Diverse albedos of small trans-neptunian objects

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    ne 17Discovery of trans-neptunian object (TNO) satellites and determination of their orbits has recently enabled estimation of the size andalbedo of several small TNOs, extending the size range of objects having known size and albedo down into the sub-100 km range. In thispaper we compute albedo and size estimates or limits for 20 TNOs, using a consistent method for all binary objects and a consistent methodfor all objects having reported thermal fluxes. As is true for larger TNOs, the small objects show a remarkable diversity of albedos. Althoughthe sample is limited, there do not yet appear to be any trends relating albedo to other observable properties or to dynamical class, withthe possible exception of inclination. The observed albedo diversity of TNOs has important implications for computing the size-frequencydistribution, the mass, and other global properties of the Kuiper belt derived from observations of objects apparent magnitudes and may alsopoint the way toward an improved compositional taxonomy based on albedo in addition to color. 2005 Elsevier Inc. All rights reserved.

    Keywords: Kuiper belt objects; Trans-neptunian objects; Composition; Surfaces

    1. Introduction

    Considerable progress has been made in understandingthe dynamical structure of the trans-neptunian Solar System,but the physical characteristics of the small bodies that in-habit that region remain poorly determined. Measurementsof colors, absolute magnitudes, and lightcurves have accu-mulated for a number of objects, and near infrared spec-tra have been reported for some of the brightest ones, butknowledge of fundamental properties such as size, mass,albedo, and density remain woefully sparse. Determiningthese properties for a representative sample of TNOs is cru-cial for estimating the total mass of material in the trans-neptunian region, for relating magnitude-frequency distrib-utions to size- and mass-frequency distributions, for eluci-dating patterns of compositional taxonomy, for quantitative

    * Corresponding author. Fax: +1-928-774-6296.

    interpretation of infrared spectra, and for constraining theinternal compositions of these objects. All of these objec-tives have important implications for physical and chemicalconditions in the outer proto-planetary nebula and for the ac-cretion of solid objects in the outer Solar System.

    Data constraining TNO sizes, albedos, and colors arecompiled in this paper in an effort to search for possible pat-terns related to TNO origins or subsequent surface process-ing. We use a consistent set of models to derive size andalbedo constraints from thermal emission observations andfrom binary orbits reported by various authors, thereby ex-panding the range of diameters included in the sample tomore than an order of magnitude.

    Although they likely do derive from the TNO popula-tion, Centaurs (by which we mean non-resonant objectswhich cross the orbits of major planets) and comet nucleiwere excluded from this study because we lack knowledgeabout what part of the trans-neptunian system they origi-nated from. Comet nuclei also have systematically differentIcarus 176 (2005

    Diverse albedos of sma

    W.M. Grundy a,, K.S.a Lowell Observatory, 1400 W. M

    b Space Telescope Science Institute, 3700

    Received 1 July 2004

    Available onli

    AbstractE-mail address: w.grundy@lowell.edu (W.M. Grundy).1 Now at Johns Hopkins University, Baltimore, MD 21218, USA.

    0019-1035/$ see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2005.01.007191www.elsevier.com/locate/icarus

    rans-neptunian objectsll b, D.C. Stephens b,1

    ill Rd., Flagstaff, AZ 86001, USAMartin Dr., Baltimore, MD 21218, USA

    ed 9 December 2004

    March 2005colors from TNOs (e.g., Jewitt, 2002), inferred to result fromvolatile loss processes which may also affect Centaurs.

  • TNODiverse

    2. Size and albedo

    Size and albedo are fundamental properties of bodieswhich constrain composition and internal structure. If sys-tematic patterns were found to exist among these properties,they could reveal much about the accretion of TNOs in theproto-planetary nebula and about the subsequent process-ing of their surfaces. Different types of TNOs could haveformed at different rates or in different source regions re-sulting in different bulk compositions. Objects in differenttypes of orbits could experience different cratering rates orradiolytic fluxes (e.g., Stern, 2002a; Cooper et al., 2003).Smaller objects may see their surfaces eroded by impactswhile larger objects retain impact ejecta (e.g., Stern, 1995;Durda and Stern, 2000). Larger objects could also have theirsurface compositions altered by thermal processes such asvolatile transport or differentiation (e.g., De Sanctis et al.,2001). It has been postulated that TNO colors reflect sur-face age; that their surfaces darken and redden over time viaradiolysis and photolysis, in contrast with fresher surfaceswhich feature recently exposed, bright ices, and so appearmore gray (e.g., Luu and Jewitt, 1996; Gil-Hutton, 2002;Peixinho et al., 2003; Strazzulla et al., 2003; Thbault andDoressoundiram, 2003; Delsanti et al., 2004). Alternatively,gray surfaces could be so highly radiation damaged and de-volatilized that they are extremely dark, with an almost purecarbon composition (e.g., Johnson et al., 1987; Moroz et al.,2003). Impacts by micrometeoroids could also play an im-portant role in churning up gray or red subsurface materials(Cooper et al., 2003). Comparing albedo with other parame-ters for a large sample of TNOs could potentially provideuseful information for these types of inquiries.

