THE EFFECTS OF ROTATION ON THE MAIN-SEQUENCE TURNOFF OF INTERMEDIATE-AGE MASSIVE STAR CLUSTERS

Abstract

The double or extended main-sequence turnoffs (MSTOs) in the color-magnitude\ndiagram (CMD) of intermediate-age massive star clusters in the Large Magellanic\nCloud are generally interpreted as age spreads of a few hundred Myr. However,\nsuch age spreads do not exist in younger clusters (i.e., 40-300 Myr), which\nchallenges this interpretation. The effects of rotation on the MSTOs of star\nclusters have been studied in previous works, but the results obtained are\nconflicting. Compared with previous works, we consider the effects of rotation\non the MS lifetime of stars. Our calculations show that rotating models have a\nfainter and redder MSTO with respect to non-rotating counterparts with ages\nbetween about 0.8 and 2.2 Gyr, but have a brighter and bluer MSTO when age is\nlarger than 2.4 Gyr. The spread of the MSTO caused by a typical rotation rate\nis equivalent to the effect of an age spread of about 200 Myr. Rotation could\nlead to the double or extended MSTOs in the CMD of the star clusters with ages\nbetween about 0.8 and 2.2 Gyr. However, the extension is not significant; and\nit does not even exist in younger clusters. If the efficiency of the mixing\nwere high enough, the effects of the mixing would counteract the effect of the\ncentrifugal support in the late stage of evolution; and the rotationally\ninduced extension would disappear in the old intermediate-age star clusters;\nbut younger clusters would have an extended MSTO. Moreover, the effects of\nrotation might aid in understanding the formation of some "multiple\npopulations" in globular clusters.\n