Investigating Unusually Extended Main-Sequence Turnoff of Galactic Open Clusters

Khushboo Kunwar Rao^1*, Wen-Ping Chen¹

¹Institute of Astronomy, National Central University, No. 300, Zhongda Rd, Zhongli District, Taoyuan City, 320, Taiwan

* Presenter:Khushboo Kunwar Rao, email:khushboo@astro.ncu.edu.tw

Main sequence turnoffs in the color-magnitude diagrams of open and globular clusters alike are usually well-defined to render a determination of their ages. However, in intermediate-age open clusters within the Milky Way galaxy and the Magellanic Clouds, their main sequence turnoffs appear unusually broad, aka, 'extended main-sequence turnoff' (eMSTO), defying stellar evolution models of coeval single stars. Several studies have been conducted over the past decade to understand this unusual eMSTO. Among the proposed competing mechanisms to account for the eMSTO, stellar rotation seems to stand out. However, it does not completely explain the eMSTO phenomenon. Therefore, we carry out an extensive search of the eMSTO phenomenon in intermediate-age galactic open clusters. Using the ML-MOC algorithm on Gaia DR3 data, we identify members of 100 open clusters and visually examine their main sequence turnoff region. We further thoroughly analyse 13 of these clusters using projected rotational velocities from Gaia DR3 and data from the Gaia Spectroscopic Survey. Our analysis indicates that the eMSTO phenomenon begins to appear at approximately 0.1 Gyr and disappears around 2 Gyr, within the stellar mass range of 1.5 to 5 solar masses. Additionally, we find that eMSTO is more pronounced in massive star clusters.

Keywords: astrophysics, star clusters, stellar evolution, stellar rotation