Characterization and photometric membership of the open cluster NGC 1981

Abstract

Open clusters belonging to star-forming complexes are the leftovers from the initial stellar generations. The study of these young systems provides constraints to models of star formation and evolution as well as to the properties of the Galactic disc. We aimed at investigating NGC 1981, a young open cluster in the Orion nebula region, using near-IR and BV(RI)C photometric data. We devised a method that accounts for the field contamination and allows to derive photometric membership for the cluster stars. A new cluster centre was determined by Gaussian fittings to the 2D stellar distribution on the sky, and has been used to obtain the radial stellar density profile and the structural parameters. Mass functions were computed for stars inside the cluster limiting radius and total mass estimated from them. Although more easily distinguished by its grouping of six relatively bright stars, an underlying population of faint pre-main-sequence stars is evident in the cluster area. We showed that this population is related to the cluster itself rather than to the nearby Orion nebula cluster. Additionally, a fraction of the cluster low mass stars may have been evaporated from the region in its early evolution leading to the present sparse, loose structure. The estimated parameters of NGC 1981 are core radius Rc= 0.09 ± 0.04 pc, limiting radius Rlim= 1.21 ± 0.11 pc, age t= 5 ± 1 Myr, distance modulus (m-M)0= 7.9 ± 0.1 (380 ± 17 pc), reddening E(B-V) = 0.07 ± 0.03 and total mass m= 137 ± 14 M⊙. © 2010 The Authors. Journal compilation © 2010 RAS.