Observational templates of star cluster disruption - The stellar group NGC 1901 in front of the Large Magellanic Cloud

Abstract

Context. Observations indicate that present-day star formation in the Milky Way disk takes place in stellar ensembles or clusters rather than in isolation. Bound, long-lived stellar groups are known as open clusters. They gradually lose stars and are severely disrupted in their final evolutionary stages, leaving an open cluster remnant made up of a few stars. Aims. In this paper, we study in detail the stellar content and kinematics of the poorly populated star cluster NGC 1901. This object appears projected against the Large Magellanic Cloud. The aim of the present work is to derive the current evolutionary status, binary fraction, age, and mass of this stellar group. These are fundamental quantities to compare with those from N-body models in order to study the most general topic of star cluster evolution and dissolution. Methods. The analysis is performed using wide-field photometry in the UBVI pass-band, proper motions from the UCAC. 2 catalog, and 3 epochs of high-resolution spectroscopy, as well as results from extensive N-body calculations. Results. The star group NGC 1901 is found to be an ensemble of solar metallicity stars, 400 +/- 100 Myr old, with a core radius of 0.23 pc, a tidal radius of 1.0 pc, and a location at 400 +/- 50 pc from the Sun. Out of 13 confirmed members, only 5 single stars have been found. Its estimated present-day binary fraction is at least 62%. The calculated heliocentric space motion of the cluster is not compatible with possible membership in the Hyades stream. Conclusions. Our results show that NGC 1901 is a clear prototype of an open cluster remnant characterized by a high value of the binary fraction and a significant depletion of low-mass stars. In light of numerical simulations, this is compatible with NGC 1901 being what remains of a larger system initially made of 500 - 750 stars.