Rotation periods for very low mass stars in Praesepe

TL;DR

This study measures rotation periods of very low mass stars in Praesepe to empirically constrain their magnetic wind-driven spindown, revealing a steep increase in spindown timescale towards lower masses and confirming a period-mass break at M~0.3-0.6 Msol.

Contribution

It provides the first large sample of VLM star rotation periods in Praesepe, constrains mass-dependent spindown timescales, and confirms a period-mass break at M~0.3-0.6 Msol.

Findings

Most VLM stars are fast rotators with periods <2.5 days.

Spindown timescale increases steeply towards lower masses.

Period-mass distribution shows a break at M~0.3-0.6 Msol.

Abstract

We investigate the rotation periods of fully convective very low mass stars (VLM, M<0.3 Msol), with the aim to derive empirical constraints for the spindown due to magnetically driven stellar winds. Our analysis is based on a new sample of rotation periods in the main-sequence cluster Praesepe (age 600 Myr). From photometric lightcurves obtained with the Isaac Newton Telescope, we measure rotation periods for 49 objects, among them 26 in the VLM domain. This enlarges the period sample in this mass and age regime by a factor of 6. Almost all VLM objects in our sample are fast rotators with periods <2.5 d, in contrast to the stars with M>0.6 Msol in this cluster which have periods of 7-14 d. Thus, we confirm that the period-mass distribution in Praesepe exhibits a radical break at M~0.3-0.6 Msol. Our data indicate a positive period-mass trend in the VLM regime, similar to younger clusters. In addition, the scatter of the periods increases with mass. For the M>0.3 Msol objects in our sample the period distribution is probably affected by binarity. By comparing the Praesepe periods with literature samples in the cluster NGC2516 (age ~150 Myr) we constrain the spindown in the VLM regime. An exponential rotational braking law P ~ exp(t/tau) with a mass-dependent tau is required to reproduce the data. The spindown timescale tau increases steeply towards lower masses; we derive tau~0.5 Gyr for 0.3 Msol and >1 Gyr for 0.1 Msol. These constraints are consistent with the current paradigm of the spindown due to wind braking. We discuss possible physical origins of this behaviour and prospects for future work.