Wuming Yang, Shaolan Bi, Xiangcun Meng, Zhie Liu
The double or extended main-sequence turnoffs (MSTOs) in the color-magnitude diagram (CMD) of intermediate-age massive star clusters in the Large Magellanic Cloud are always interpreted as age spreads. However, it has been confirmed that the age spreads do not exist in young massive star clusters in the Large Magellanic Cloud, which is challenging the scenario of the age spreads. The result of effects of rotation on the MSTOs of star clusters is conflicting in previous works. Compared with previous works, we consider the effects of rotation on the MS lifetime and take a different efficiency of rotational mixing. Our calculations show that rotating models have a fainter and redder turnoff with respect to non-rotating counterparts with ages between about 0.8 and 2.2 Gyr, but they have a brighter and bluer turnoff when age is larger than 2.4 Gyr. The spread of the MSTO caused by a typical rotation rate is equivalent to the effect of an age spread of about 200 Myr in the CMD of intermediate-age star clusters. Rotation could lead to the double or extended MSTOs in the CMD of the star clusters with ages between about 0.8 and 2.2 Gyr. But the extension is not significant, even does not exist in young clusters. If the efficiency of the mixing were high enough, the effects of the mixing would counteract the effect of the centrifugal support in the late stage of evolutions, the rotationally induced extension would disappear in the old intermediate-age star clusters; and young clusters would have a blueward extended MSTO.
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http://arxiv.org/abs/1304.5865
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