The mass function of the Coma Berenices open cluster below 0.2 M⊙: a search for low-mass stellar and substellar members

Context. Little is known about the population of the Coma Berenices open cluster (age similar to 500 Myr) below 0.2 M-circle dot, and statistics show that there is a prominent deficit of very low-mass objects in this mass range compared to younger open clusters with ages of <250 Myr. Aims. We search for very low-mass stars and substellar objects (brown dwarfs) in the Coma open cluster to derive the present-day cluster mass function below 0.2 M-circle dot. Methods. An imaging survey of the Coma open cluster in the R- and I-bands was carried out with the 2k x 2k CCD Schmidt camera at the 2-m telescope in Tautenburg. We performed a photometric selection of the cluster member candidates by combining results of our survey with 2MASS JHK(s) photometry. We also analysed low-resolution optical spectroscopic observations of a sample of 14 of our stellar cluster member candidates to estimate their spectral types and luminosity. Results. We present a photometric survey covering 22.5 deg(2) around the Coma open cluster centre with a completeness limit of 21.(m)5 in the R-band and 20.(m)5 in the I-band. Using optical/IR colour-magnitude diagrams, we identify 82 very low-mass cluster member candidates in the magnitude range 14.(m)7 < I < 20.(m)1. Five of them have luminosities and colour indices consistent with brown dwarfs. We calculate a mass spectrum of the very low-mass end of the cluster under the assumption that all selected candidates are probable cluster members. The calculated present-day mass function (dN/dm approximate to m(-alpha)) can be divided into two parts with a slope of alpha = 0.6 in the mass interval 0.2 > M-star > 0.14 M-circle dot and alpha similar to 0 in 0.14 > M-star > 0.06 M-circle dot. Conclusions. Our analysis of the spectroscopy of 14 objects shows that they are very low-mass MV-dwarfs of spectral type and luminosity consistent with Coma open cluster membership. This suggests that the membership probability of our other candidates may be high. Our results suggest that the mass function of the Coma open cluster can be traced towards substellar objects, but comparison with a mass spectra of younger clusters indicates that the Coma open cluster has probably lost its lowest mass members by means of dynamical evolution.