On 14 Dec 2023 at 10:17:12.943 UT, the 13.6-magnitude outer main belt asteroid 790 Pretoria was in the middle of covering up the 10.9-magnitude star TYC 1299-730-1 (UCAC4 532-018101) in the constellation Taurus, as seen from my vantage point on the west side of Tucson, Arizona.
I observed a 6.6-second occultation, but noticed a single-point event with the same magnitude drop as the main occultation, but 0.5 second earlier. A close up of that section of the light curve is shown below.
If 790 Pretoria has a moon that also occulted the star, then you would expect the magnitude drop when the star is covered up to be the same—the magnitude of the asteroid (and its satellite) alone. This is indeed the case here.
An analysis of all the data points prior to these two occultation events indicates that there is only a 1/130,000 chance that the single-point dip was caused by noise. The time-resolution of each data point is 0.03 seconds, so it seems likely that the moonlet occultation event was very close to 0.03 seconds in duration, as the data points on either side fall comfortably within the baseline.
Originally, there were four other observers signed up for this event (three in Arizona and two in Texas), but the other two Arizona observers were not able to observe and the two Texas observers were clouded out, leaving only me with my 8-inch Meade LX90 telescope on the back patio of my home to observe the event.
No moon of 790 Pretoria has yet been reported, and since no one else observed the 14 Dec 2023 event, my observation can only serve as a suggestion that 790 Pretoria might have a moon, but not proof.
Prior to my observation there were 14 stellar occultation events of 790 Pretoria observed, the first in 1998 and the last in 2021. We don’t yet have a shape model for 790 Pretoria.
Only three light curves from previous 790 Pretoria occultation events are available through the VizieR archive. These are listed and shown below.
Here is a list of all previous occultation chords for 790 Pretoria.
And details of the North American events, including the 14 Dec 2023 event I observed.
The 19 Jul 2009 event resulted in the discovery that TYC 2255-01354-1 is a double star.
Many thanks to Dave Gault in Australia, who determined that the purported moon is spurious, an unexpected artifact of the software I used to convert the star images in the .avi file into a light curve .csv.
Here is Dave Gault’s note to me:
I downloaded the .avi and put it through Tangra. Alas, the blink is not there.
See attached light curves with and without a comp. star.
Also attached is the Tangra.lc file.
I can only conclude that PyMovie had a tracking “moment”,
Regards
Dave Gault
Kuriwa Observatory
And my response back to him:
Thanks, Dave. I guess this just goes to show the importance of looking at unusual events with more than one software. Do you know of any other instances similar to this where Tangra and PyMovie disagree? In the back of my mind when I originally reported the “blink” I did wonder why it was only one point and no evidence of a partial drop in the point on either side. Seemed awfully fortuitous that I would see a full mag-drop blink of exactly one point duration.
Dave Oesper
Here is the light curve of the occulted star Dave Gault got from my .avi file using Tangra software:
And here is that light curve superimposed upon the light curve of a nearby comparison star: