Short-term rotational variability in the large TNO 2005FY9

Abstract

Context. Despite the fact that 2005FY 9 is one of the largest trans-Neptunian objects (TNOs) and a dwarf planet candidate, little is known about this object apart from constraints on its surface composition. Aims. The goal of this work is to study 2005FY 9 's short-term variability in order to determine the amplitude of the lightcurve, which can be linked to the degree of elongation of the body or to the degree of albedo heterogeneity on the surface. Besides, the rotation period can be determined. Methods. CCD photometric observations of the trans-Neptunian object 2005FY 9 in R band on 21 nights spanning several months were carried out using the 1.5 m telescope at Sierra Nevada Observatory and the 2.2 m telescope at Calar Alto Observatory. Results. The time-series analysis leads to confident detection of short-term variability. The most significant periodicities are 11.24 ± 0.01 h and its double, but other possibilities cannot be ruled out. The 22.48 h double-peaked rotational phase curve is slightly preferred by our analysis. As for the amplitude of the lightcurve, we get a peak-to-peak variability of 0.03 + 0.01 mag. This result is compatible with a nearly spherical body that has a very homogeneous surface. There is also the possibility that the body is rotating nearly pole on, but we believe this is less likely. Very weak constraints are obtained for the density and internal strength based on the rotational properties derived from the photometry.