Image of the Week

Tales of two clusters retold by Gaia

   
  Image 1  

The celestial object NGC 2451 is a loose grouping of fairly bright stars in the southern Milky Way which was first described as a star cluster by John Herschel in 1835. In 1888, Johan Ludvig Emil Dreyer entered it in his famous sky inventory of star clusters and nebulae, the New General Catalogue (NGC), under the running number 2451.

Image 2: NGC 2451 in the southern Milky Way

Towards the end of the 20th century, doubts arose whether this group of stars really constitutes a cluster, i.e. a gravitationally bound collection of sibling stars of the same origin and age. These doubts were prompted by measurements of the brightness and colours of the putative members which did not quite match. In 1994, S. Röser and U. Bastian used their newly compiled Positions and Proper Motions Catalogue (PPM) to decide this question: for the PPM they had combined more than 100 years of position measurements to compute precise proper motions for almost half a million stars on the whole sky. As a little science application they checked the motions of the putative cluster members. As a cluster, those would have to move at the same speed and direction, to within the uncertainty of the measured proper motions. But they did not. Instead, in a diagram plotting the east-west motion versus the north-south motion the putative members were scattered all over the place instead of being concentrated in a tight group. The cluster was thus finally proven to be non-existent.

The plot which Röser and Bastian used to kill NGC 2451 is shown at the top left in Image 1. In its outskirts it shows some nearby stars having large apparent motions, and towards its centre the image shows an accumulation of farther-away stars which have small apparent motions - due to their large distances. Looking more closely at the image, one might imagine a barely discernible density enhancement to the top left of the central clumping. To guide the eye, it is marked by a circle in the middle panel of Image 1. Could this be a hitherto unrecognised star cluster happening to lie in the direction (and behind) the non-existent NGC 2451? This clearly cannot be decided from just the few scattered points in the diagram.

Röser and Bastian scanned through the astronomical literature to find precise colours and magnitudes of the stars belonging to the slight density enhancement. They plotted a colour-magnitude diagram (CMD) from the data that they found, and with just three exceptions the stars nicely fitted to the assumption that all of them were at the same distance of 220 parsec (700 lightyears). A real star cluster had apparently been discovered. But there were even more surprises to come: the CMD of all stars in the sky region of NGC 2451 indicated that there was another - completely unrelated - new star cluster far "behind" the one discovered from the motion diagram. It was estimated to be at about 400 parsec (1300 lightyears) from the Sun. 

The publication of this result prompted other astronomers to make additional and more precise magnitude and colour measurements in that area of the sky, and within just a few years the existence of both star clusters was general astronomical consensus. The names of NGC 2451A and NGC 2451B became customary for them.

Enter ESA's Gaia astrometry satellite, almost 20 years later. During the last few months the astronomers who are busy with the processing of the Gaia data have started to make first rough calibrations and solutions. Very preliminary trial positions, parallaxes and motions of 2 million stars were produced. Here, "trial" means computing test results, checking, improving, re-doing, and checking again. Star clusters can be helpful in this process. One of last July's trial solutions was used to produce a plot similar to the plot that Röser and Bastian produced back in 1994. It is shown (on a slightly different scale) in the bottom right panel of Image 1 where cluster NGC 2451A stands out prominently. So much more precise are the Gaia measurements of just the first 10 months of the mission compared to what more than 100 years of ground-based astronomy could achieve.

No CMD is needed anymore to confirm that NGC 2451A exists. But what about NGC 2451B, the second new cluster? Its apparent motion happens to lie right in the dense part of the galactic "background" clump in Image 1. It could thus not be discovered (nor confirmed) in the old 1994 motion data. It is also not visible in the Gaia panel in Image 1.

Enter Gaia's distance measurements. A look at the distribution of the Gaia distances (not shown) of all 1100 stars in the NGC 2451 area - among the 2 million in the whole trial solution - indicated two very slight bumps (maxima): one at about 190 parsec, and one at about 360 parsec. These agree with the supposed distances of the two clusters, within the uncertainties these distances have. Image 3 shows the same Gaia data as the right-hand panel of Image 1, but with some additional colour coding. Blue shows the stars with apparent motions consistent with membership in NGC 2451A. Magenta indicates the stars with Gaia parallaxes within the mentioned 360-parsec bump. Green are simply all other stars.

Image 3
What do the magenta-coloured points tell us? At first sight, these points appear to be very scattered. Of course there are stars unrelated to any cluster which just happen to be at about 1300 lightyears away from the Sun and in the right direction as seen from us. But note the strong concentration of such points at the left rim of the galactic "background" clump. It is too dense and distinct to be just a coincidence. Those are the members of NGC 2451B. They share a common motion, within the uncertainties of the present Gaia data. If you like, you can read the apparent motion of the cluster from the image.

But do not trust it yet! The data shown are very preliminary. Well calibrated - while still preliminary - Gaia motions and parallaxes of 2 million stars, including the ones shown here, are still to be produced and verified over the next few months. They will not become available before summer 2016.             

Credits: ESA/Gaia/DPAC/DPCE/DPCB/CU3/U. Bastian (ARI, Heidelberg)

Image 2: By Roberto Mura (Own work) [CC BY-SA 3.0], via Wikimedia Commons

[Published:18/12/2015]

 

Image of the Week Archive

2017
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.