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In 1977, while Apple II and Atari computers were being sold, a tiny dot was observed in an inconvenient orbit. The minor body 1977 UB, to be named (2060) Chiron, with an orbit between Saturn and Uranus, became the first Centaur, a new class of minor bodies orbiting roughly between Jupiter and Neptune. The observed overabundance of short-period comets lead to the downfall of the Oort cloud as exclusive source of comets and to the rise of the need for a Trans-Neptunian comet belt. Centaurs were rapidly seen as the transition phase between Kuiper belt objects, also known as Trans-Neptunian objects (TNOs) and the Jupiter-family comets (JFCs). Since then, a lot more has been discovered about Centaurs: They can have cometary activity and outbursts, satellites, and even rings. Over the past four decades since the discovery of the first Centaur, rotation periods, surface colors, reflectivity spectra, and albedos have been measured and analyzed. However, despite such a large number of studies and complementary techniques, the Centaur population remains a mystery as they are in so many ways different from the TNOs and even more so from the JFCs. 

Abstract Dynamically excited objects within the Kuiper Belt show a bimodal distribution in their surface colors, and these differing surface colors may be a tracer of where these objects formed. In this work, we explore radial color distributions in the primordial planetesimal disk and implications for the positions of ice line/color transitions within the Kuiper Belt’s progenitor populations. We combine a full dynamical model of the Kuiper Belt’s evolution due to Neptune’s migration with precise surface colors measured by the Colours of the Outer Solar System Origins Survey in order to examine the true color ratios within the Kuiper Belt and the ice lines within the primordial disk. We investigate the position of a dominant, surface color–changing ice line, with two possible surface color layouts within the initial disk: (1) inner neutral surfaces and outer red and (2) inner red surfaces and outer neutral. We performed simulations with a primordial disk that truncates at 30 au. By radially stepping the color transition out through 0.5 au intervals, we show that both disk configurations are consistent with the observed color fraction. For an inner neutral, outer red primordial disk, we find that the color transition can be at ${28}_{-3}^{+2}$au at a 95% confidence level. For an inner red, outer neutral primordial disk, the color transition can be at ${27}_{-3}^{+3}$au at a 95% confidence level. 

Abstract We present an analysis of microlensing event OGLE-2019-BLG-0825. This event was identified as a planetary candidate by preliminary modeling. We find that significant residuals from the best-fit static binary-lens model exist and a xallarap effect can fit the residuals very well and significantly improvesχ²values. On the other hand, by including the xallarap effect in our models, we find that binary-lens parameters such as mass ratio,q, and separation,s, cannot be constrained well. However, we also find that the parameters for the source system such as the orbital period and semimajor axis are consistent between all the models we analyzed. We therefore constrain the properties of the source system better than the properties of the lens system. The source system comprises a G-type main-sequence star orbited by a brown dwarf with a period ofP∼ 5 days. This analysis is the first to demonstrate that the xallarap effect does affect binary-lens parameters in planetary events. It would not be common for the presence or absence of the xallarap effect to affect lens parameters in events with long orbital periods of the source system or events with transits to caustics, but in other cases, such as this event, the xallarap effect can affect binary-lens parameters.