IoW_20170707 - Gaia

Image of the Week

Chariklo stellar occultation follow-up

Figure 1: Newly predicted occultation path by Chariklo on 22 June 2017 based on the preliminary Gaia DR2 stellar position [Image credit: ERC Lucky Star project]

On 23 May 2017 the astrometry for three Gaia DR2 stars was released to support the unique occultation events of Chariklo and Triton. Triton is the largest moon of Neptune and occultation events for this body are rare, while Chariklo is the largest known centaur that happens to have a ring system around it. This makes it an object of special interest.

Following our news item, the predicted paths for the occultation events of Chariklo on 22 June and 23 July 2017 were updated. Also an update for the predicted path for the occultation by Triton on 5 October 2017 is now available. Currently the predicted occultation paths given by the ERC Lucky Star project are based on Gaia DR1 data. For some interesting occultations though several sources of proper motion are used to refine the star position and the prediction. Given that for these three occultation events now the preliminary Gaia DR2 star positions and proper motions are available, even more refined occultation paths were predicted.

For the June 22 occultation event by Chariklo, this means that the path moved 5 milliarcsecond to the North with respect to the path that was based on Gaia DR1 data. This change can be seen in the animation available for download below. For the other occultations the path does not change a lot but the timing is different (several seconds). Also the prediction uncertainty got better with the usage of the preliminary Gaia DR2 positions and proper motions. For the previous prediction of the 22 June event, the prediction uncertainty was still 15 to 20 milliarcsecond which relates to about 150 to 200 km on Earth. Now the total uncertainty (star + Chariklo) is about 8 milliarcsecond (representing 80 km on Earth).

A test was performed to compare the different sources of proper motions: UCAC4, UCAC5, HSOY and the Herald method (a method by Dave Herald which uses the difference between the Gaia DR1 positions and the respective UCAC4 positions to derive the proper motions). For this test the preliminary Gaia DR2 data were used as the nominal values. The results of this test can be found in Figure 2 and 3. Obviously, this test only shows the result for three stars so definitive conclusions cannot be drawn from it. It is quite informative though. In particular the Herald method and the UCAC5 method seem to be close to the preliminary Gaia DR2 data.

Figure 2: A demonstration of the differences in the proper motions in right ascension of the three stars discussed when comparing the preliminary Gaia DR2 data with the proper motions obtained with the Herald method, and from UCAC5, UCAC4 and HSOY. [Image credit: Josselin Desmars]

Figure 3: A demonstration of the differences in the proper motions in declination of the three stars discussed when comparing the preliminary Gaia DR2 data with the proper motions obtained with the Herald method, and from UCAC5, UCAC4 and HSOY. [Image credit: Josselin Desmars]

Campaigns for the observation of an occultation event usually involve months of preparation and the need to transport people and equipment to the right places. There is always an element of luck involved with the weather as well. So being at the most optimal spot for making the observations is very essential to the astronomers involved. However, without the dedicated astrometry (by many individual observers but also by many dedicated collaborations) and the precise orbit calculations (by Josselin Desmars, LESIA, Paris) these campaigns could not be performed.

For the 22 June occulation event, a campaign with five stations distributed approximately every 60 km from the North to the South was organised in Namibia. Due to the updated prediction path and due to perfect weather conditions, the stations could be placed very accurately to make the observations.

"Of course now it was exciting to see how good the prediction would be with complete (i.e. including proper motions) Gaia data." said Mike Kretlow.

For the first time ever 4 positive chords on Chariklo's body (meaning 8 points distributed along the limb of the body) and 5 chords on the rings (meaning 10 points along the ring orbit) were recorded. A video of a successful observation of the occultation by Chariklo on 22 June can be found below. These observations will greatly help when trying to solve the long-awaited scientific questions like "What is the shape of the central body?" since it is an important parameter for the ring dynamics and "Does the centre of the rings coincide with the centre of the body?" which might give information on the eccentricity of the rings.

"According to preliminary results, the precision of the prediction was about 30km in distance (representing about 3 mas) and 3 seconds in time (representing about 6 mas) which is the most accurate prediction for a TNO/Centaur occultation. Such a precision would have been impossible without the Gaia catalogue." said Josselin Desmars.

This occultation event also provides for a very accurate astrometric position of Chariklo at the date of the occultation, as it only depends on the precision of the position of the star occulted. Given that the star position is a preliminary DR2 position, the precision is about 0.2-0.3 mas. This helps to refine the orbit of Chariklo and thus also to compute the next predictions to an accuracy of a few mas.

The next occultation event by Chariklo with a predicted occultation path based on preliminary Gaia DR2 stellar position and motion will happen on 23 July 2017. More information on the many occultation events happening can be found from the Lucky Star website.

Credits: ESA/Gaia/DPAC, ERC Lucky Star Project, IOTA-ES, Josselin Desmars, Bruno Sicardy, Paolo Tanga, Diane Berard, Mike Kretlow

[Published: 07/07/2017]