The magellanic bridge cluster NGC 796: deep optical AO imaging reveals the stellar content and initial mass function of a massive open cluster

Abstract

NGC 796 is a massive young cluster located 59 kpc from us in the diffuse intergalactic medium of the 1/5-1/10 Z(circle dot) Magellanic Bridge, allowing us to probe variations in star formation and stellar evolution processes as a function of metallicity in a resolved fashion, and providing a link between resolved studies of nearby solar-metallicity and unresolved distant metal-poor clusters located in high-redshift galaxies. In this paper, we present adaptive optics griH alpha imaging of NGC 796 (at 0''.5, which is similar to 0.14 pc at the cluster distance) along with optical spectroscopy of two bright members to quantify the cluster properties. Our aim is to explore whether star formation and stellar evolution vary as a function of metallicity by comparing the properties of NGC 796 to higher-metallicity clusters. We find an age of 20(-5)(+12) Myr from isochronal fitting of the cluster main sequence in the color-magnitude diagram. Based on the cluster luminosity function, we derive a top-heavy stellar initial mass function (IMF) with a slope alpha = 1.99 +/- 0.2, hinting at a metallicity and/or environmental dependence of the IMF, which may lead to a top-heavy IMF in the early universe. Study of the H alpha emission-line stars reveals that classical Be stars constitute a higher fraction of the total B-type stars when compared with similar clusters at greater metallicity, providing some support to the chemically homogeneous theory of stellar evolution. Overall, NGC 796 has a total estimated mass of 990 +/- 200 M-circle dot, and a core radius of 1.4 +/- 0.3 pc, which classifies it as a massive young open cluster, unique in the diffuse interstellar medium of the Magellanic Bridge.