    Size and albedo are, unfortunately, extremely difficult tomeasure for TNOs. To date, data enabling estimates or limitsfor 20 TNOs have been obtained from a variety of sourcesand methods. Despite potential risks in comparing data hav-ing different systematic uncertainties, the growing body ofobjects having constrained size and albedo offers an oppor-tunity to look for potential correlations between albedo andorbital parameters, size, or color. In the following subsec-tions we describe the sources of size and albedo constraintsused in this paper along with their characteristic uncertain-ties.

    2.1. Thermal observations

    Simultaneously observing thermal emission and reflectedsunlight is a well-established technique which has beenwidely used to estimate asteroid size and albedo (e.g.,Lebofsky and Spencer, 1989). A thermal model (subject tovarious assumptions) can be inverted to solve for a size and

    an albedo which are consistent with observed emitted andreflected fluxes. Principal sources of uncertainty in TNOradiometric diameters include lack of knowledge of pole ori-entation and thermal inertia.albedos 185

    Thermal emission from trans-neptunian objects is ex-tremely difficult to detect, owing to their great distance,small size, and low surface temperatures. Blackbody radi-ation from TNOs typically peaks in the vicinity of 100 m, awavelength where ground-based observations are precludedby the opacity of the terrestrial atmosphere. Nevertheless,thermal flux measurements have now been reported for 4TNOs, and upper limits have been reported for 9 more.Thomas et al. (2000) reported a 90 m thermal measure-ment from the ISO spacecraft for (15789) 1993 SC alongwith a tentative detection of (15874) 1996 TL66 which wetake to be an upper limit. Additionally, various ground-based observers have detected the RayleighJeans tail ofthermal emission from TNOs at sub-mm wavelengths, in-cluding (20000) Varuna, observed by Jewitt et al. (2001) andby Lellouch et al. (2002). Margot et al. (2002) have reported1.2 mm observations of (55565) 2002 AW197. Altenhoff etal. (2004) reported 1.2 mm observations of (47171) 1999TC36 and upper limits for (28978) Ixion, (24835) 1995SM55, (19308) 1996 TO66, (19521) Chaos, (38628) Huya,and (84522) 2002 TC302. Ortiz et al. (2004) have reportedan upper limit for (55636) 2002 TX300. Brown et al. (2004)have reported an upper limit for (90377) Sedna. Many ad-ditional thermal observations of TNOs can be expected overthe next few years from the Spitzer Space Telescope (SST),although SST targets tend to be biased towards the larger andnearer objects which are more readily detected at thermalinfrared wavelengths. An additional advantage of Spitzerobservations is its ability to observe more than one ther-mal wavelength, which can provide the additional constraintneeded to overcome many of the uncertainties associatedwith thermal properties and pole orientation (e.g., Lebofskyand Spencer, 1989; Stansberry et al., 2004).

    For this paper, we fitted a consistent suite of thermal mod-els to reported thermal fluxes in conjunction with publishedvisual photometry (from sources detailed in Section 3). Up-per limits on diameters (and lower limits on albedos) werederived by using an infinite thermal inertia (fast-rotator)model without beaming, in equator-on orientation, while theopposite limits were derived from a zero thermal inertia(slow-rotator) model augmented with a beaming parame-ter of 0.8. These two cases conservatively encompass thegamut of plausible thermal models. The most probable ther-mal model was taken to be a fast-rotator tilted 30 away fromequatorial orientation.

    2.2. Direct imaging

    The diameter of the large Classical KBO (50000) Quaoarhas been determined via direct observation by Brownand Trujillo (2004), using the Hubble Space TelescopesAdvanced Camera for Surveys High Resolution Camera

    (HST/ACS HRC). An upper limit to the diameter of (90377)Sedna can be set from HRC observations that failed to re-solve that object. At the time of the HST observation, Sednawas at a geocentric distance of 90.375 AU. At this distance,

  • carus186 W.M. Grundy et al. / I

    the ACS/HRC y-axis pixel scale of 0.0247 arcsec pixel1corresponds to a linear distance of 1620 km. The cor-responding x-axis values are 0.02855 arcsec pixel1 and1870 km. An object of this size or larger would show ex-tension perpendicular to the motion vector (the observationsdid not track Sedna). An approximate upper limit to thediameter can be set at d < 1800 km from the ACS/HRCdata, similar to the upper limit diameter inferred from thereported non-detection by SST (Brown et al., 2004). Theprincipal source of uncertainty in direct imaging diametermeasurements comes from lack of knowledge of a targetscenter-to-limb brightness profile.

