Accretion-induced luminosity spreads in young clusters: evidence from stellar rotation

TL;DR

This study finds a correlation between stellar rotation rates and their position in the color-magnitude diagram in young clusters, suggesting that accretion history influences stellar rotation more than age, challenging existing models.

Contribution

It introduces the idea that accretion history, not age, primarily determines a star's position in the CMD and its rotation rate in young clusters.

Findings

Rapid rotators tend to be above the median pre-main sequence in CMD.

Position in the CMD correlates with accretion history rather than age.

The results challenge traditional angular momentum evolution models.

Abstract

We present an analysis of the rotation of young stars in the associations Cepheus OB3b, NGC 2264, NGC 2362 and the Orion Nebula Cluster (ONC). We discover a correlation between rotation rate and position in a colour-magnitude diagram (CMD) such that stars which lie above an empirically determined median pre-main sequence rotate more rapidly than stars which lie below this sequence. The same correlation is seen, with a high degree of statistical significance, in each association studied here. If position within the CMD is interpreted as being due to genuine age spreads within a cluster, then the stars above the median pre-main sequence would be the youngest stars. This would in turn imply that the most rapidly rotating stars in an association are the youngest, and hence those with the largest moments of inertia and highest likelihood of ongoing accretion. Such a result does not fit naturally into the existing picture of angular momentum evolution in young stars, where the stars are braked effectively by their accretion discs until the disc disperses. Instead, we argue that, for a given association of young stars, position within the CMD is not primarily a function of age, but of accretion history. We show that this hypothesis could explain the correlation we observe between rotation rate and position within the CMD.