Tumbling hometown - An asteroid's lightcurve

tl;dr - Maximilian Komar, 'senior member' of our students astronomy group, determined the rotational period of the main belt asteroid '11538 Brunico'  during his internship at 'Planetarium Südtirol'. He worked on data collected in September 2020 by his teacher Christof Wiedemair in the school's observatory and after two weeks of analysis he nailed down the result: the distant rock spins with a period of 9.5 hours. Sadly, the next opportunity to monitor Bruneck in detail will only be in 2027, when the asteroid will again be close enough to bring it into reach of our telescope.

Bruneck is not only the most beautiful and picturesque town in the vicinity of the 'astrocusanus observatory' (It is the only town in the vicinity and the hometown of Astrocusanus 😉) but also the German name of the asteroid 11538 Brunico, named so in 2011 in recognition of our ongoing collaboration with Christiaan Sterken from Brussels University. 

After our leap in telescope diameter which happened almost two years ago, our desire for more research work and challenging astronomical projects grew. So we searched for fruitful topics where we could try out our new toy and learn some advanced skills. Unfortunately, the corona pandemic severely thwarted our zeal. However, finding out more about 11538 Brunico, was still top priority. Especially, the determination of the unknown rotational period was something we were sure we could do.

Maximilian Komar at work

To get decent observational data of an asteroid one should observe it around the time of opposition, while it is close to earth. So we looked up the upcoming oppositions of the 11538 Brunico on minorplanet.info and found that a great one was close at hand. Not just that, but the opposition on the 29 August 2020 was one of the five brightest between 1995 and 2050 and the closest approach would be only 1.0004 AU away from earth. If that is not 11538 Brunico screaming “Observe me”, what is it? 

So it was plain as a pikestaff that by the end of August we would try to get as much frames of our celestial counterpart as possible, in order to do photometry. The steady rotation of the asteroid changes its earth-facing cross section and so we expected its brightness to change periodically. This subtle effect was what we were after. By the end of the campaign we had managed to get 26.2 h worth of image data. The frames gathered during our last observation at the end of September were of poor quality. Alas, the asteroid was already too faint for reliable photometry.

Observation log

As a nasty consequence of the global pandemic the fun of observing was not shared between three or four observers, as usually. Instead, our teacher Christof Wiedemair had to spend long and lonely nights at the observatory where he used his routine to take frames of the best possible quality.

Asteroid Bruneck moving through the constellation Aquarius on the 5th September 2020.
Setup: 16 inch RC telescope, paired with a SBIG ST8-XME camera.

The data then lingered on our hard disk for several months until I planned to do a internship at the 'Planetarium Südtirol' in Gummer. The planetarium was not open for the public (due to the pandemic, again), so I was kindly allowed to continue the work on the Brunico project. 

At first I did some introductory reading about the so far unfamiliar subject of asteroid photometry. Through Luca Ciprari, my supervisor at the planetarium, I got in touch with Lorenzo Franco, one of Europe’s leading amateur astronomers in the field of asteroid photometry. During a very useful online-meeting with Mr. Franco he kindly warned me about the common and major pitfalls. A big thank you goes out to him for his time and his great help.

After a few days of preparation I felt ready to begin the endeavor. This was not only the first time astrocusanus measured the amount of light coming from an asteroid but it was the first time we tried 'moving object photometry'. As expected, the data calibration itself wasn't a problem however, a first look at the light curves revealed some troubles. On several occasions the asteroid got in front of background stars which corrupted the photometry of those images. This effect is shown in the following animation: 

Fortunately, Mr. Franco had warned me that this would happen. After sorting out those troublesome frames and getting clean light curves I drew closer to the finish line. 

Light curves obtained on six nights of observations. 
Can you join the parts of the puzzle?

With the trial version of the period analysis tool Peranso I extracted the rotational period of 11538 Brunico. I got 9.4866 ± 0.0025 h which is only 0.12% off from the current literature value of 9.47538 ± 0.00005 h, published by the TESS survey group in March 2020, hence only a few months ahead of me. Confirming their findings is a valuable scientific contribution, therefore we are planing to publish our findings in a 'minor planet bulletin'.

Folded rotational light curve of asteroid 11538 Brunico.
The profile is a clear indication of the non-spherical shape of the body.

Now that is the preliminary end of the story. Unfortunately, 11538 Brunico is going to delve into the remote parts of the solar system for the years to come and will come into reach of our telescope only in 2027. That is a real pity since thorough observations during three or more different oppositions would give us the opportunity to determine the shape of the distant rock. Surely, it is as good looking as its terrestrial counterpart.

During our waiting time we plan to make use of our recently learned skills on other asteroids. One candidate eagerly volunteered, treading freely and easily through our field of view during the observation of the 14 September 2020. 

However, this bold fella probably knew he was well beyond our reach... For now. 😜

Maximilian Komar


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