Occultation by Saturn on January 25, 2006

On Wednesday, January 25, a remarkable occultation of a star by a planet will occur: The 7.9 mag star BY Cancri (SAO 98054, TYC 1395-1113, PPM 125631, HIP 42705, HD 74050) will be occulted by Saturn and its rings. This star is a delta Scuti variable with a very small amplitude. Its characteristics are: V = 7.91 mag, Spectral class A7V, B-V = +0.21

Occultations of stars brighter than 8th magnitude by Saturn are extremely rare. In the entire 21^st century there are only three further observable events world-wide:

· 2032 April 7      5.9 mag    E-Pacific, Alaska

· 2054 April 12     7.1 mag    partly in Brazil

· 2099 June 21      7.8 mag    Ring passage in Asia and W-Australia,
                             grazing occultation by Saturn partly in Africa

For Europe the upcoming event is thus a unique event during this century!

The Course of the Occultation

The following diagram shows the progress of the occultation as viewed from the star, as well as the nomenclature of the rings:

The approximate event times for Central Europe:

Disappearance at the outer edge of ring A       18:45 UT

Reappearance at the inner edge of ring A        18:57 UT

          Cassini division

     Disappearance at the outer edge of ring B       19:02 UT

     Occultation by Saturn begins                     20:08 UT

     (evtl. short reappearance in Cassini division)

     Reappearance near Saturn’s South pole           20:50 UT

The variations within Europe do not exceed a few minutes. Times for many locations within the area of the map below are listed in this Table .

Important remark: Uncertainties up to several minutes should be taken into account!

In Europe the star traverses behind the A ring within 12 minutes and then emerges in the Cassini division for another 5 minutes. Subsequently it disappears behind the B ring. However it does not traverse, but runs along behind it. Until Saturn will occult the star, the further progress differs regionally:

• In Northern Europe the disappearance behind Saturn is unobservable, because the star is still behind the B ring.
  
  
• Within an about 1200 km wide zone across Central Europe and South Asia, the star will re-enter the Cassini division before the occultation by Saturn begins. Along the north rim of this zone the reappearance from the B ring and the disappearance at Saturn’s rim will be momentary and therefore will give only a theoretical chance for observation. But near the southern rim the star will emerge for up to five minutes. Please do not take the edges of this zone too strictly. According to informations from Jet Propulsion Laboratory (JPL) the relative positions of the main bodies in the Saturn system including the rings are well-known, but not the absolute position of Saturn. There are uncertainties of 100 to 200 km! Thus the uncertainty of the zone plotted above is even more uncertain.
  
  
• South of this "Cassini Strip",  i.e. in the Mediterranean region and North Africa, the star has crossed the Cassini division for the second time and is already covered by the A-ring again. The disappearance at Saturn’s rim is thus as unobservable as well as in Northern Europe.
  
  
• In a strip across Central Africa and W-Australia the star completes the occultation sequence by the rings, before Saturn will graze it. In S-Africa and on Madagascar the star will even pass Saturn’s south pole without any occultation by the planet itself.

Goals of Observation

Bruno Sicardy, manager of the IR VIMS camera aboard the Cassini spacecraft, recorded many stellar occultations with this instrument and will continue these experiments. Due to the substantially more favorable signal-noise ratio from the space probe, he does not see an outstanding scientific interest in this event. Nevertheless it should be considered that the probe can always gain only one chord, while observations from different places on earth can supply several chords at the same time and therefore test the symmetry of ring patterns.

· As described above there are uncertainties in the absolute position of Saturn. By reduction of observations of the contact times of the rings from different locations, this position may be improvable. Since the Cassini probe is within the Saturn system, it cannot contribute to the solution of this problem.

· The Cassini division is not empty at all. High resolution images obtained by the Cassini spacecraft clearly show numerous so-called ringlets within the Cassini division: http://photojournal.jpl.nasa.gov/catalog/PIA06175
Video recordings from the earth possibly can detect such fine ring pattern and photometrically derive their optical density. Observations on several chords from different places on earth could test the circularity of these ringlets.

· The F ring, discovered by Pioneer 11 in 1978, is situated about 3400 km outside of the outer edge of ring A. Its main concentration is as narrow as about 50 km, but there is some matter in the entire range between A and F ring. The F ring is probably the most complex and most dynamic feature of the ring system. On one hand the shepherd satellites hold it together, on the other hand they disturbe, bend, and twist it. The remarkable density in the area around the main concentration implies a detectability by the occultation. See fig. 3a on
http://www.planetary.org/saturn/tpr_vol02no1_lane.html
The resolution of this diagram is well comparable with that of the upcoming occultation, because the effective diameter of the star at Saturn’s distance is in the order of one kilometer, too.  The F ring traverses the star three minutes before the beginning of the occultation the A ring.  The passage through the densest part only takes a few seconds.

