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  4. Disappearance of the extended main sequence turn-off in intermediate age clusters as a consequence of magnetic braking
 
research article

Disappearance of the extended main sequence turn-off in intermediate age clusters as a consequence of magnetic braking

Georgy, C.
•
Charbonnel, C.
•
Amard, L.
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January 29, 2019
Astronomy & Astrophysics

Context. Extended main sequence turn-offs are features commonly found in the colour-magnitude diagrams of young and intermediate age (less than about 2 Gyr) massive star clusters, where the main sequence turn-off is broader than can be explained by photometric uncertainties, crowding, or binarity. Rotation is suspected to be the cause of this feature, by accumulating fast rotating stars, strongly affected by gravity darkening and rotation-induced mixing, near the main sequence turn-off. This scenario successfully reproduces the tight relation between the age and the actual extent in luminosity of the extended main sequence turn-off of observed clusters.

Aims. Below a given mass (dependent on the metallicity), stars are e ffi ciently braked early on the main sequence due to the interaction of stellar winds and the surface magnetic field, making their tracks converge towards those of non-rotating tracks in the HertzsprungRussell diagram. When these stars are located at the turn-off of a cluster, their slow rotation causes the extended main sequence turn-off feature to disappear. We investigate the maximal mass for which this braking occurs at di ff erent metallicities, and determine the age above which no extended main sequence turn-off is expected in clusters.

Methods. We used two sets of stellar models (computed with two di ff erent stellar evolution codes: STAREVOL and the Geneva stellar evolution code) including the effects of rotation and magnetic braking, at three di ff erent metallicities. We implemented them in the S yclist toolbox to compute isochrones and then determined the extent of the extended main sequence turn-off at different ages.

Results. Our models predict that the extended main sequence turn-off phenomenon disappears at ages older than about 2 Gyr. There is a trend with the metallicity, the age at which the disappearance occurs becoming older at higher metallicity. These results are robust between the two codes used in this work, despite some di ff erences in the input physics and in particular in the detailed description of rotation-induced internal processes and of angular momentum extraction by stellar winds.

Conclusions. Comparing our results with clusters in the Large Magellanic Cloud and Galaxy shows a very good fit to the observations. This strengthens the rotation scenario to explain the cause of the extended main sequence turn-off phenomenon.

  • Details
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Type
research article
DOI
10.1051/0004-6361/201834505
Web of Science ID

WOS:000456970400002

Author(s)
Georgy, C.
Charbonnel, C.
Amard, L.
Bastian, N.
Ekstrom, S.
Lardo, C.  
Palacios, A.  
Eggenberger, P.
Cabrera-Ziri, I.
Gallet, F.
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Date Issued

2019-01-29

Publisher

EDP SCIENCES S A

Published in
Astronomy & Astrophysics
Volume

622

Start page

A66

Subjects

Astronomy & Astrophysics

•

stars: evolution

•

hertzsprung-russell and c-m diagrams

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stars: rotation

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galaxies: star clusters: general

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multiple stellar populations

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magellanic cloud clusters

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angular-momentum evolution

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rotating massive stars

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young cluster

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circulation

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models

Editorial or Peer reviewed

REVIEWED

Written at

EPFL

EPFL units
LASTRO  
LHE  
Available on Infoscience
June 18, 2019
Use this identifier to reference this record
https://infoscience.epfl.ch/handle/20.500.14299/158098
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