Populations of rotating stars III. SYCLIST, the new Geneva Population Synthesis code

TL;DR

This paper introduces SYCLIST, a new Geneva-based tool for creating synthetic colour-magnitude diagrams that incorporate stellar rotation effects, aiding in more accurate cluster age and property determinations.

Contribution

The paper presents a novel, publicly accessible tool that models the impact of stellar rotation on population synthesis, including interpolation of stellar tracks and isochrones for various parameters.

Findings

Fast rotators are most prevalent just below the turnoff in young clusters.

Rotation increases the main sequence lifetime, affecting the distribution of stellar populations.

Using rotating models improves age estimates of star clusters.

Abstract

(abridged) We present a new tool for building synthetic colour-magnitude diagrams of coeval stellar populations. We study, from a theoretical point of view, the impact of axial rotation of stars on various observed properties of single-aged stellar populations: magnitude at the turnoff, photometric properties of evolved stars, surface velocities, surface abundances, and the impact of rotation on the age determination of clusters by an isochrone fitting. Stellar models for different initial masses, metallicities, and zero-age main sequence (ZAMS) rotational velocities are used for building interpolated stellar tracks, isochrones, and synthetic clusters for various ages and metallicities. The synthetic populations account for the effects of the initial distribution of the rotational velocities on the ZAMS, the impact of the inclination angle and the effects of gravity and limb darkening, unresolved binaries and photometric errors. Interpolated tracks, isochrones, and synthetic clusters can be computed through a public web interface. For clusters with a metallicity in the range [0.002, 0.014] and an age between 30 Myr and 1 Gyr, the fraction of fast rotators on the main sequence (MS) band is the largest just below the turnoff. This is a natural consequence of the increase in the MS lifetime due to rotation. The fraction of fast rotators one magnitude below the turnoff also increases with the age of the cluster between 30 Myr and 1 Gyr. There is an increase in the fraction of enriched stars when the metallicity decreases. We show that the use of isochrones computed from rotating stellar models with an initial rotation that is representative of the average initial rotation of the stars in clusters provides a reasonable estimate of the age, even though stars in a real cluster did not start their evolution with an identical initial rotation.