The prediction and observation of the 1997 July 18 stellar occultation by Triton: More evidence for distortion and increasing pressure in Triton's atmosphere

A variety of CCD astrometric data was used to predict the location of the p ath for the occultation of the star we have denoted "Tr176" by Triton, whic h occurred on 1997 July 18, and was visible from locations in northern Aust ralia and southern North America. A network of fixed and portable telescope s equipped with highspeed photometric equipment was set up to observe the e vent, with the following observational goals: (i) mapping the central flash (to establish the global shape of Triton's atmosphere at about 20-km altit ude by modeling the detailed shape of the central hash), (ii) obtaining one or more Light curves of high signal-to-noise ratio from a large telescope (to accurately determine the thermal structure of Triton's atmosphere), and (iii) obtaining light curves distributed across Triton's disk (to probe th e thermal structure of Triton's atmosphere above different areas and to est ablish the shape of the atmosphere at about 100-km altitude by modeling the half-light surface). Although the large, fixed telescopes proved to be out side of the occultation shadow and observations with some of the portable t elescopes were foiled by clouds, light curves were successfully recorded fr om Brownsville, Texas, and Chillagoe, Queensland. These were combined with data from another group to determine the radius and shape of the half-light surface in Triton's atmosphere and the equivalent-isothermal temperatures at the sub-occultation latitudes on Triton. A circular solution for the hal f-light surface (projected into Triton's shadow) yielded a radius of 1439 /- 10 km. However, the data are indicative of a global shape more complex t han a sphere. Such. a figure is most likely caused by strong winds. Light-c urve models corresponding to the best fitting circular and elliptical atmos pheres were fit to the data. The mean pressure at 1400-km radius (48-km alt itude) derived from all of the data was 2.23 +/- 0.28 mu bar for the circul ar model and 2.45 +/- 0.32 mu bar for the elliptical model. These values su ggest a global pressure increase at this level since a previous Triton occu ltation in 1995 August. The mean equivalent-isothermal temperature at 1400 km was 43.6 +/- 3.7 K for the circular model and 42.0 +/- 3.6 K for the ell iptical model. Within their (sometimes large) uncertainties, the equivalent -isothermal temperatures agree for all Triton latitudes probed. (C) 2000 Ac ademic Press.