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Further to the earlier posting listed lower down the page, the following additional information has been provided by Jean Lecacheux and forwarded by David Dunham>
About the incoming exceptional Pluto / SAO 160793 appulse, Aleksey Losyuk asked: " When it is necessary to begin photometry of of occultated star > and when to finish it?"
1/ By analogy with major planets, we could hope a main ring existing well inside the orbit of Charon, the innermost satellite of Pluto. The orbital radius of Charon is 19 400 km.
According to the current prediction by the MIT, based upon the UCAC2 position of "P507"= SAO 160793 and recent USNO astrometry of Pluto, the closest approach should occur near 14:48 UT on Thursday 27 at 7 700 ± 1 100 km above the Pluto surface. This is well inside the Charon orbit, as roughly midway between Pluto and Charon. Nevertheless such an appulse at about 7.5 equatorial radii from Pluto's center might be not close enough, leading to a miss of the ring. (Something really difficult to forecast, as we don't know what this hypothetical ring would be like...)
2/ Faint diffuse rings, narrow ringlets, discrete arcs, etc. also could exist in the region of the outer satellites Nix and Hydra.
Assuming the MIT appulse prediction, I have computed the approximate GEOCENTRIC instants of the orbit crossings by the beam from the star :
- Star entering the HYDRA orbit : 13:43 UT
- Star entering the NIX orbit : 14:01
- Star entering the CHARON orbit : 14:37
- Closest approach of the star at 0".39 : 14:48 UT
- Crossing the Pluto-Charon line : 14:55
- Star leaving the CHARON orbit : 15:13 UT
- Star leaving the NIX orbit : 15:48
- Star leaving the HYDRA orbit : 16:07
These geocentric times are valid also from Uzbekistan. ADD 1-3 min from India, 2 min from La Réunion and Mauritius, about 6 min from south-east Asia, 8 min from west Australia. The uncertainty probably lies under 5 minutes.
In my opinion any complete monitoring should begin near 12h30 UT, if possible from the site (for example from Australia), i.e. at about 75 equatorial radii ESE from Pluto, and should end near 17 h (for example from South-Africa), at the same distance WNW.
3/ Sure, other faint rings could exist even very farther, maybe at 400 equatoral radii as pointed out by David, but seeking them would consume two or three complete nights, undoubtly with a low chance of detection.
4/ Some other useful informations to prepare this observation :
The velocity relative to Earth of the Pluto system will be 11.1 km/s or 0.49 mas/s. This seems favourable, as occultations by Pluto often are faster. (Note : 1 mas = 1 milli-arc second will correspond to 22.82 km at the current Pluto distance).
The probable apparent diameter of the giant target star should be close to 0.5 mas or 11 km. (Note : I assumed a K4 III star at 400 parsecs). The Fresnel diffraction scale will be 1.1 km in visible light or 2.3 km for 2.2 microns (infrared) sensors.
It results that any quick event (for example a brief occultation by some "Uranian-style" narrow ringlet, or by some "Neptunian-style" arc) will be lengthened to about one second by the stellar diameter. Besides - the occultation drop will suffer reduction, - any diffraction effect should be almost erased.
To be able to detect by further image addition the slow crossing of a faint tenuous ring of very small optical depth, please take the following precautions : - keep in the field of the camera the brighter SAO 160798, abput five arc min. east (a V= 8.0, R= 7.8 white-yellow subgiant star); - to beware of any signal saturation, either by this bright comparison or by the red target itself.
Note1 : This above suggested bright comparison star is a close visual binary where one component is a magnetic variable star. Its luminosity period is unknown, but probably equal to the star rotation. Moreover Hipparcos found only a 0.011 mag. scatter from 57 widely time-spaced measurements. So one may assume the magnitude of SAO 160798 will not vary at few minutes scale, thus it can be used as reliable comparison along the Pluto event. Note2 : Some fainter comparisons within 0.1 degree also are convenient for large telescope users.
Good luck to every observer.
The email below was received from Dr David Dunham and draws attention to an occultation opportunity around the 27th to detect possible ring systems around Pluto.
Observers in those and adjacent areas, such as Japan, eastern Australia, New Zealand, and South Africa, are encouraged to record the star with CCD, video, or photometric equipment for a few hours around the time of closest approach; such records can be analyzed later to find evidence of any fadings of the star's light that might be due to Plutonian rings; these would be of great interest to scientists involved with the New Horizons mission to Pluto. Walker Vanning, San Rafael, California, claims that observations of past occultations show that Pluto has an extensive ring system that could cause fadings of SAO 160793's light even more than 12h from the time of closest approach, so he encourages observers worldwide to monitor the star while it is high enough above the local horizon; he thinks that very distant rings may exist that could pass over the star even 3 days from the Pluto closest approach time. The star, also known as B.D. -16 deg. 4607, P507, and UCAC2 25587116, is at J2000 position is R.A. 17h 44m 38.4s, Dec. -16 deg. 46' 35" its spectral type is K5.
This event was mentioned in a Sky and Telescope Astroalert at
http://www.skyandtelescope.com/resources/proamcollab/astroalert/AstroAlert2007May11.htmland information about it is also given at the Massachusetts Institute of Technology's Web site at
http://occult.mit.edu/research/occultations/Pluto/P507-preds/David Dunham
2007 September 24
Below are two finder charts showing the position of Pluto on the evening of September 27 and the location of the target star.
Observers are urged to monitor for any variations in the brightness of the target star and if possible carry out analysis using Limovie. Results should be forwarded to Steve Kerr at srkak@iinet.net.au .
Good Hunting! Steve Kerr.
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