Rotation periods for cool stars in the open cluster NGC 3532. The transition from fast to slow rotation

TL;DR

This study measures and analyzes rotation periods of stars in the 300-million-year-old open cluster NGC 3532 to understand the transition from fast to slow stellar rotation and evaluate angular momentum loss models.

Contribution

It provides a large dataset of 279 rotation periods for NGC 3532 stars and compares observations with models, highlighting shortcomings in current spin-down theories.

Findings

Identified a well-structured slow rotator sequence in NGC 3532.

Observed stars in transition from fast to slow rotation, indicating a shorter crossing time.

Found discrepancies between observed period distributions and existing angular momentum evolution models.

Abstract

A cluster intermediate in age between the Pleiades (150 Myr) and the Hyades (600 Myr) is needed to probe the rotational evolution, especially the transition between fast and slow rotation that occurs between the two ages. We study the rich 300 Myr-old open cluster NGC 3532 to provide constraints on angular momentum loss. Measuring the rotation periods builds on our prior work of providing spectroscopic membership information for the cluster, and it supports the chromospheric activity measurements of cluster stars that we provide in a companion paper. Using 42 d-long photometric time series observations, we measured rotation periods for members of NGC 3532 and compared them with the predictions of angular momentum evolution models. We directly measured 176 photometric rotation periods for the cluster members. An additional 113 photometric rotation periods were identified using activity information resulting in a total sample containing 279 rotation periods for FGKM stars in NGC 3532. The colour-period diagram constructed from this rich data set shows a well-populated and structured slow rotator sequence, and a fast rotator sequence evolved beyond zero-age main sequence age whose stars are in transition from fast to slow rotation. We also identify an extended slow rotator sequence, apparently the analogue of the one we previously identified in NGC 2516. We compare our period distribution to rotational isochrones in colour-period space and find that all considered models have certain shortcomings. Using more detailed spin-down models, we evolve the rotation periods of the younger NGC 2516 forward in time and find that the spindown of the models is too aggressive with respect to the slow rotators. In contrast, stars on the evolved fast rotator sequence are not spun down strongly enough by these models. Our observations suggest a shorter crossing time for the rotational gap. (Abridged)