Blue Straggler Stars in Old Open Clusters and the Kraft Break

TL;DR

This study measures the rotational velocities of blue straggler stars in old open clusters, revealing a Kraft break in their rotation distribution and insights into their binary evolution and magnetic activity.

Contribution

It provides the first detailed analysis of BSS rotation in open clusters, showing the influence of temperature and environment on their rotational behavior.

Findings

BSSs hotter than the Kraft break are rapidly rotating.

BSSs below the Kraft break have slow rotation rates.

Rotation distributions are similar across different cluster environments.

Abstract

We measure the projected rotational velocities ($v \sin i$) of the solar-like blue straggler stars (BSSs) in the old ($\geq4$ Gyr) open clusters M67, NGC 188, and NGC 6791. We find that the BSS rotation distribution shows a Kraft break similar to that found in the field. The main-sequence progenitors of these BSSs were cooler than the Kraft break and have spun down by their age. The binary interactions that create BSSs are expected to spin up these progenitors, so current BSS rotation rates are due to transformation and any subsequent spin-down. We observe that BSSs hotter than the Kraft break are rapidly rotating, showing that binary evolution spins up these, and likely all, BSSs to initial rotational periods below two days, still below critical velocity. BSSs below the Kraft break currently have slow rotation rates, and those within the Kraft break have a mixture of rotation rates suggesting rotational transition. This dependence of rotation on effective temperature indicates that BSS envelopes behave like those of single stars, becoming convective and generating magnetic fields at the same temperatures. For globular cluster BSSs with [Fe/H]$\sim-1.5$, we find evidence of a BSS rotation transition region that is 100-250 K hotter than at solar metallicity. We find the $v \sin i$ distributions of BSSs in open clusters have similar characteristics to both high- and low-density globular clusters, indicating the density of environment is not the only factor that can determine rotational distributions. We suggest that velocity dispersion plays an important role.