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B. Sicardy, O. Mousis (Obs. \& Univ. Paris-Despa, Meudon, France), W. Beisker, E. Hummel (IOTA-European Section, Hannover, Germany), W.B. Hubbard, R. Hill (LPL Uni. Arizona, Tucson, USA), H.J. Reitsema (Ball Aerospace, Boulder, Colorado, USA), P. Anderson, L. Ball, B. Downs, S. Hutcheon, M. Moy, G. Nielsen (Astron. Assoc. of Queensland, St Lucia, Qld, Australia), I. Pink, R. Walters (Bundaberg Astronomical Society Inc.)
The occultation of the star Tr176 by Triton (Mc Donald & Elliot, {\it AJ} {\bf 109}, 1352, 1995) was observed on 18 July 1997 from three stations in Queensland, Australia (Bundaberg, Ducabrook and Lochington) and one station in Texas, USA (Brownsville). All observations were made with CCD (no filter) and with portable C14 telescopes, except at Bundaberg, where a fixed 48-cm telescope was used. Time sampling rate ranges from 0.33 sec (Bundaberg) to 0.66 sec (Ducabrook and Lochington), with the intermediate value 0.5 sec at Brownsville.
Isothermal fits were performed to the lightcurves in order to determine the isothermal temperature, T\rm iso, and the radius at half-level, R1/2, of Triton's atmosphere (assumed to be composed of pure N2).
Considering the level of noise, we cannot detect any departure from isothermal profiles, and we do not see any deviations from spherical shape. A global fit yields T\rm iso = 53.7 ±2 K and R1/2 = 1456 ±3 km. We also derive the pressure at 1400 km: p1400 = 1.9 ±0.3 \mubars.
We will discuss these results and compare them with previous works obtained by {\it Voyager} teams from the 1989 observations, and by Olkin {\it et al.} ({\it Icarus} {\bf 129}, 178, 1997), who analyze two Triton occultations observed in July 1993 (Tr60) and August 1995 (Tr148). We observe a general increase of pressure at 1400 km, since Olkin {\it et al.} derive p1400 = 1.4 ±0.1 \mubars from the Tr148 event. This result is actually confirmed by a recent work by Elliot {\it et al.}, ({\it Nature} {\bf 393}, 765 1998), who note a global warming on Triton, based in particular on a new HST occultation observation in November 1997 (Tr180).