Three stellar positions released to support unique occultation events

On 22 June and 23 July 2017 relatively brights stars will be occulted by the largest known centaur Chariklo. The object is unique due to the ring system around it. By observing the occultation, a better shape of Chariklo and the detailed structure of the rings can be obtained. Knowing these characteristics improves our understanding of the ring stability and formation.

Figure 1: Artist impression of a close-up of the rings around Chariklo
(Image credit: ESO/L. Calçada/M. Kornmesser/Nick Risinger (


Even more unique is the occultation of a star on 5 October 2017 by Triton, the largest moon of Neptune. Triton occultations of suitable stars are extremely rare and can be used to study its atmosphere. While Gaia DR1 positions for these stars are very accurate, there are no proper motions available. This leads to large uncertainties in planning for the ground-based occultation observing campaigns. In order to help preparations for these unique events, we make the preliminary astrometric solutions for these stars, prepared for Gaia DR2, public.

Figure 2: Image of Triton, Neptune's largest satellite, taken on 22 August 1989 by Voyager 2 
(Image credit: NASA/JPL)


In the table below Gaia presents the three stellar positions to the astronomy community, taken from preliminary Gaia DR2 data, for epoch 2015.5 in the ICRF reference frame.

Object to be observed Description Chariklo Chariklo Triton
Date of occulation event   22 June 2017 23 July 2017 5 October 2017
Gaia DR2 source ID   6760223758801661440 6737020112089260672 2610107911326516992
Epoch / Reference frame   2015.5 / ICRF 2015.5 / ICRF 2015.5 / ICRF
Right Ascension (RA) [degrees] \alpha 283.81521653659905 282.038419778835 343.5768010149869
RA uncertainty [mas] \sigma_{\alpha *} 0.0522238599934819 0.03963777263392541 0.026564074030629107
Declination (DEC) [degrees] \delta -31.522685159905006 -31.442346425165297 -8.002309358044462
DEC uncertainty [mas] \sigma_{\delta} 0.05225161762414635 0.03900893618378824 0.022142168821058843
Proper Motion in Right Ascension
(PM in RA) [mas/yr]
\mu_{\alpha *} 3.2099344713972338 4.026596025279473 27.513673140123508
Uncertainty of
PM in RA [mas/yr]
\sigma_{\mu\alpha *} 0.12086068017989549 0.07293067963207914 0.053522387226693335
Proper Motion in Declination
(PM in DEC) [mas/yr]
\mu_{\delta} -2.010735262831512 -6.219325688340016 -1.8210553401369338
Uncertainty of
PM in DEC [mas/yr]
\sigma_{\mu\delta} 0.10194645902097235 0.0655658281047265 0.045857682351872736
Parallax [mas] \varpi 0.1572409701013944 0.14967444548457415 1.8956730873515917
Parallax uncertainty [mas] \sigma_{\varpi} 0.06086506896517232 0.04228961058438762 0.028530840302218616
Table: Preliminary data from Gaia DR2 on three stars provided for occultation observations of Chariklo and Triton. (Credit: ESA/Gaia/DPAC, please follow the acknowledgment guidelines as given here and please also cite the Gaia mission paper)
Back in 2016 before Gaia's first data release a similar effort was done for the occultation observation of Pluto, a story that can be found here. The Gaia Data Release 2 (Gaia DR2) is planned for April 2018 and will provide the community with accurate positions, proper motions, and parallaxes for more than a billion stars leading to much better predictions for occultation paths on Earth. This will greatly enhance the chances of catching the occultation event during observation campaigns.

[published: 23/05/2017]