Physics – Optics
Scientific paper
Aug 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002eso..presp..21.&link_type=abstract
ESO Press Release, 08/2002
Physics
Optics
Scientific paper
High-Resolution VLT Image of Pluto Event on July 20, 2002
A rare celestial phenomenon involving the distant planet Pluto has occurred twice within the past month. Seen from the Earth, this planet moved in front of two different stars on July 20 and August 21, respectively, providing observers at various observatories in South America and in the Pacific area with a long awaited and most welcome opportunity to learn more about the tenuous atmosphere of that cold planet. On the first date, a series of very sharp images of a small sky field with Pluto and the star was obtained with the NAOS-CONICA (NACO) adaptive optics (AO) camera mounted on the ESO VLT 8.2-m YEPUN telescope at the Paranal Observatory.
With a diameter of about 2300 km, Pluto is about six times smaller than the Earth. Like our own planet, it possesses a relatively large moon, Charon , measuring 1200 km across and circling Pluto at a distance of about 19,600 km once every 6.4 days. In fact, because of the similarity of the two bodies, the Pluto-Charon system is often referred to as a double planet .
At the current distance of nearly 4,500 million km from the Earth, Pluto's disk subtends a very small angle in the sky, 0.107 arcsec. It is therefore very seldom that Pluto - during its orbital motion - passes exactly in front of a comparatively bright star. Such events are known as "occultations" , and it is difficult to predict exactly when and where on the Earth's surface they are visible. Stellar occultations
When Pluto moves in front of a star, it casts a "shadow" on the Earth's surface within which an observer cannot see the star, much like the Earth's Moon hides the Sun during a total solar eclipse. During the occultation event, Pluto's "shadow" also moves across the Earth's surface. The width of this shadow is equal to Pluto's diameter, i.e. about 2300 km.
One such occultation event was observed in 1988, and two others were expected to occur in 2002, according to predictions published in 2000 by American astronomers Steve W. McDonald and James L. Elliot (Massachussetts Institute of Technology [MIT], Cambridge, USA). Further refinements provided by other observers later showed that the first event would be visible from South America on July 20, 2002 , while a second one on August 21 was expected to be observable in the Pacific basin, from the western coast of North America down to Hawaii and New Zealand.
A stellar occultation provides a useful opportunity to study the planetary atmosphere, by means of accurate photometric measurements of the dimming of the stellar light, as the planet moves in front of the star. The observed variation of the light intensity and colour provides crucial information about the structure (atmospheric layers) and composition of different gases and aerosols. More information is available in the Appendix below. The July 20 occultation
ESO PR Photo 21a/02
ESO PR Photo 21a/02
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Caption : PR Photo 21c/02 shows the path of Pluto's shadow (grey region) during the July 20, 2002 occultation. The shadow has a diameter of about 2300 km and moves from right to left; the timings along the central line are indicated in one-minute intervals (Universal Time - UT). The width of the gray area corresponds to the regions where more than 50% of the light from the star P126 A was attenuated by Pluto's atmosphere. The dotted lines indicate where the stellar flux was attenuated by more than 10%. The maximum duration of the occultation (for observers located at the middle of the shadow track) was about 3 min. The plot is based on astrometric measurements posted at the MIT site. Once completely analyzed, the VLT NACO images will provide significantly better accuracy on the location of this track and therefore a solid basis for the interpretation of the photometric observations obtained with other telescopes.
In order to profit from the rare opportunity to learn more about Pluto and its atmosphere, a large campaign involving more than twenty scientists and engineers from the Paris Observatory and associated institutions [1] was organized to observe the July 20, 2002, event involving an occultation of a star of visual magnitude 11 (i.e., about 100 times fainter than what can be perceived with then unaided eye), referred to as "P126" in McDonald and Elliot's catalogue.
In May 2002, preparatory observations showed that star to be double, with the brighter component of the system ( "P126 A" ) being likely to be occulted by Pluto, as seen from South America. However, because of the duplicity, the predictions of exactly where the shadow of Pluto would sweep the ground were uncertain by about 0.1 arcsec in the sky, corresponding to more than 2000 km on the ground. The NACO images
ESO PR Photo 21b/02
ESO PR Photo 21b/02
[Preview - JPEG: 400 x 469 pix - 47k] [Normal - JPEG: 800 x 937 pix - 208k]
ESO PR Photo 21c/02
ESO PR Photo 21c/02
[Preview - JPEG: 400 x 467 pix - 53k] [Normal - JPEG: 800 x 933 pix - 232k]
Caption : PR Photo 21b/02 shows one of the images obtained with the NAOS-CONICA (NACO) adaptive optics (AO) camera mounted on the ESO VLT 8.2-m YEPUN telescope at the Paranal Observatory in connection with a stellar occultation by Pluto on July 20, 2002. The star was found to be triple - the three components (A, B and C), as well as Pluto and its moon, Charon, are indicated in PR Photo 21c/02 for easy orientation. The images are based on data available from the NACO data webpage. See the text for details.
In the end, the close approach (an "appulse" in astronomical terminology) of Pluto and P126 A was indeed observed from various sites in South America, with several mobile telescopes and also including major facilities at the ESO La Silla and Paranal Observatories. In particular, unique and very sharp images were obtained with the NAOS-CONICA (NACO) adaptive optics (AO) camera mounted on the ESO VLT 8.2-m YEPUN telescope . One of the NACO images is shown in PR Photos 21b-c/02 . These images were made during the final adjustments of the NACO instrument and in anticipation of the upcoming science verification observations. All frames are now publicly available from the NACO data webpage on the ESO site.
The NACO image shown was obtained in infrared light (in the K-band at wavelength 2.2 µm) on July 20, 2002, some 45 min before Pluto's shadow passed north of Paranal ( Photo 21a/02) . The orientation is such that North is up, and East is left. The small sky field measures about 7 x 7 arcsec 2. The pixel size is 0.027 arcsec, and the achieved image sharpness corresponds to the theoretical limit (the diffraction limit) with a telescope of this size and at this wavelength (0.07 arcsec).
The object at the centre is the star P126 A of K-magnitude 9.5 (see also Photo 21c/02 where the objects are identified), while the brighter object at the right is the companion star P126 B , 2.25 arcsec away. As P126 B is very red (of stellar type M), it appears brighter than P126 A at this infrared wavelength - the opposite is true in visible light. The intensity of the left part of the image has been multiplied by a factor of approximately 35 in order to better display Pluto and its moon Charon , located some 0.5 arcsec to the lower left (SE) of the planet. Note also the faint star "P126 C" , at this moment very close to Pluto; it is probably a (physical) member of the P126 system.
A closer inspection of the original image shows that the disk of Pluto (with a diameter of 0.107 arscec and covering 16 NACO pixels) is (barely) resolved.
Many images were taken by NACO prior to the occultation. They will allow to retrace the precise motion of Pluto relative to P126 A, thereby improving the mapping of the motion of Pluto's shadow on the ground, cf. Photo 21a/02 . This is important in order to apply the correct geometrical circumstances for the interpretation of the photometric observations. A first evaluation of the NACO data indicates that the Paranal site "missed" the upper layers of Pluto's atmosphere by a mere 200 km or so - this is equivalent to no more
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