Image of the Week

Triton observation campaign



Figure 1: Left - The reconstructed occultation geometry with the centrality line in red, compared to the latest prediction based on the preliminary DR2 star field prior to the event in pink. The two lines are separated by about 2 milli-arcsec when projected in the sky plane. The blue and pale-blue lines at the bottom are the respective limits (reconstructed and predicted) of the occultation shadow, below which no occultation was visible. The yellow pins mark the location of the stations that sent reports, with almost 80 of them actually recording the event. (Image credit: Google, INEGI, ORION-ME. Image annotation: ERC Lucky Star Project). Right - The central flash observed at Constancia (Portugal, courtesy Rui Gonçalves) and the C2PU 1.04m telescope of the Observatoire de la Côte d'Azur in south-east of France (Image copyright C2PU-OCA, 2017).

The stellar occultation by Triton, the largest of Neptune's natural satellites, on 5 October 2017 was a rare occasion. Triton passed in front of a star of magnitude 12.6 allowing to observe the phenomenon called occultation. To help out the observation campaign of this rare and special phenomenon, Gaia released on 23 May 2017 the preliminary Gaia DR2 position of the star to be occulted by Triton.

On 5 October Triton’s shadow first swept Europe and North Africa, and a few minutes later, the US East Coast (see Figures 1 and 2). On all three continents, a total of more than one hundred stations were ready to record the event, which had a maximum duration of three minutes.

One of the main scientific goals of these observations was to derive Triton’s atmospheric structure from a few kilometres above the surface up to altitudes of about 150 km. Comparison with results obtained during the NASA Voyager 2 flyby in 1989 was of paramount importance, as Triton suffered an “extreme solstice” around 2000, when the Sun illuminated Triton’s southern polar cap. Seasonal effects may then have triggered a drastic surge of pressure that could still be going on – or not – until today.

During the occultation, the stellar rays are refracted, and thus deviated by Triton's atmosphere. The latter thus acts as a lens that focuses the signal to the shadow center, creating a flash. This flash, called the central flash, can however only be detected in a small band bracketing the centrality. With a width of about 100 km, this band subtends about 5 milli-arcseconds (mas) when projected at Triton’s distance. Needless to say, planning the observation of the flash thus requires extremely accurate predictions that are now becoming possible thanks to the upcoming Gaia DR2 catalogue...


Figure 2: Predicting the Triton occultation of 5 October 2017, using the preliminary star position of Gaia DR2 and the JPL DE435/NEP081 ephemeris for the Triton's position in the sky (image credit: ERC Lucky Star Project, Bruno Sicardy).

At that level of accuracy, most of the uncertainty comes from the precision of Triton's position in the sky. Triton's ephemeris was computed using the JPL ephemeris DE435 for Neptune and NEP081 for Triton’s motion relative to Neptune. This ephemeris has not been not updated since 2009, and thus required updates for better predictions.

Improving Triton's position

With this in mind, a large campaign of observations was performed at the Observatório do Pico dos Dias (Brazil) by Marcelo Assafin and collaborators. Gathering data during eight nights from 15 to 23 September 2017, they obtained astrometric observations of Triton as it completed about one orbit around Neptune. These observations were reduced against the Gaia DR1 catalogue and used to determine an offset between the ephemeris and the observations. This offset was then used to refine Triton’s position at the time of the occultation.

At his point (end of September), the Gaia team released the positions of 453 stars, providing their positions taken from the preliminary Gaia DR2 catalogue, and covering the path of Triton on the celestial sphere during September 2017 (see Gaia News of 30 September and 2 October 2017). Using this improved catalogue, M. Assafin and colleagues were able to further refine Triton’s offset, with estimated accuracies of 5.4 milli-arcsecond in right ascension and 2.6 milli-arcsecond in declination, respectively.

Combined with the accuracy on the star position, this resulted in a final prediction with typical crosstrack uncertainty of 3 milli-arcsecond (corresponding to 60 km on Earth) and 8 seconds in time. This is an important result per se: even if the Gaia observations of the occulting body (here Triton) are not yet available in Gaia DR2, retrieving its accurate position at a few milli-arcsecond accuracy level is possible using classical CCD astrometry, backed up by the Gaia DR2 stellar catalogue. It is worth mentioning that Triton has been observed 22 times during the period planned to be included in Gaia DR3 and the anticipation is to achieve sub-milliarcsec position determination. This is good omen for future occultations, and an improvement of one order of magnitude compared to the pre-Gaia era.

Figure 3: The same as Figure 2 but using the preliminary Gaia DR2 position of the occulted star, plus a Triton's offset derived for astrometric observations reduced against 453 preliminary DR2 star positions present in astrometric fields of view around Triton. This was the latest prediction made by the ERC Lucky Star Project, which was very close to what actually happened (see Figure 1 and Figure 4) (Image credit: ERC Lucky Star Project / Bruno Sicardy).

While the Chariklo occultations of June and July made use of preliminary Gaia DR2 positions of the occulted stars only (see Gaia News of 23 May 2017 and story on the follow-up), this was the first occultation that used a preliminary Gaia DR2 star field to improve the astrometry of the object itself before the event. Also used of course was the preliminary Gaia DR2 position of the star that was released on 23 May 2017 as well. This eventually led to one of the most successful stellar occultations ever observed.


The most observed occultation ever

The event was eventually attempted from more than one hundred sites in Europe, North Africa and eastern USA. Thanks to the accurate prediction and the good weather conditions generally prevailing over Europe, it was successfully monitored from more than 70 sites, 25 of them detecting the much wanted central flash.

A reconstruction of the occultation geometry, using the best observations, shows that the latest prediction was accurate to within 8 seconds in time and 2 milli-arcsecond in distance, in agreement with the latest prediction described above, see Figures 2 and 3 for comparison, as well as Figure 4.

Due to the large deployment of both professional and amateur stations, almost 80 positive records of the occultation (mostly in Europe) have been reported so far. Among them, about 25 detected an increase of light near the centre of the event. Some of them are marked in the left panel of Figure 1 and examples of central flashes are shown in the right panel.

The shape of the flash is extremely sensitive to the global shape of the atmosphere that surrounds Triton at an altitude of about ten kilometers. Any large-scale distortions by a few kilometers would result in cusps in the central flash structure that are not observed, indicating an atmosphere close to spherical. However, some low-scale features observed in some flashes (see Figure 1) do suggest the presence of irregularities that will be quantified after comparing flashes observed at various distances to centrality.

In the same vein, a comparison between the amplitudes of the flashes observed in various wavelengths will be important to assess the presence of hazes near Triton’s surface. Depending on the size of aerosol particles, the absorption of a stellar ray that traverses the atmosphere may depend on wavelength, thus informing us about the distribution size of those putative aerosols.

This large success is comforting, as stellar occultations by Triton are rare (Neptune’s system is currently moving in low density stellar fields). The next opportunity is still far away into the future, occurring on 6 October 2022. It involves a G=11 star and will be visible from India, China and Japan, among others.

Figure 4: Post occultation path of Triton's occultation using preliminary reduction of some of the observations, see Figure 3 for comparison with the latest update made before the event (Image credit: ERC Lucky Star Project / Bruno Sicardy).


Credits: ESA/Gaia/DPAC, Bruno Sicardy, ERC Lucky Star Project, C2PU-OCA, Rui Gonçalves

[Published: 27/02/2018]


Image of the Week Archive

16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
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
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
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
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
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
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
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
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
27/08: Quest for the Sun's siblings
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