Extended main sequence turn-off originating from a broad range of stellar rotational velocities

Abstract

Star clusters have long been considered to comprise a simple stellar population, but this paradigm is being challenged, since apart from multiple populations in Galactic globular clusters 1,2 , a number of intermediate-age star clusters exhibit a significant colour spread at the main sequence turn-off (MSTO) 3,4,5,6,7,8,9,10,11 . A sequential evolution of multiple generations of stars formed over 100-200 million years is a natural explanation of this colour spread 12 . Another approach to explain this feature is to introduce the effect of stellar rotation 13 . However, its effectiveness has not yet been proven due to the lack of direct measurements of rotational velocities. Here we report the distribution of projected rotational velocities (V sin i) of stars in the Galactic open cluster M11, measured through a Fourier transform analysis. Cluster members display a broad V sin i distribution, and fast rotators including Be stars have redder colours than slow rotators. Monte Carlo simulations infer that cluster members have highly aligned spin axes and a broad distribution of equatorial velocities biased towards high velocities. Our findings demonstrate how stellar rotation affects the colours of cluster members, suggesting that the colour spread observed in populous clusters can be understood in the context of stellar evolution even without introducing multiple stellar populations.

Tidal-locking-induced Stellar Rotation Dichotomy in the Open Cluster NGC 2287?

“…The distribution of projected rotational velocities is an effective tool to constrain the underlying distribution of v eq and i (Lim et al 2019). Here, we consider three cases.…”

“…In the simulation, the maximum v eq was fixed at 414 km s −1 , corresponding to the maximum v crit in our synthetic clusters (t = 150 Myr). A stochastic orientation in three-dimensional space can be represented by a uniform distribution in cos i (Lim et al 2019). Case 2 has two free parameters to describe its Gaussian profile-the peak inclination angle i peak (5 • to 90 • in steps of 5 • ) and the dispersion σ i (1 • to 46 • in steps of 5 • ).…”

UBVI CCD Photometry of Berkeley 55 Open Cluster

“…The E12 isochrone (high rotation) fittings to The spread of the brightness of the RSGs/RBGs in the HRD/CMDs indicates a much large spread in age. This feature may be the result of the diversity of stellar rotation among evolved cluster members (Lim et al , 2019;Lim et al, 2016;Sung et al, 1997), which indicate the necessity of cluster isochrone with nonsingle rotational velocity distribution. The stars in OCs do not have the same rotational velocity (Lim et al , 2019).…”

“…This feature may be the result of the diversity of stellar rotation among evolved cluster members (Lim et al , 2019;Lim et al, 2016;Sung et al, 1997), which indicate the necessity of cluster isochrone with nonsingle rotational velocity distribution. The stars in OCs do not have the same rotational velocity (Lim et al , 2019). Some stars have very low rotational velocities, and some may be fast rotators.…”

“…Note that Be 55 has a extended main sequence. Due to the diversity of the rotational velocity even in a cluster (Lim et al , 2019), MS turn-off broadens. Broadening in the main sequence of the CMD could be also due to binarity, peculiarity, rotation in the stars.…”