Abstract
A stellar occultation by a trans-Neptunian object (TNO) provides an opportunity to probe its size and shape. Very few occultations by TNOs have been sampled simultaneously from multiple locations, while a robust estimation of shadow size has been possible for only two objects. We present the first observation of an occultation by the TNO 2007 UK126 on 15 November 2014, measured by three observers, one nearly on and two almost symmetrical to the shadow's centerline. This is the first multi-chord dataset obtained for a so-called detached object, a TNO subgroup with perihelion distances so large that the giant planets have likely not perturbed their orbits. We revisit Herschel/PACS far-infrared data, applying a new reduction method to improve the accuracy of the measured fluxes. Combining both datasets allows us to comprehensively characterize 2007 UK126. We use error-in-variable regression to solve the non-linear problem of propagating timing errors into uncertainties of the ellipse parameters. Based on the shadow's size and a previously reported rotation period, we expect a shape of a Maclaurin spheroid and derive a geometrically plausible size range. To refine our size estimate of 2007 UK126, we model its thermal emission using a thermophysical model code. We conduct a parametric study to predict far-infrared fluxes and compare them to the Herschel/PACS measurements. The favorable geometry of our occultation chords, combined with minimal dead-time imaging, and precise GPS time measurements, allow for an accurate estimation of the shadow size (best-fitting ellipse with axes 645.80 $\pm$ 5.68 km $\times$ 597.81 $\pm$ 12.74 km) and the visual geometric albedo (15.0 $\pm$ 1.6 %). By combining our analyses of the occultation and the far-infrared data, we can constrain the effective diameter of 2007 UK126 to 599 - 629 km. We conclude that subsolar surface temperatures are $\approx$ 50 - 55 K.
Highlights
Stellar occultations are the best opportunity to directly and accurately determine the size and shape of a solar system body remotely from Earth
The reconstruction of shadow size and shape is done in the geocentric fundamental (Besselian) plane, which is perpendicular to the line connecting the apparent position of the occulted star UCAC4 448-006503 and the position of 2007 UK126 as given by the ephemeris
It can be seen that the occultation data is able to improve the previous size estimate of dSphere,eff = 599 ± 77 km for an equivalent sphere that was derived by Santos-Sanz et al (2012) solely through thermophysical modeling based on Herschel/PACS data
Summary
(50000) Quaoar (Braga-Ribas et al 2013) is currently the best sampled TNO, having five unique chords (i.e. chords that are geographically sufficiently distant to each other to sample the occulting object at distinguishable locations) that were recorded during a single event (04 May 2011) In this dataset, the time resolution and accuracy of the two chords above the centerline have been low, resulting in large uncertainties of ingress and egress times and in some ambiguity of the ellipse fit. In addition to being the first comprehensive study of an object of this dynamical class, the dataset presented in this paper is extremely rare compared to previously achieved observations of occultations by TNOs: to our knowledge, it is only the second after (50000) Quaoar that provides a sufficient number of chords (at least three chords are required for an ellipse fit) that are well spaced (in the present case sampling the target simultaneously above, very close, and below the centerline by extremely favorably distributed, quasi symmetrically located observers). This combined analysis allows us to comprehensively characterize 2007 UK126
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