Image of the Week

secrets of the "Wild duck cluster" revealed

 

Figure 1: Top: Colour-magnitude diagram of the young open cluster M11. Blue, green, and red dots represent slow (Vsini < 100 km/s), moderate (100 km/s <= Vsini < 200 km/s), and fast rotators (Vsini > 200 km/s), respectively. The solid line is the Geneva isochrone for 2.5 million-year-old stars with a relative rotation rate of 0.7. Middle: Correlation between colours and Vsini, indicating faster rotating stars have redder colours than slower rotating stars. Bottom: Distribution of observed and synthetic colours, demonstrating the good match between the two.

Stars are rarely born in isolation, and the formation of stellar clusters is still mostly to be understood. One of the remaining questions is whether star clusters comprise a single stellar population or result from multiple formation events. Indeed, while the former view was accepted in the past, the detection in the last decade of a colour spread at the main-sequence turn-off led researchers to envisage the presence of mutiple populations, formed over 100-200 million years.

This, however, poses a problem for open clusters: their total mass generally is insufficient to be able to sustain multiple formation events, though a colour spread has been reported for them too. A new analysis done by a team of Korean and Belgian astronomers of the M11 cluster, located at 2 kiloparsec and harbouring about 3000 stars, sheds new light on this issue.

First, the researchers assessed the membership of stars in the area using proper motions from Gaia data release 2. Then rotational velocities of cluster members were measured from high-resolution spectra taken at ESO in the framework of the GAIA-ESO public spectroscopic survey. Comparing them to the known colours of the stars already unveils a correlation between colour and rotation rates: the redder stars also are the fastest rotators.

In fact, rotational mixing inside the stars brings fresh hydrogen to the core, thereby extending the main sequence lifetime by several tens of percent, explaining the colour spread at turn-off. But the astronomers wanted to dig further to even better understand what is happening. While the measured rotational velocities are projected velocities (V sin(i), with i the inclination angle and V the true rotational velocity) along our line-of-sight, the important parameters are the actual rotation rates. To constrain these, several possible distributions of true (equatorial) rotational velocities and inclination angles were considered.

The best fit to the observations was found for stars with highly aligned spin axes and a broad distribution of equatorial velocities skewed towards high rotation rates. In other words: the spin axes of the stars in M11 are not randomly oriented. This is likely related to the formation process, and recent simulations have indeed suggested this can happen if the parent molecular cloud of the cluster has a strong rotation to begin with.

Finally, also simulations for a synthetic cluster were performed using this best-fit underlying rotation distribution, the gravity darkening, the differential reddening and the Geneva stellar evolutionary models. The colour-magnitude diagram of M11 could then be reproduced using a single stellar population. The presence of multiple stellar populations is thus not required to explain the colour spread at the turn-off.

The target of this study, M11, is also known as the "wild duck cluster" and was discovered in 1681. It is located at about 6,500 light years from us. Its name derives from the fact that the object occupies the 11th place in the catalogue compiled by Charles Messier. It can be easily observed with binoculars in the Scutum constellation.

More info also in the press release of the University of Liège, and in the press release by the University of Arizona.

The paper "Extended main sequence turn-off originating from a broad range of stellar rotational velocities" discussing this research was published on 5 November in Nature Astronomy.

Credits: ESA/Gaia/DPAC, Beomdu Lim, Gregor Rauw, Yaël Nazé, Hwankyung Sung, Narae Hwang, and Byeong-Gon Park

[Published: 06/11/2018]

Image of the Week Archive

2018
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
2017
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
04/10: 1:1 Gaia model arrives at ESAC
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
27/08: Quest for the Sun's siblings
 
Please note: Entries from the period 2003-2010 are available in this PDF document.