    2.3. Binaries

    An exciting new source of TNO sizes and albedos is as-trometric observations of TNOs with natural satellites (e.g.,Noll, 2003). Determining a satellites orbital period andsemimajor axis yields the system mass. Total system volumecan then be computed for an assumed density. For this paperwe used densities of 0.5 and 2 g cm3 to bracket the plau-sible range of system densities. We then used relative visualphotometry between primary and secondary to compute theirrelative sizes, assuming both have the same albedo, and usedpublished absolute photometry to constrain that albedo. Be-cause the majority of the surface area in a binary is presentedby the primary body, its diameter and albedo are better con-strained than are those of the secondary. For this reason,sizes and albedos of binary secondaries are not consideredin this paper.

    Binaries are especially valuable in providing data for ob-jects much too small to detect with current thermal infraredor direct imaging technologies. The first binary orbit was re-ported by Veillet et al. (2002) for 1998 WW31. Since then,orbits have been determined for (66652) 1999 RZ253 (Nollet al., 2004a), (58534) 1997 CQ29 (Margot et al., 2004;Noll et al., 2004b), (88611) 2001 QT297 (Osip et al., 2003),(26308) 1998 SM165, (47171) 1999 TC36, and 2001 QC298(Margot et al., 2004). Future observations of thermal emis-sion from binaries, mutual events, or stellar occultationswould greatly help in constraining TNO densities, by pro-viding independent determinations of their sizes (e.g., Noll,2003; Elliot and Kern, 2003).

    3. Data processing

    By using a consistent set of thermal models to re-computesizes and albedos from reported thermal and visual fluxesand a consistent set of densities to estimate sizes and albedosfrom system masses and visual photometry we attemptedto provide as consistent as possible a basis for compari-

    son. We also used photometry from the literature to computethe spectral slope parameter s, defined as the percent in-crease in reflectance per 100 nm of wavelength relative tothe V central wavelength 550 nm, for wavelengths ranging176 (2005) 184191

    from V to I (e.g., Boehnhardt et al., 2001). We computeds from all available photometry in that wavelength range,combined according to reported uncertainties. Values of sfor TNOs range from less than 5 for gray objects to greaterthan 10 for red objects. Table 1 lists dynamical class, spectralslope s, method of constraining size and albedo, diameter d ,and R albedo for the 20 TNOs having published size andalbedo constraints, as well as the planet Pluto and its satel-lite Charon. Data sources for specific objects are detailed inTable 2 and for objects requiring non-standard treatment, inthe remainder of this section.

    For (15874) 1996 TL66, we took the radiometric flux re-ported by Thomas et al. (2000) to be an upper limit becausenothing was seen at the objects ephemeris location.

    For (20000) Varuna, we used radiometric fluxes fromJewitt et al. (2001) and Lellouch et al. (2002) separately, andthen took the resulting albedo and diameter limits to encom-pass the range of possible values.

    For the binary (47171) 1999 TC36, we used the Margot etal. (2004) mass and differential photometry. A radiometricflux measurement by Altenhoff et al. (2004) offers an in-dependent measure of the projected surface area of (47171)1999 TC36 which can be combined with the system mass tocompute an apparently absurd density of 0.15+0.170.05 g cm3for the primary. However, the Altenhoff et al. (2004) sizehas been called into question by Stansberry et al. (2004),who find a size consistent with our size estimates from thesystem mass (Stansberry, personal communication, 2004).

    For (50000) Quaoar, we used the direct imaging diameterof Brown and Trujillo (2004).

    For (55565) 2002 AW197, Margot et al. (2002) reported aradiometric diameter and R albedo but did not report the ob-served thermal flux, so we could not apply our standard ther-mal models to this object, and instead used the albedo andsize reported by Margot et al. Recent results from SpitzerSpace Telescope reported by Stansberry et al. (2004) sug-gest that this object has a somewhat smaller diameter andhigher albedo than found by Margot et al. (2002).

    For (84522) 2002 TC302, we were unable to find photo-metric colors to compute a spectral slope. The only availablephotometry for this object was reported in MPECs 2002-V26 and 2002-X65.

    For (90377) Sedna we used the Brown et al. (2004) radio-metric limits and photometry, which are consistent with di-rect imaging limits based on HST/ACS observations. Brownet al. did not...

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