[BAA 00379] Unusual occultation by a Binary Asteroid visible from England and W

[Rick]

BAA electronic bulletin No. 00379 -- http://www.britastro.org/

The following alert by Andrew Elliott concerns an extremely rare event and I strongly recommend you try to observe it.  If clear, the shadow of two objects will cross England in quick succession - Please try and time the event and in particular be prepared for the star to 'disappear' a second time.

Good Luck,

Richard Miles

At around 2307 UT on Thursday night, 2009 February 19, the *binary* asteroid (90) Antiope is predicted to occult the 12th magnitude star TYC 0849-01304-1.  In the UK, this star will be at about 42º elevation in azimuth 142º (SE).  This is in Leo, about half way between Saturn and Regulus, or 0.6º east of 5.3 magnitude star 53 Leonis. 

The maximum duration of the occultation for observers in the actual shadow path will be ~5.5 seconds *for each body*, and the visual brightness will drop by 1.6 magnitudes.  The shadow tracks of the two components overlap and, according to the latest prediction, observers in the middle third of the combined track should see two separate occultations 5-7 seconds apart.  There will be no interference from the moon or twilight.

The current update (2009 Feb 06) shows the combined 143 km wide track crossing from east to west over south east England, the south Midlands, Wales, and central RoI, and crossing most major conurbations in the south east.  The formal probability of a positive observation is quite high being 71% for observers on the centre line reducing to 48% at the shadow edges.

In more detail, in GB, the main shadow crosses Greater London, the Home Counties, the south and central Midlands, and parts of Sussex, Hants, Wilts, Oxon, Gloucs, (former) Avon, and all of Wales except N Wales.  The uncertainty zone extends these areas to parts of East Anglia, the northern Midlands, Cheshire, the IoW, Dorset and Somerset. Click on the following link to see the precise tracks plotted on a Google map, and zoom in:-

http://www.poyntsource.com/New/Google/20090219_17281.HTM

The predicted centre line is shown in green, the shadow track edges in blue, and the 1 standard deviation uncertainty track in red.  (Ignore the movable grey lines.)

Good planetarium software will aid identification of the correct star. More detailed information, with links to a track map and star charts can be found here:-

http://asteroidoccultation.com/2009_02/0219_90_17281.htm

Another useful map of the UK showing both components and the area of uncertainty is on the following page (with other occultations):-

http://call4obs.iota-es.de/

Click on the small UK map for an enlarged version.  There is also a link to a Word document containing much more detailed information about the circumstances of the double occultation.

As with all predictions of this type, there is some uncertainty in the precise location of the track, although in this case the prediction is expected to be quite accurate.

For visual observers, the star should be just visible in 4" aperture telescopes, but to reliably observe the occultation(s) a larger aperture would be preferable.  Those with integrating CCD video cameras e.g. Mintron, Stellacam, Watec 120N, or integrating CCDs would probably need larger apertures, above 8", to achieve acceptable time resolution.  Sensitive CCD video cameras e.g. Watec 902H, would require larger apertures still, but are capable of producing the highest timing accuracy.  The star should appear brighter (Mr 11.0) in CCD cameras and the magnitude drop correspondingly greater (2.1).

Scientifically valuable information can be obtained from accurate timings of the disappearance *and* reappearance of occulted stars.  A single positive timing can yield a position relative to the star of several times higher precision than standard astrometry.  Combined timings from observers distributed across the track can yield the size and shape of the asteroid.  Even more valuable information can be obtained from this particular occultation to define the size, shape, separation and orientation of the binary components.

Observers wishing to make serious *visual* timings (to better than 1 second) will need a multi-lap stopwatch or a tape/digital voice recorder linked with an accurate UT time signal.  Those with video and CCD systems sensitive enough to record the events, will need some method of accurately time-stamping the events, or otherwise relating them to UT.  Integrating CCDs can be used in fast frame readout or 'drift scan' mode.

Observations of this event from suitably equipped observers have been requested by professional colleagues.  So serious timings of positive occultations would be welcomed by the undersigned, as would definite negative events from within or just outside the predicted shadow track. It is normal practice to observe or record for 2 minutes either side of the predicted occultation time above, although the formal time uncertainty is expected to be only a few seconds.  A specimen report form can be found here:-

http://astrosurf.com/eaon/Report%20form.htm

Please email any reports to me, or contact me if you need assistance with timing methods or completing the form.  The required accurate latitude and longitude (WGS84 datum) can be obtained from the Google Map above (at the highest zoom level on the 'Satellite' view), before or after the event.  Elevation above Mean Sea Level is also required, preferably from an OS map.

Good luck and clear skies!

Andrew Elliott

Assistant Director (Occultations), Asteroids and Remote Planets Section
Email:  ae [at] f2s [dot] com