The wild duck delivers its secrets


In Research

Star birth rarely occurs in isolation, hence galaxies abound in groups of stars (called stellar clusters). Astronomers still wonder how exactly such clusters are formed. A spread in star's colours was taken to indicate the presence of multiple generations of stars. By studying the wild duck cluster (also called "M11"), a team of Korean and Belgian astronomers now shows that a single generation is sufficient to explain observations. The colours actually come from a range in rotation speeds, that naturally occurs at birth. In addition, they found that the stars in the cluster have highly aligned spin axes, which constrain the formation processes.

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hen astronomers compare the luminosity of stars to their colours, they found a typical shape : first a diagonal (from faint red stars to bright blue stars), then a turn towards red colors indicating stars which are beginning to die. This turning point is very important as its position indicates the age of the cluster. But a strange observation was reported : the turning line is not narrow but it's spreading amongst colours. This was interpreted as the signature of multiple generations of stars, as each one would have its own turning point at a slightly different position, spreading the feature. But is it really correct? To assess it, a team of Korean and Belgian astronomers decided to closely study the "Wild duck cluster" (also called M11). They first selected stars truly belonging to the cluster using data from the European space mission GAIA. Then they investigate whether "normal" stellar properties could explain the strange spread.

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Luminosity vs colour in M11 (c) B. Lim – note how stars appear first along a nearly vertical diagonal (the main sequence) then they distribute to the right of the diagram. It is the turning point which has been studied here.

To this aim, they used spectra, which are the distribution of the stellar light with energy and displays the signatures of chemical elements. These signatures (the so-called "spectral lines") are narrow if the star slowly rotates or broad in case of fast rotation. Using data collected at ESO and MMT, the astronomers derived the rotation speeds of stars in M11. Comparing them to the colours already indicated that the fast rotators were appearing redder. But the astronomers wanted to go further. They simulated theoretical clusters similar to M11 using Geneva evolutionary models which take into account the effect of rotation. In this way, they were able to prove that the "colour spread" of M11 can be fully explained with a single generation of stars.

In the process, the astronomers also found that the spin axes of the stars in M11 were not random: they were actually aligned. This is related to their formation process : recent simulations have indeed suggested this can happen if the parent cloud has a strong rotation to begin with. The target of the study is the "wild duck cluster", discovered in 1681 and located at about 6500 light-years from us. It appears at the 11th place in the catalogue written by Charles Messier (hence
its name "M11"). With its ~3000 stars, it is the richest young cluster in the Messier catalogue. It can be easily observed with binoculars in the Scutum constellation.

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Youtube

Zooming in on the open cluster Messier 11

This video takes you on a journey to the open cluster Messier 11 as seen with the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope at the La Silla Observatory.

Scientific reference

Extended main sequence turn-off originating from a broad range of stellar rotational velocities, Nature Astronomy, 6 Novembre 2018.

Contact

The High-Energy Astrophysics Group (GAPHE)- UR STAR

Dr Beomdu Lim blim@uliege.be  

Yael Nazé - ynaze@uliege.be

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