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arXiv · 1812.05544

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

C. Georgy·C. Charbonnel·L. Amard·N. Bastian·S. Ekström·C. Lardo·A. Palacios·P. Eggenberger·I. Cabrera-Ziri·F. Gallet·N. Lagarde
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Abstract

Extended MS 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 MS 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 MS turn-off. This scenario successfully reproduces the tight relation between the age and the actual extent in luminosity of the extended MS turn-off of observed clusters. Below a given mass (dependent on the metallicity), stars are efficiently braked early on the MS due to the interaction of stellar winds and the surface magnetic field, making their tracks converge towards those of non-rotating tracks in the HRD. When these stars are located at the turn-off of a cluster, their slow rotation causes the extended MS turn-off feature to disappear. We investigate the maximal mass for which this braking occurs at different metallicities, and determine the age above which no extended MS turn-off is expected in clusters. Our models predict that the extended MS 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 differences in the input physics and in particular in the detailed description of rotation-induced internal processes and of angular momentum extraction by stellar winds. Comparing our results with clusters in the LMC and Galaxy shows a very good fit to the observations. This strengthens the rotation scenario to explain the cause of the extended MS turn-off phenomenon.

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C. Georgy, C. Charbonnel, L. Amard, N. Bastian, S. Ekström, C. Lardo, A. Palacios, P. Eggenberger, I. Cabrera-Ziri, F. Gallet, N. Lagarde. 2018-12-19. Disappearance of the extended main sequence turn-off in intermediate age clusters as a consequence of magnetic braking. https://doi.org/10.1051/0004-6361%2F201834505

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