Detection of Kuiper Belt Objects By Stellar Occultation George Georgevits (UNSW) Will Saunders (AAO) Michael C.B. Ashley (UNSW)

Looking for Occultations by Small Distant Objects The choice of class of star to observe is IMPORTANT. O, B and A class stars of the appropriate magnitude can project a small KBO-size disk onto the Kuiper Belt plane at ~40AU. eg. 10 th mag O5 star projects a ~300m radius disk onto the KBO plane at 40AU, and proportionally smaller for closer-in distances.

What can be achieved? KBO radius 1.2m >~150m KBO 0.5m >~200m KBO

Why KBO occultations are hard: Small time durations!!! The above velocity value applies when observing close to quadrature. For opposition, differential velocity is ~24km/sec so occultation events are proportionally shorter, but also more frequent. So, for opposition, 1km radius KBO, 1km R *, t occ ~= 170mSec But we really need to sample MUCH faster than this in order to capture the event details and to help differentiate between possible KBO events and other things, such as events due to asteroids and also artifacts.

Fresnel diffraction - Simulations using a point source star At 450nm, 1 FSU = 1.0km, and at 650nm, 1 FSU = 1.2km

3d simulation plots IMPORTANT: Fresnel diffraction is wavelength dependent, so when observing with no filter, the fringes tend to blur. Also, irregular shape objects tend to break up the diffraction pattern, which makes it much more difficult to see in the presence of noise. Rotated 180 deg

University of NSW Automated Patrol Telescope (APT) (Michael Ashley) Results from a few nights of observing ~100 blue stars: A few 1 and 2 pixel events found – encouraging, but far from convincing! So, what to do ??

Observing using the AAO UKST and 6df

Choice of stars – limiting criteria

Next, select the best 150 candidates, and use 6df “postage stamp” software to fine tune the star selection process manually. This permits us to reject any candidates which may suffer from confusion etc.

Fibre pick and place robot – places 100 fibres

Close-up of 6df spectrograph fibre plate and buttons

6df field plate fibre placement map 2 degree radius

6df Spectrograph in straight thru mode

Typical KBO occultation light curve Challenge: How to detect (and extract such events from noisy data)?

Secs Note: timescale 0 to 10 seconds, backwards! Note: 10mSec/pixel = 100Hz sampling rate! Event edges are nicely resolved

Secs

More sample events:

Have I convinced you?

Well, to be honest, I wasn’t convinced either!

Target field (Aug17-24) Sanity Check Field (Aug25)

Seeing – Typically ~2 arcSec

ARTIFACTS 1. Single Pixel Events 2. Telescope Vibration (a blessing in disguise) 3. Correlated fibre events

OTHER POSSIBILITIES: 4. Asteroids? – no, too rare, and event timings very different. 5. Satellites & space junk? – again, wrong timings 6. Telescope tracking errors? – no, would result in correlated events 7. Light spillage due to fibre placement errors? – no, placement very accurate and plenty of clearance & should not be star type dependent 8. Gremlins in the electronics (see 15. below)? – no way! 9. Cosmic rays? – no, should give single pixel events 10. Very fast localized seeing variations? – blue and red stars statistics do not appear to support this, but need to check this further. Also, many stars are very close, and on nights with good seeing, what mechanism could cause consistent very localized, fast, violent changes in seeing? Seems highly improbable?

11. A fly inside the camera/optics? – no, too clean and too cold 12. High flying large owls? – no such thing! 13. Or perhaps microbats? – none seen, and should give correlated events most of the time – there are relatively very few correlated events 14. Dust particles in the camera? – the camera is in a clean room, so extremely unlikely, also wrong size, and doesn’t show up on flats! 15. Planes - Planes do not fly over Siding Spring in the middle of the night! 16. Something else out there? – What???*** IN SUMMARY: Its not haunted, and there is no magic! So … what could it be ???

The Future: We have a conceptual design for a 2,000 fibre purpose-built instrument using a fast video camera. We are presently seeking collaborators and funding to mount a full-scale survey of the Kuiper Belt using the 1.2m UK Schmidt telescope at Siding Spring Observatory. Estimated project cost $0.5 million Acknowledgements: I wish to thank the Anglo Australian Observatory for providing telescope time and support staff to enable me to run with this proof of concept set of observing runs.