Low mass stars, brown dwarf candidates and the mass function of the young open cluster NGC 2547

Abstract

We present a catalogue of RIZ photometry over an area of 0.855 square degrees, centred on NGC 2547. The survey is substantially complete to limits of R_c=21.5, I_c=19.5, Z=19.5. We use the catalogue to define NGC 2547 candidates with masses of about 0.05-1.0 M_sun. After correcting for incompleteness and contamination by foreground field dwarfs, we investigate the mass function of the cluster, its binary content, and search for evidence of mass segregation among lower mass stars. There is evidence for mass segregation between high (>3 M_sun) and lower mass stars, but over the range 0.1<M<0.7 M_sun, the data are consistent with no further mass segregation. By fitting King profiles we conclude that at least 60 percent of the low-mass stellar population are contained in our survey. The cluster mass function is remarkably similar to the Pleiades for 0.075<M<0.7 M_sun. Because of its age (~30 Myr), we demonstrate that this mass function is robust to several systematic uncertainties likely to affect older and younger clusters and is therefore one of the best available estimates for the initial mass function in young disc populations. For 0.05<M<0.075 M_sun there is some evidence for a deficit of brown dwarfs in NGC 2547 compared with other clusters. This deficit may extend to lower masses or may only be a dip, perhaps caused by an imperfect understanding of the mass-magnitude relationship at temperatures of 2800K.Incompleteness in our survey, and the luminosity functions from which we estimate contamination, leave this question open. The binary fraction for systems with mass ratios greater than about 0.5 is 20-35 per cent for M-dwarfs in NGC 2547, quite consistent with that found in the field (abridged).