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Register a new userCol-OSSOS: Color and Inclination are Correlated Throughout the Kuiper Belt
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Micha\\\"el Marsset
Publication date
2018
fields
Physics
and research's language is
English
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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.
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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.
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.
We re-examine the correlation between the colors and the inclinations of the Classical Kuiper Belt Objects (CKBOs) with an enlarged sample of optical measurements. The correlation is strong (rho=-0.7) and highly significant (>8 sigma) in the range 0-34 deg. Nonetheless, the optical colors are independent of inclination below ~12 deg, showing no evidence for a break at the reported boundary between the so-called dynamically hot and cold populations near ~5 deg. The commonly accepted parity between the dynamically cold CKBOs and the red CKBOs is observationally unsubstantiated, since the group of red CKBOs extends to higher inclinations. Our data suggest, however, the existence of a different color break. We find that the functional form of the color-inclination relation is most satisfactorily described by a non-linear and stepwise behavior with a color break at ~12 deg. Objects with inclinations >12 deg show bluish colors which are either weakly correlated with inclination or are simply homogeneously blue, whereas objects with inclinations <12 deg are homogeneously red.
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 outer Solar System object (486958) Arrokoth (provisional designation 2014 MU$_{69}$) has been largely undisturbed since its formation. We study its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through radiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/ or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, suggesting Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29$pm$5 K.
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