Notes on Pluto stellar occulation of July 20th A-M. Lagrange (11/6/2002) ============================================================================= THE PLUTO STELLAR OCCULTATIONS OF JULY 20TH, 2002 ------------------------------------------------- A. Context Pluto's atmosphere was seen **only once**, after a discovery observation made in Australia by american teams during a stellar occultation on July 9, 1988, see Hubbard et al. Nature 336, 452 (1988) and Elliot et al. Icarus 77, 148 (1989). Now, the only way to study Pluto's atmosphere from the Earth is to observe a stellar occultation, when it occurs. Furthermore the July occultation will be particularly important to the New Horizons space mission to Pluto-Charon, since this will be the last occultation that will be able to impact the design of the mission instruments. B. The July 20th, 2002 occultation Due to the very small Pluto's angular diameter on the sky (2400 km = 0.1 arcsec), occultations by this planet are **extremely rare** and difficult to predict well in advance. This is why no occultation by Pluto has been observed since 1988. Such an event will occur on July 20th, 2002, and will be visible from Southern America, with a central line in the Atacama region, see updates on: http://despa.obspm.fr/~sicardy/pluton/pluton.html (The next favourable event is due on May 22, 2005. It could be visible anywhere from New Guinea, Australia or New Zealand, but not from Chile). An international campaign is organized to follow this exceptional event. American teams from MIT, Boulder and Flagstaff will observe from Cerro Tololo (4-m + 0.9-m), Las Campanas (6-m Magellan) and with portable telescopes to catch the central flash (see below) in northern Chile. A European team (mainly from Paris Observatory, plus an observer from Germany) will conduct observations at La Silla (NTT/EMMI, 3.6-m/Adonis) on DDT. We will also set up portable stations in northern Chile. Finally, we got time in Argentina (San Juan, 2-m) and Brazil (Itajuba, 1.6-m + 0.6-m) C. NACO observations NACO observations could yield by far the highest S/N ratio among all the observations made during this large campaign. The aim is to record the event at a rate of 5-10 images/sec during the 3 minutes or so of the occultation. This time resolution allows us to get a spatial resolution better than 5 km at Pluto, as the star "scans" the planet surrounding at a relative velocity of about 17 km/sec. In particular, the tenuous atmosphere of Pluton (a few microbars of N2) will refract the stellar rays and cause a dimming of the star. Putative aerosols in the planet atmosphere could add to this dimming. Adaptive optics is very important because: (1) it concentrates the star image into a narrow spot, thus minimizing the contribution of the sky and boosting the photometric S/N on the occulted star. (2) it allows us to neatly remove the "polluting" contributions of Charon (0.5 arcsec away) and of a nearby star (2 arcsec from the occulted star). In contrast, classical imagery with regular seeing will integrate all these contributions into a single spot, thus adding to the noise. Furthermore Paranal is ideally located because: (3) Pluton will be near the zenith (distance 12 deg) at the moment of the event. (4) the central flash line is so far expected in northern Chile. (5) Finally, the occulted star is bright in the IR, with K ~ 9.5, about 25 times brighter than Pluto, providing an excellent contrast for the occultation. AO observations will be attempted at La Silla/Adonis, but probably with a lower S/N and coarser time resolution. In any case, La Silla will scan a different region of Pluto's atmosphere, 500 km away from Paranal (while Pluto's radius is about 1200 km), allowing us to detect inhomogeneities in the atmosphere and to reconstruct Pluto's shape. D. Scientific importance After the 1988 occultation, Pluto's atmosphere is still poorly understood. Results from then show that the atmosphere (essentially nitrogen, N2) either (i) has a strong temperature gradient near the surface (5-10 K/km), possibly due to radiative heating by methane, or (ii) possesses a thick aerosol layer, of photochemical origin. As a consequence, Pluto's radius R and the atmospheric surface pressure p_s are poorly determined too: R ranges from 1160 to 1210 km, and p_s ranges from 3 to 50 microbar. Thus: (1) Observing the event in widely separated sites will allow us to **reconstruct Pluto's shape with unprecedented accuracy of few km**, while the planet will be at more than 4 billions km from the Earth. (2) Observing the event in widely separated wavelengths (V and IR) at various telescopes will **discriminate between Pluto's atmospheric models**: a thermal gradient will not have chromatic effects during the event, while aerosols will absorb differentially the stellar flux in the V and IR ("red sunset effect"). (3) Since 1988, Pluto has receded from the Sun by 3%, enough to cool down the surface by 0.6 K, enough also to significantly "shrink" the N2 atmosphere by 50%, if the latter is in thermodynamical equilibrium. The **time evolution*** of Pluto's atmosphere is thus important to follow up. (4) In a +/- 100 km region around the centrality of the occultation, a "flash" could be observed for a few seconds. It is caused by the focusing of the stellar rays refracted by Pluto's atmosphere towards the Earth. The central flash is very sensitive to the shape of the atmosphere and could be a very unique diagnosis of **winds** in the planet atmosphere. For the moment, the central flash is expected somewhere in northern Chile, but predictions can still change. In any case, it is important to diversify the observation sites to increase our chances to catch this central flash. In conclusion, this event is rare and could have a rich and unique scientific return concerning Pluto's atmosphere. --------------------------------------------------------------------------- Exp. time: Imaging: short exposures 0.1 second or shorter if necessary to avoid saturation , windowing if necessary for imaging. Total time is about 20 minutes, plus 10 minutes for images taken some time prior to or after the occultation (see below). Spectroscopy: a total of 2 hours is probably needed. So total time is 3 hours is needed. ---------------------------------------------------------------------------- Atmospheric conditions: being photometric is not required as there will be another star in the FoV, so relative photometry is ok. concerning the seeing conditions, we can still work under medium seeing and up to 1.1". --------------------------------------------------------------------------- Objective: The aim is 1) to take benefit from an occultation by Pluton of a GSC star to study with an unprecedent spatial resolution Pluto's atmosphere and 2) to take benefit from the high spatial resolution to record Pluton and Charon spectra in the 1.8 to 3.6 micron range. Note: observing strategy - Servo on a R=15,6 magnitude star located at ra= 255.074631 dec= -12.694848 J2000, ie 21 arcsec south-east of the occulted star. - Start recording images 5 minutes before the occultation up to 5 minutes afterwards. These observations outside the occultation will allow getting an idea on the statistics of the signal and low frequency variability. - Record images well prior (a few hours) the occultation while Pluton and the GSC are well separated in order to get a very precise estimate of the relative magnitudes. Pluto's motion is 2.8"/hour. - Be carefull on disks space. - Record an image well prior the July 20 to check the FoV (2 stars) and their respective magnitudes. =============================================================================