Brown dwarfs and very low mass stars in the Praesepe open cluster: a dynamically unevolved mass function?

Context. Determination of the mass functions of open clusters of different ages allows us to infer the efficiency with which brown dwarfs are evaporated from clusters to populate the field. Aims. In this paper we present the results of a photometric survey to identify low mass and brown dwarf members of the old open cluster Praesepe (age 590(-120)(+150) Myr, distance 190(-5.8)(+6.0) pc) from which we estimate its mass function and compare this with that of other clusters. Methods. We performed an optical (I-c-band) and near-infrared (J and K-s-band) photometric survey of Praesepe covering 3.1 deg(2). With 5 sigma detection limits of I-c = 23.4 and J = 20.0, our survey is predicted to be sensitive to objects with masses from 0.6 to 0.05 M-circle dot. Results. We photometrically identify 123 cluster member candidates based on dust- free atmospheric models and 27 candidates based on dusty atmospheric models. The mass function rises from 0.6 M-circle dot down to 0.1 M-circle dot (a power law fit of the mass function gives alpha = 1.8 +/- 0.1; xi(M)alpha M-alpha), and then turns over at similar to 0.1 M-circle dot. This rise agrees with the mass function inferred by previous studies, including a survey based on proper motion and photometry. In contrast, the mass function differs significantly from that measured for the Hyades, an open cluster with a similar age (tau similar to 600 Myr). Possible reasons are that the clusters did not have the same initial mass function, or that dynamical evolution (e. g. evaporation of low mass members) has proceeded differently in the two clusters. Although different binary fractions could cause the observed (i.e. system) mass functions to differ, there is no evidence for differing binary fractions from measurements published in the literature. Of our cluster candidates, six have masses predicted to be equal to or below the stellar/substellar boundary at 0.072 M-circle dot.