A multi-chord stellar occultation by the large trans-Neptunian object (174567) Varda

TL;DR

This study reports the first stellar occultation observation of trans-Neptunian object Varda, determining its size, shape, albedo, and density, and exploring its rotational properties and possible shape models.

Contribution

It provides the first occultation-based measurements of Varda's size, shape, and density, and analyzes its rotational state with multiple period hypotheses.

Findings

Varda's equivalent radius is approximately 370 km.

The geometric albedo of Varda is about 0.099.

Bulk density estimates range from 1.23 to 1.78 g/cm³ depending on rotation period.

Abstract

We present results from the first recorded stellar occultation by the large trans-Neptunian object (174567) Varda that was observed on September 10$^{\rm th}$, 2018. Varda belongs to the high-inclination dynamically excited population, and has a satellite, Ilmar\"e, which is half the size of Varda. We determine the size and albedo of Varda and constrain its 3D shape and density. Thirteen different sites in the USA monitored the event, five of which detected an occultation by the main body. A best-fitting ellipse to the occultation chords provides the instantaneous limb of the body, from which the geometric albedo is computed. The size and shape of Varda are evaluated, and its bulk density is constrained, using Varda's mass known from previous works. The best-fitting elliptical limb has semi-major (equatorial) axis of $(383 \pm 3)$km and an apparent oblateness $0.066\pm0.047$ corresponding to an apparent area-equivalent radius $R'_{\rm equiv}= (370\pm7)$km and geometric albedo $p_v=0.099\pm 0.002 $ assuming a visual absolute magnitude $H_V=3.81\pm0.01$. Using three possible rotational periods for the body (4.76h, 5.91h, and 7.87h), we derive corresponding MacLaurin solutions. Furthermore, given the low-amplitude ($0.06\pm0.01$) mag of the single-peaked rotational light-curve for the aforementioned periods, we consider the double periods. For the 5.91h period (the most probable) and its double (11.82h), we find bulk densities and true oblateness of $\rho=(1.78\pm0.06)$ g cm$^{-3}$, $\epsilon=0.235\pm0.050$ and $\rho=(1.23\pm0.04)$ g cm$^{-3}$, $\epsilon=0.080\pm0.049$. However, it must be noted that the other solutions cannot be excluded just yet.