Study of the plutino object (208996) 2003 AZ84 from stellar occultations: size, shape and topographic features

Abstract

We present results derived from four stellar occultations by the plutino object (208996) 2003~AZ${}_{84}$, detected at January 8, 2011 (single-chord event), February 3, 2012 (multi-chord), December 2, 2013 (single-chord) and November 15, 2014 (multi-chord). Our observations rule out an oblate spheroid solution for 2003~AZ${}_{84}$'s shape. Instead, assuming hydrostatic equilibrium, we find that a Jacobi triaxial solution with semi axes $(470\pm 20)\times (383\pm 10)\times (245\pm 8)$~km % axis ratios $b/a=0.82\pm 0.05$ and $c/a=0.52\pm 0.02$, can better account for all our occultation observations. Combining these dimensions with the rotation period of the body (6.75~h) and the amplitude of its rotation light curve, we derive a density $\ddot{I}=0.87\pm 0.01$~g~cm${}^{-3}$ a geometric albedo $p_V=0.097\pm 0.009$. A grazing chord observed during the 2014 occultation reveals a topographic feature along 2003~AZ${}_{84}$'s limb, that can be interpreted as an abrupt chasm of width $\sim 23$~km and depth $>8$~km or a smooth depression of width $\sim 80$~km and depth $\sim 13$~km (or an intermediate feature between those two extremes).