AAQ Members discover a double star

The occultation of TYC 1397-00920-1 by 271 Penthesilea was a well attended event with AAQ members Jonathan Bradshaw, Peter Anderson and Tim Napier-Munn looking after the North, and with John Broughton to the South with two stations planned.

The conditions were typically Qld Summer – variable to say the least and rather spoilt John's attempts the South. Fortune smiled on those North of the river. Tim, an experienced observer, but completely new to the occultation game and armed only with a stopwatch caught a 4.3 second duration only event. Peter unfortunately missed the beginning of the event but reported a good time for R and also noted a gradual reappearance. Jonathan managed to bag the whole thing at 4.56 seconds.

Jonathan was initially sceptical about any possible `step' event, but with some encouragement from John Talbot at the RASNZ, put a bit more effort into his analysis and managed to show quite clearly that there was a good case to be put for a double star discovery – a very close star though!

The good news is that the target star is well and truly on the lunar occultation path so that it is possible take some further measurements to confirm whether or not it is a double. The bad news is that the next occurence happens in 2018, by which time the near neighbour may well be tucked behind.

Good teamwork chaps!

During the wee small hours of Sunday morning, a certain occultation obsessive decided to be the first person ever to witness an event involving the 12.8 magnitude asteroid 32 Pomona. It was a pretty unlikely event – low probability and during a full moon. Fortified by red wine, he toughed it out and was rewarded by a very strange happening (insert joke here). http://www.youtube.com/watch?v=ZctDMdF52hU

Graze occultations happen every now and then and Jonathan got lucky to see this ‘double flasher’. The occultation community has got a little animated at the thought of this unusual object, and the following CBET (Central Bureau Electronic Telegram) is planned to go out to the pro observatories:

32 Pomona. David Herald (International Occultation Timing Association) reports that Jonathan Bradshaw (Samford, Queensland, Australia) recorded an unusual occultation by the asteroid 32 Pomona on 2008 August 16. The expected maximum duration of the occultation was 7.1 secs.

The video recording shows two separate occultations of equal depth each lasting 1.2 seconds, separated by 0.8 secs. Those durations convert to chord lengths at the asteroid of 15km, 10km, and 15km - for a total length of 40km. The IRAS diameter for Pomona is 86.3km A possible explanation is that the star (UCAC2 38196247) is a double star with a separation of about 0.013".

However the equal duration of the two occultations combined with the geometry of the occultation would require the PA of the components to very closely match the direction of motion of the asteroid. This, together with the equal depth of the occultations, indicates that the double star explanation is unlikely. The most likely explanation is that the asteroid is either binary (including a contact binary), or is a unitary asteroid with a significant concave region on its surface.

The video of this occultation can be viewed at: http://www.youtube.com/watch?v=ZctDMdF52hU

It turns out that 32 Pomona, even though it has been known about since 1854, has never been studied in any great detail. There are two identified sets of photometry of the object, the first in 1975: http://adsabs.harvard.edu/abs/1976A&A....53..115S , the second in 2001: http://www.rni.helsinki.fi/~mjk/IcarPIII.pdf, but neither came to any conclusion.