Rotating Stellar Models Can Account for the Extended Main Sequence Turnoffs in Intermediate Age Clusters

TL;DR

This study demonstrates that variable stellar rotation in a coeval population can explain the extended main sequence turnoffs observed in intermediate age clusters, challenging the age spread hypothesis.

Contribution

The paper introduces rotating stellar evolution models that reproduce extended turnoffs without requiring age spreads in clusters.

Findings

Extended turnoffs appear naturally at ages under 1 Gyr.

A single-age population can explain observed turnoff extents.

Turnoff color shows almost no correlation with stellar rotation velocity.

Abstract

We show that the extended main sequence turnoffs seen in intermediate age Large Magellanic Cloud (LMC) clusters, often attributed to age spreads of several hundred Myr, may be easily accounted for by variable stellar rotation in a coeval population. We compute synthetic photometry for grids of rotating stellar evolution models and interpolate them to produce isochrones at a variety of rotation rates and orientations. An extended main sequence turnoff naturally appears in color-magnitude diagrams at ages just under 1 Gyr, peaks in extent between ~1 and 1.5 Gyr, and gradually disappears by around 2 Gyr in age. We then fit our interpolated isochrones by eye to four LMC clusters with very extended main sequence turnoffs: NGC 1783, 1806, 1846, and 1987. In each case, stellar populations with a single age and metallicity can comfortably account for the observed extent of the turnoff region. The new stellar models predict almost no correlation of turnoff color with rotational vsini: the red edge of the turnoff is populated by a combination of slow rotators and edge-on rapid rotators.