· In 1987 Bruno Sicardy (Observatoire de Paris) et al. published a paper on „Stellar occultations by small bodies - Diffraction effects“, i. e. rings,  in Astronomical Journal:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1987AJ.....93.1549R
According to this publication diffraction fringes may occur. Depending on the star diameter and the size of the particles involved, the width of the fringes may be several kilometers. As Raymond Dusser expessed it: “According to the size of bodies in various rings, the occultation could be somewhat strange.”
Of course, to record quick variations of the star’s brightness, sufficient time resolution is necessary (for details about frame rates, see next chapter).

· The star will reappear near Saturn’s south pole. Recording this reappearance with a sufficient frame rate is interesting, because this atmospheric area is less intensely explored than at other latitudes.

Remarks:

No satellite of Saturn will occult the star.

The passage of the star trough the G ring takes place about 24 minutes before the entrance into the A ring, but there is no chance to detect it with an occultation. This applies all the more for the very extended, extremely feeble E ring.

Observation Requirements

The star does not really look spectacular near Saturn. The glare of Saturn and its rings will complicate observations with small telescopes. A minimum aperture cannot be specified because of the dependence on atmospheric conditions, filters, camera etc., but also because sufficient empirical values are not available. The author asks for feedback, which observations under which conditions were possible.  Comparisons between video and visual observations are interesting, too!

The brightness of the star is comparable with the brightness of a square arcsecond of the ring surface. The observability can be improved by filters: The ring material reflects the sunlight, but not unchanged in all spectral regions. Assuming an average color close to that of Saturn’s large atmosphereless moons, the values should be approximately B-V=+0.74 and U-B=+0.32, while the corresponding color indices of the star are +0.21 and +0.32, resp. Therefore a blue filter will enhance the color contrast. But dependening on the telescope or camera the over-all reduction of brightness may increase noise or worsen the visual recognizability of the star. Previous tests are recommended.

It is advantageous that the Saturn’s south pole area, where the star will finally emerge, is darker. The use of a methane band filter would be ideal. Jean Lecacheux (Observatoire de Paris) recently provided valuable detailed informations .

The photometric resolution is limited by the star’s diameter to approximately one kilometer. Saturn approaches its opposition moving retrograde with 12.3 seconds of arc per hour. The resulting relative velocity is 20 km/s. Depending on the angle of a passage 10 to 20 frames per second are required to reach maximum resolution. The star reappears near Saturn’s pole under a very narrow angle. Therefore about 5 frames per second are sufficient for full resolution, limited by the star diameter. At lower frame rates the resolution of fine ring pattern or diffraction fringes decreases, but nevertheless such observations are useful for the reduction of contact times at the ring edges. For example, at a rate of one frame per second the passage of the F ring would be detected on only two or three pictures.

A device, which inserts a the DCF or GPS time code into a video, would be ideal for timing. But in this case timing tolerance is larger than in other occultation events. If necessary, the minimum required accuracy may be met by recording a display of a DCF clock or a GPS receiver uninterruptedly before and after the event. Simple radio clocks may deviate because they do not receive the radio signal permanently. But a setup can be forced by removing the battery before. On the other hand GPS devices should receive the satellite signal for several minutes to make sure that time is displayed correctly. When using a webcam, an acoustic time signal may be recorded simultaneously. Important note: Time synchronity can not at all be guaranteed by transfering an acoustic signal via a different USB or sound port. Therefore a built-in microphone of the webcam must be used to transfer the acoustic signal via the same USB port. The alarm of a radio clock pulsating in 1-second intervals is usable for tape recording during visual observations as well.

For the video analysis of small changes in brightness suitable comparison stars are necessary. SAO 98052 (F2, V=9.4, B-V=+0.47) is located in a distance of 3.4’ at position angle 246°. Closer to Saturn several satellites are suitable for comparison, particularly Titan has nearly the same brightness as the target star. The following diagram provides an overview:

All significant calculations were performed with Dave Herald’s program winOCCULT 3.1.0, ephemeris base DE405.

Clear skies and successful observation of the unique event!

Alfons Gabel

 IOTA/ES

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