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Register a new userCol-OSSOS: Compositional homogeneity of three Kuiper belt binaries
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Added by
Micha\\\"el Marsset
Publication date
2020
fields
Physics
and research's language is
English
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The surface characterization of Trans-Neptunian Binaries (TNBs) is key to understanding the properties of the disk of planetesimals from which these objects formed. In the optical wavelengths, it has been demonstrated that most equal-sized component systems share similar colors, suggesting they have a similar composition. The color homogeneity of binary pairs contrasts with the overall diversity of colors in the Kuiper belt, which was interpreted as evidence that Trans-Neptunian Objects (TNOs) formed from a locally homogeneous and globally heterogeneous protoplanetary disk. In this paradigm, binary pairs must have formed early, before the dynamically hot TNOs were scattered out from their formation location. The latter inferences, however, relied on the assumption that the matching colors of the binary components imply matching composition. Here, we test this assumption by examining the component-resolved photometry of three TNBs found in the Outer Solar System Origins Survey: 505447 (2013 SQ99), 511551 (2014 UD225) and 506121 (2016 BP81), across the visible and J-band near-infrared wavelength range. We report similar colors within 2 sigma for the binary pairs suggestive of similar reflectance spectra and hence surface composition. This advocates for gravitational collapse of pebble clouds as a possible TNO formation route. We however stress that several similarly small TNOs, including at least one binary, have been shown to exhibit substantial spectral variability in the near-infrared, implying color equality of binary pairs is likely to be violated in some cases.
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Both physical and dynamical properties must be considered to constrain the origins of the dynamically excited distant Solar System populations. We present high-precision (g-r) colors for 25 small (Hr>5) dynamically excited Trans-Neptunian Objects (TNOs) and centaurs acquired as part of the Colours of the Outer Solar System Origins Survey (Col-OSSOS). We combine our dataset with previously published measurements and consider a set of 229 colors of outer Solar System objects on dynamically excited orbits. The overall color distribution is bimodal and can be decomposed into two distinct classes, termed `gray and `red, that each has a normal color distribution. The two color classes have different inclination distributions: red objects have lower inclinations than the gray ones. This trend holds for all dynamically excited TNO populations. Even in the worst-case scenario, biases in the discovery surveys cannot account for this trend: it is intrinsic to the TNO population. Considering that TNOs are the precursors of centaurs, and that their inclinations are roughly preserved as they become centaurs, our finding solves the conundrum of centaurs being the only outer Solar System population identified so far to exhibit this property (Tegler et al. 2016). The different orbital distributions of the gray and red dynamically excited TNOs provide strong evidence that their colors are due to different formation locations in a disk of planetesimals with a compositional gradient.
Observations show that 100-km-class Kuiper belt objects (KBOs) can be divided in (at least) two color groups, hereafter red (R, g-i<1.2) and very red (VR, g-i>1.2), reflecting a difference in their surface composition. This is thought to imply that KBOs formed over a relatively wide range of radial distance, r. The cold classicals at 42<r<47 au are predominantly VR and known Neptune Trojans at r=30 au are mostly R. Intriguingly, however, the dynamically hot KBOs show a mix of R and VR colors and no correlation of color with r. Here we perform migration/instability simulations where the Kuiper belt is populated from an extended planetesimal disk. We find that the color observations can be best understood if R objects formed at r<r* and VR objects at r>r*, with 30<r*<40 au. The proposed transition at 30<r*<40 au would explain why the VR objects in the dynamically hot population have smaller orbital inclinations than the R objects, because the orbital excitation from Neptune weakens for orbits starting beyond 30 au. Possible causes of the R-VR color bimodality are discussed.
The cold main classical Kuiper Belt consists of those small solar system bodies with low orbital inclinations and orbital semi-major axes between 42.4 and 47.7~au. Various arguments suggest that these objects formed textit{in situ} and the original population has experienced minimal collisional modification since their formation. Using the Outer Solar System Origins Survey (OSSOS) ensemble sample and characterization, combined with constraints on the number of small cold classical objects from deeper surveys and supported by evidence from the Minor Planet Center catalog, we determine the absolute magnitude $H_r$ distribution of the cold classical belt from $H_rsimeq5$ to 12 (roughly diameters of 400 km to 20 km). We conclude that the cold populations size distribution exhibits an exponential cutoff at large sizes. Exponential cutoffs at large sizes are not a natural outcome of pair-wise particle accretion but exponentially tapered power-law size distributions are a feature of numerical simulations of planetesimal formation via a streaming instability. Our observation of an exponential cutoff agrees with previous observational inferences that no large objects ($D gtrsim 400$~km) exist in the cold population. Studies of the transneptunian region are providing the parameters that will enable future streaming-instability studies to determine the initial conditions of planetesimal formation in the $approx 45$~au region of the Suns protoplanetary disk.
The cold classical Kuiper belt objects have low inclinations and eccentricities and are the only Kuiper belt population suspected to have formed in situ. Compared with the dynamically excited populations, which exhibit a broad range of colours and a low binary fraction of ~10% cold classical Kuiper belt objects typically have red optical colours with ~30% of the population found in binary pairs; the origin of these differences remains unclear. We report the detection of a population of blue-coloured, tenuously bound binaries residing among the cold classical Kuiper belt objects. Here we show that widely separated binaries could have survived push-out into the cold classical region during the early phases of Neptunes migration. The blue binaries may be contaminants, originating at ~38 au, and could provide a unique probe of the formative conditions in a region now nearly devoid of objects. The idea that the blue objects, which are predominantly binary, are the products of push-out requires that the planetesimals formed entirely as multiples. Plausible formation routes include planetesimal formation via pebble accretion and subsequent binary production through dynamic friction and binary formation during the collapse of a cloud of solids.
Here we present observations of 7 large Kuiper Belt Objects. From these observations, we extract a point source catalog with $sim0.01$ precision, and astrometry of our target Kuiper Belt Objects with $0.04-0.08$ precision within that catalog. We have developed a new technique to predict the future occurrence of stellar occultations by Kuiper Belt Objects. The technique makes use of a maximum likelihood approach which determines the best-fit adjustment to cataloged orbital elements of an object. Using simulations of a theoretical object, we discuss the merits and weaknesses of this technique compared to the commonly adopted ephemeris offset approach. We demonstrate that both methods suffer from separate weaknesses, and thus, together provide a fair assessment of the true uncertainty in a particular prediction. We present occultation predictions made by both methods for the 7 tracked objects, with dates as late as 2015. Finally, we discuss observations of three separate close passages of Quaoar to field stars, which reveal the accuracy of the element adjustment approach, and which also demonstrate the necessity of considering the uncertainty in stellar position when assessing potential occultations.
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