January 5th and 6th, 2013: Capturing Varuna
The biggest known transneptunian objects (Wikipedia)
It has yet not been classified as a dwarf planet, but some reasearchers point out that it should be included into that category. To do so it is necessary to make telescopical observations of the TNO. The most important observations occur when the object crosses over a star reducing its brightness.
Some days ago we received a notification of Varuna would be eclipsing a magnitude 16 star near the Gemini constelation. The brightness of the star would be reduced more than 4 magnitudes and what it would be better, the occultation could have been visible from our latitude.
Inital prediction of the projected shadow of Varuna in the possible occultation. It passes quite near Andalucsia (our location). Courtesy of Jose Luis Ortíz (IAA)
We inmedialty thought that this event could be quite interesting and a wonderful ocassion to test our new Obsession Rover. To avoid last minute surprises we decided to go out on the night of January 5th, completely ignoring the Three Wise Man and to familiarise with the occultation star field and instrumentation. The night was quite productive but after several hours taking exposures batteries did not resist coldness and we run out of energy. Thus, we packet everything up and we returned home without finishing all the test we planned.
We decided that the next day we have to go out again to finish everything that was pending from this first night. We charged all batteries (and we got some spare extra ones) and early in the evening we set up the scope and began taking shots. This time the setup and aligning was much more easy (every time we are more experienced!).
As the occultation was two days away we decided to try something more difficult: to capture Varuna in a photograph. It is important to remark that Varuna is a magnitude 20 star, that is, it brights 400 million times less that Sirius (one of the most brightest stars in the sky). To capture Varune birghtness we needed long exposures. We decided to take shots of a 60 and 30 seconds long and afterwards integrate them. And finally we managed to get Varuna. It is a very faint and small dot in a small photograph... but it is visible:
Varuna (red) and the occultation candidate star on January 8th, 2013 (green)
The still shows many neighbouring stars including the star that would be eclipsed. In reality that star is a double one -it is not clearly seen as it is saturated due to its intrinsic brightness-. Moreover, in the image it is possible to appreciate a faint galaxy (center bottom).
To ensure that what we were seeing was Varuna we made a series of 6 shots with 30 minutes (aproximately) between takes to show that the TNO is moving respect to the background stars (and clearly in direction to the occultation star):
Animation in which the movement of Varuna with respect to the background stars in a 3 hours time lapse (if you do not see it properly you might want to adjust the brightness of your screen)
While we edited the images we noticed that in the star field was something else interesting: an object much faster than Varuna was moving. In fact it was so fast that it appeared as a faint line:
Searching in online databases we have confirmed that it is an asteroid from the main asteroid belt. Particularly it is the asteroid 176474 (2001 XX154).
Finally we have to mention that very accurate last minute measurements of the orbit of Varuna have discarded that the occultation might take place from our location. But we are sure there will be other ocassions. However we are happy with the experience of being able to spot a so tiny and dim object.
We want to thank Jose Luis Ortíz (del IAA) for the information about the possible occultation and for his support and encouragement to do this kind of observations.