A search for substellar members in the Praesepe and σ Orionis clusters

We have conducted deep photometric searches for substellar members of the Praesepe (0.5-1 Gyr) and sigma Orionis (3 Myr) star clusters using the Sloan i' and z' broad-band filters, with the 3.5-m and the 5-m Hale telescopes on the Calar Alto and Palomar Observatories. The total area surveyed was 1177 arcmin(2) and 1122 arcmin(2) towards the central regions of Praesepe and s Orionis, respectively. The 5-sigma detection limit of our survey is measured at i' = 24.5 and z' = 24 mag, which according to state-of-the-art evolutionary models corresponds to masses of 50-55 M-Jup (Praesepe) and 6 M-Jup (sigma Orionis), i.e., well within the substellar regime. Besides recovering previously known cluster members reported in the literature, we have identified new photometric candidates in both clusters whose masses expand the full range covered by our study. In s Orionis, follow-up near-infrared photometry has allowed us to confirm the likely cluster membership of three newly discovered planetary-mass objects. The substellar mass function of sigma Orionis, which is complete from the star-brown dwarf borderline down to 7 M-Jup, rises smoothly with a slope of alpha = 0.6(-0.1)(+0.5) (dN/dM similar to M-alpha). One of the faintest Praesepe candidates for which we have obtained follow-up near-infrared JHK(s) photometry closely fits the expected optical and infrared photometric sequence of the cluster. From its colors, we have estimated its spectral type to be between L4 and L6. If confirmed as a true Praesepe member, it would become the first L-type brown dwarf (50-60 M-Jup) identified in an intermediate-age star cluster. Our derivation of the Praesepe mass function, which is based on state-of-the-art evolutionary models, depends strongly on the cluster age. For the youngest possible ages (500-700 Myr), our results suggest that there is a deficit of Praesepe brown dwarfs in the central regions of the cluster, while the similarity between the Praesepe and sigma Orionis mass functions increases qualitatively for models older than 800 Myr.