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]

Image 4 shows the comparison of the final position for the 22 June occultation event by Chariklo. When comparing the preliminary Gaia DR2 result with the results found from Gaia DR1, Gaia DR1 + Herald method and Gaia DR1 + UCAC5, we see that there is less than 10 milliarcsecond between the final position obtained from Gaia DR2 and from the one obtained using Gaia DR1 + Herald method or Gaia DR1 + UCAC5.
Figure 4: A comparison of the final position for the 22 June occultation event by Chariklo [Image credit: Josselin Demars].

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.

Video of the observation of the occultation by Chariklo on 22 June 2017. Credits: Mike Kretlow (IOTA-ES)

 

"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.

This animation (available through download) shows how the predicted occultation path for the 22 June occultation event by Chariklo moves dependent on which data was used. First a comparison is shown between the predictions based on Gaia DR1 (without proper motions taken into account) and Gaia DR2, then a comparison is shown for the predictions based on Gaia DR1 + Herald method and Gaia DR2. At last it is shown how the prediction path moves through the predictions using Gaia DR1, then Gaia DR1 + Herald method and at last Gaia DR2.

 

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

[Published: 07/07/2017]

 

Image of the Week Archive

2017
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
 
Please note: Entries from the period 2003-2010 are available in this PDF document.