Exploring the Lower Mass Function in the young open Cluster IC 4665

Abstract

We present a study of the young (30-100Myr) open cluster IC 4665 with the aim to determine the shape of the mass function well into the brown dwarf regime. We photometrically select 691 low-mass stellar and 94 brown dwarf candidate members over an area of 3.82 square degrees centred on the cluster. K-band follow-up photometry and Two-Micron All-Sky Survey data allow a first filtering of contaminant objects from our catalogues. A second filtering is performed for the brightest stars using proper motion data provided by the Tycho-2 and UCAC2 public catalogues. Contamination by the field population for the lowest mass objects is estimated using same latitude control fields. We fit observed surface densities of various cluster populations with King profiles and find a consistent tidal radius of 1.0 degree. The presence of possible mass segregation is discussed. In most respects investigated, IC 4665 is similar to other young open clusters at this age: (1) a power law fit to the mass function between 1 and 0.04Msun results in best fit for a slope of -0.6; (2) a cusp in the mass function is noticed at about the substellar boundary with respect to the power law description, the interpretation of which is discussed; (3) a fraction between 10-19% for BDs with M > 0.03Msun to total members; (4) a best-fit lognormal function to the full mass distribution shows an average member mass of 0.32Msun, if IC 4665 has an age of 50Myr.