On the photometric signature of fast rotators

Abstract

Rapidly rotating stars have been recently recognized as having a major role in the interpretation of colour-magnitude diagrams of young and intermediate-age star clusters in the Magellanic Clouds and in the Milky Way. In this work, we evaluate the distinctive spectra and distributions in colour-colour space that follow from the presence of a substantial range in effective temperatures across the surface of fast rotators. The calculations are inserted in a formalism similar to the one usually adopted for non-rotating stars, which allows us to derive tables of bolometric corrections as a function not only of a reference effective temperature, surface gravity and metallicity, but also of the rotational speed with respect to the break-up value, $\omega$, and the inclination angle, $i$. We find that only very fast rotators ($\omega >0.95$) observed nearly equator-on ($i>45^{\circ }$) present sizable deviations from the colour-colour relations of non-rotating stars. In light of these results, we discuss the photometry of the $\sim$ 200-Myr-old cluster NGC 1866 and its split main sequence, which has been attributed to the simultaneous presence of slow and fast rotators. The small dispersion of its stars in colour-colour diagrams allow us to conclude that fast rotators in this cluster either have rotational velocities $\omega <0.95$, or are all observed nearly pole-on. Such geometric colour-colour effects, although small, might be potentially detectable in the huge, high-quality photometric samples in the post-Gaia era, in addition to the evolutionary effects caused by rotation-induced mixing.

Accepted for publication in MNRAS, 10 pages, 7 figures

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• PARSEC V2.0: Stellar tracks and isochrones of low and intermediate mass stars with rotation
• YBC, a stellar bolometric corrections database with variable extinction coefficients: an application to PARSEC isochrones
• Multiple stellar populations in NGC 1866. New clues from Cepheids and Colour-Magnitude Diagram
• Rubin Observatory LSST Stars Milky Way and Local Volume Star Clusters Roadmap
• PARSEC V2.0: Rotating tracks and isochrones for seven addtional metallicities in the range Z=0.0001-0.03
• On the Precision of Full-Spectrum Fitting of Simple Stellar Populations. IV. A Systematic Comparison with Results from Colour-Magnitude Diagrams
• The Predicted Properties of Helium-Enriched Globular Cluster Progenitors at High Redshift