Physical properties of TNO 1996 TO 66 ? Lightcurves and possible cometary activity

We describe observations of the Trans-Neptunian Object (TNO) 1996 TO 66 performed during three observing runs (August 1997, October 1997 and September 1998). They show significant brightness variations that indicate a rotation period of 6.25±0.03h. In the the 1997 data, the phased lightcurve displays a nearly symmetrical double peak with a peak-to-peak amplitude of 0.12 mag, while in the data of 1998, it shows a single maximum with a full amplitude of 0.33 mag. Possible causes for this change of shape are explored, the simplest explanation being that 1996 TO 66 experienced a phase of cometary activity during the interval between these observations. This hypothesis, if confirmed, could be of interest for the pending question of the observed color diversity of TNOs. The average magnitude (R = 21.15) was converted into a mean radius of 326±7 km (assuming albedo p = 0.04), making 1996 TO 66 the largest known TNO after Pluto and Charon. The object is among the bluest in the outer Solar System and the colour, as measured at different epochs, shows marginally significant changes. Deep, composite images, totaling 13,500s integration time with the NTT and 4500s with the UH 2.2m in the R filter, were searched for possible signatures of a faint coma. None were found and the photometric profile of 1996 TO 66 perfectly matches a stellar one, down to the 29 mag/sq.arcsec level. We also present an apparently featureless 8100s NTT +EMMI spectrum (6000-9100Å) with a neutral reflectivity in the 6000– 7600Å range, and with a marginally significant red gradient S = 30 ± 10%/1000Å in the 7600–9000Å range.