Abstract

On September 8, 2001 around 2 h UT, the largest uranian moon, Titania, occulted Hipparcos star 106829 (alias SAO 164538, a V = 7.2, KO III star). This was the first-ever observed occultation by this satellite. a rare event as Titania subtends only 0.11 arcsec on the sky. The star's unusual brightness allowed many observers, both amateurs or professionals, to monitor this unique event, providing fifty-seven occultations chords over three continents, all reported here. Selecting the best 27 occultation chords, and assuming a circular limb, we derive Titania's radius: R-T = 788.4 +/- 0.6 km (1-sigma error bar). This implies a density of rho = 1.711 +/- 0.005 g cm(-3) using the value GM = (2.343 +/- 0.006) x 10(11) m(3) s(-2) derived by Taylor [Taylor, D.B., 1998. Astron. Astrophys. 330, 362-374]. We do not detect any significant difference between equatorial and polar radii, in the limit r(eq) - r(po) = -1.3 +/- 2.1 km, in agreement with Voyager limb image retrieval during the 1986 flyby. Titania's offset with respect to the DE405 + URA027 (based on GUST86 theory) ephemeris is derived: Delta alpha(T) cos(delta(T)) = -108 +/- 13 mas and Delta delta(T) = -62 +/- 7 mas (ICRF J2000.0 system). Most of this offset is attributable to a Uranus' barycentric offset with respect to DE405, that we estimate to be: Delta alpha(U) cos(delta(U)) = -100 +/- 25 mas and Delta delta(U) = -85 +/- 25 mas at the moment of occultation, This offset is confirmed by another Titania stellar occultation observed on August 1st. 2003, which provides an offset of Delta alpha(T)-cos(delta(T)) = -127 +/- 20 mas and Delta delta(T) = -97 +/- 13 mas for the satellite. The combined ingress and egress data do not show any significant hint for atmospheric refraction, allowing us to set surface pressure limits at the level of 10-20 nbar. More specifically, we find an upper limit of 13 nbar (1-sigma level) at 70 K and 17 nbar at 80 K, for a putative isothermal CO2 atmosphere. We also provide an upper limit of 8 nbar for a possible CH4 atmosphere, and 22 nbar for pure N-2, again at the 1-sigma level. We finally constrain the stellar size using the time-resolved star disappearance and reappearance at ingress and egress. We find an angular diameter of 0.54 +/- 0.03 mas (corresponding to 7.5 +/- 0.4 km projected at Titania). With a distance of 170 +/- 25 parsecs, this corresponds to a radius of 9.8 +/- 0.2 solar radii for HIP 106829, typical of a KO III giant. (c) 2008 Elsevier Inc. All rights reserved.