Binary-driven stellar rotation evolution at the main-sequence turn-off in star clusters

Authors: Sun, Weijia; de Grijs, Richard; Deng, Licai; Albrow, Michael D.

The impact of stellar rotation on the morphology of star cluster colour–magnitude diagrams is widely acknowledged. However, the physics driving the distribution of the equatorial rotation velocities of main-sequence turn-off (MSTO) stars is as yet poorly understood. Using Gaia Data Release 2 photometry and new Southern African Large Telescope medium-resolution spectroscopy, we analyse the intermediate-age (∼ 1 Gyr-old) Galactic open clusters NGC 3960, NGC 6134 and IC 4756 and develop a novel method to derive their stellar rotation distributions based on SYCLIST stellar rotation models.

(left) CMD of our cluster member stars in Gaia passbands. The MSTO and MS samples are highlighted as solid dots. Grey dashed line: MSTO stars. Dark grey lines: Ridge lines used to calculate the populations’ pseudo-colours. (right) KDEs of the (top) MSTO and (bottom) MS samples. The peaks of the MS distributions have been aligned and shifted for clarity and comparison. Any misalignment is owing to minor differences in the clusters’ ages and metallicities.

Rotation rate probability distribution of the MSTO stars according to the best-fitting models. The arrows indicate the velocities adopted for selection of the slowly and rapidly rotating subsamples in each of our clusters, corresponding to the local minima of the (mostly) bimodal distributions.

Combined with literature data for the open clusters NGC 5822 and NGC 2818, we find a tight correlation between the number ratio of slow rotators and the clusters’ binary fractions. The blue-main-sequence stars in at least two of our clusters are more centrally concentrated than their red-main-sequence counterparts. The origin of the equatorial stellar rotation distribution and its evolution remains as yet unidentified. However, the observed correlation in our open cluster sample suggests a binary-driven formation mechanism.

Number fraction of slow rotators to the total number of MSTO stars in the best-fitting synthetic cluster versus their binary fractions.