Origin of extended main-sequence turn-off in open cluster NGC 2355

Abstract

The presence of extended Main-Sequence Turn-Off (eMSTO) in the open clusters has been attributed to various factors, such as spread in rotation rates, binary stars, and dust-like extinction from stellar excretion discs. We present a comprehensive analysis of the eMSTO in the open cluster NGC 2355. Using spectra from the Gaia-European Southern Observatory (ESO) archives, we find that the stars in the red part of the eMSTO have a higher mean v sin i value of 135.3 $\pm$ 4.6 km s$^{-1}$ compared to the stars in the blue part that have an average v sin i equal to 81.3 $\pm$ 5.6 km s$^{-1}$. This suggests that the eMSTO in NGC 2355 is possibly caused by the spread in rotation rates of stars. We do not find any substantial evidence of the dust-like extinction from the eMSTO stars using ultraviolet data from the Swift survey. The estimated synchronization time for low-mass ratio close binaries in the blue part of the eMSTO suggests that they would be mostly slow-rotating if present. However, the stars in the blue part of the eMSTO are preferentially located in the outer region of the cluster indicating that they may lack low-mass ratio close binaries. The spread in rotation rates of eMSTO stars in NGC 2355 is most likely caused by the star-disc interaction mechanism. The stars in the lower main sequence beyond the eMSTO region of NGC 2355 are slow rotating (mean v sin i = 26.5 $\pm$ 1.3 km s$^{-1}$) possibly due to the magnetic braking of their rotations.