A rotational age for the open cluster NGC 2281

The possibility of using the fast rotator sequence for accurate cluster age determinations^⋆

¹ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: dario.fritzewski@kuleuven.be
³ Space Science Institute, 4750 Walnut St., Boulder, CO, 80301, USA
⁴ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Straße 24/25, 14476 Potsdam, Germany

Received: 6 February 2023
Accepted: 26 April 2023

Abstract

Context. Cool star rotation periods have become an important tool in determining ages of open clusters.

Aims. We aim to estimate the age of the open cluster NGC 2281 based on the rotational properties of its low-mass members. Previous age estimates for this open cluster range from 275 Myr to 630 Myr.

Methods. Based on an eight month-long photometric time series obtained at the 1.2 m robotic STELLA telescope in Tenerife, we measured rotation periods for 126 cool star members (70% of the observed members) of NGC 2281.

Results. The large set of rotation periods allows us to construct a rich colour-period diagram for NGC 2281 with very few outliers above the slow rotator sequence. We identify an evolved fast rotator sequence which can be used to accurately age date the open cluster relative to other open clusters. Comparisons with M 37 and M 48 show that all three open clusters are roughly coeval, and we estimate the age of NGC 2281 to be 435 ± 50 Myr. Through comparisons with the younger NGC 3532 and the older Praesepe, we determine the spin down rates of mid-K and early-M fast rotators to be significantly lower than for early-K stars. We suspect that the spin down of early-K fast rotators might be governed by an additional mass dependence.

Conclusions. Finally, we show the path towards an empirical description of the evolved fast rotator sequences in open clusters.