Abstract
Abstract From CCD observations carried out with different telescopes, we present short-term photometric measurements of the large trans-Neptunian object Varuna in 10 epochs, spanning around 19 years. We observe that the amplitude of the rotational light curve has changed considerably during this period of time from 0.41 to 0.55 mag. In order to explain this variation, we constructed a model in which Varuna has a simple triaxial shape, assuming that the main effect comes from the change of the aspect angle as seen from Earth, due to Varuna’s orbital motion in the 19 year time span. The best fits to the data correspond to a family of solutions with axial ratios b/a between 0.56 and 0.60. This constrains the pole orientation in two different ranges of solutions presented here as maps. Apart from the remarkable variation of the amplitude, we have detected changes in the overall shape of the rotational light curve over shorter timescales. After the analysis of the periodogram of the residuals to a 6.343572 hr double-peaked rotational light-curve fit, we find a clear additional periodicity. We propose that these changes in the rotational light-curve shape are due to a large and close-in satellite whose rotation induces the additional periodicity. The peak-to-valley amplitude of this oscillation is in the order of 0.04 mag. We estimate that the satellite orbits Varuna with a period of 11.9819 hr (or 23.9638 hr), assuming that the satellite is tidally locked, at a distance of ∼1300 km (or ∼2000 km) from Varuna, outside the Roche limit.
Highlights
Trans-Neptunian objects (TNOs) are solar system bodies that orbit the Sun with larger semimajor axes than that of Neptune, formed quite outside the so-called “snow line” where the temperature of the protoplanetary disk was low enough to allow the survival of molecules of chemical compounds with low sublimation points
Because Varuna’s body is assumed to have an ellipsoidal shape (e.g., Jewitt & Sheppard 2002; Lellouch et al 2002), data from each light curve were fitted to a Fourier series m = Si[ai sin(2ipf) + bi cos(2ipf)], where m is the theoretical value of the relative magnitude obtained from the fit, f is the rotational phase (calculated as the fractional part of (JD − JD0)/P, where Julian date (JD) is the Julian Date, JD0 = 2451957.0 is the initial Julian Date, and P is the rotation period in days), and are the coefficients of the Fourier function
The second order is the minimum order that allows a double-peaked fit; higher orders take into account small deviations on inhomogeneous objects and can be used to fit light curves that are highly sampled
Summary
Trans-Neptunian objects (TNOs) are solar system bodies that orbit the Sun with larger semimajor axes than that of Neptune, formed quite outside the so-called “snow line” where the temperature of the protoplanetary disk was low enough to allow the survival of molecules of chemical compounds with low sublimation points. The fast period and the elongated shape require a density of ∼1000 kg m−3 (assuming hydrostatic equilibrium; Chandrasekhar 1987) This is somewhat high compared to other TNOs of similar sizes, considering that Varuna’s equivalent diameter is ∼700 km, given by thermo-physical models using Herschel Space Telescope measurements (Lellouch et al 2013); see the supplementary material in Ortiz et al (2012) to compare the density of objects with similar sizes. A stellar occultation by Varuna, detected in 2010, results in a long chord of 1004 km (Sicardy et al 2010) This value is somewhat in tension with the one given by Lellouch et al (2013), but fits better with Varuna’s density.
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