”

Density of asteroids

“…Orbit of a satellite: From optical or radar images of the components of the system, their mutual orbit can be determined and the mass derived with Kepler's third law (see, for instance, Petit et al 1997; Merline et al 1999 Merline et al , 2002 Margot et al 2002; Marchis et al 2005b Marchis et al , 2008aBrown et al 2005Brown et al , 2010Carry et al 2011;Fang et al 2011). The 28 mass estimates available for TNOs were derived from optical imaging with the Hubble space telescope or large ground-based telescopes equipped with adaptiveoptics cameras (e.g., Grundy et al 2009;Dumas et al 2011). Similarly, the 17 mass estimates for NEAs were all derived from radar (e.g., Ostro et al 2006; Shepard et al 2006), with the exception of Itokawa which was the target of the Hayabusa sample-return mission (Fujiwara et al 2006).…”