Observation of the Occultation of Tr176
by Triton from Brownsville, Texas
H.J. Reitsema (Ball Aerospace Corp.)
W.B. Hubbard (LPL, U. Az.)
R. Hill (LPL, U. Az.)
As part of a coordinated effort by the Portable CCD (PCCD) consortium,
the University of Arizona portable occultation system was deployed to
observe the 18 July 1997 occultation of star Tr176 by Triton. Although
an initial strategy called for our experiment to be located near the
predicted centerline in central Mexico, extremely poor weather
prospects there led us to shift our observing site to Brownsville,
Texas, where clear skies were predicted. According to the final
prediction update by Olkin (see adjacent late paper), Brownsville would
be 718 km from the centerline, with no prospects for observing a Triton
central flash.
Observing conditions were excellent in Brownsville, with photometric
skies and subarcsecond seeing. We obtained unfiltered data at 2 Hz on
a CCD field which included Triton/Tr176, Neptune, and a reference
star. Data cubes were also obtained 1.5 hours prior to the event to
permit separate photometry of Triton and Tr176. Thus we have good
relative photometry of the event, with immersion half-flux occurring at
10:09:34 UTC and emersion half-flux at 10:11:17 UTC (duration=103
seconds). Our data are of high quality, and the calibrated lightcurve
shows residual stellar flux at a level of about 10% in midoccultation.
However, no central flash was observed. From our chord length, Olkin
(private communication) derives a radius of closest approach of the
Brownsville station (latitude 25:58:40.95 +/- 0.10 N, longitude
97:32:11.29 +/- 0.10 W, elev. -0.9 +/- 6.7 m) to Triton of 645 km,
using a model Triton atmosphere derived from previous occultation data
(Olkin et al., in press). A fit of our data to a model atmosphere
calculated by Yelle gives a radius of closest approach of about 605 km,
while a fit to the Voyager RSS model gives a result similar to
Olkin's. Analysis of our data to obtain temperature and density
profiles of Triton's atmosphere in the microbar pressure range is
proceeding.
Our team was greatly assisted by Professors Arnulfo Mar and Mario Diaz
of the University of Texas at Brownsville. We thank Professor Diaz and
his family for allowing us to observe from their residence. The
University of Arizona occultation program is supported by NASA
Planetary Astronomy Grant NAG5-4214.
15 minute
plot of data
Higher
resolution, 3 minute plot of data
Data
compared to models
The star and Triton are to
the left of Neptune (brightest image). There is a bright reference
star below and to the right of Neptune. Notice the dimming of the
star as Triton's tenuous atmosphere occults it. Triton was 30 AU
from Earth (4.5 billion kilometers!).