Bibcode
González-García, B. M.; Zapatero Osorio, M. R.; Béjar, V. J. S.; Bihain, G.; Barrado Y Navascués, D.; Caballero, J. A.; Morales-Calderón, M.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 460, Issue 3, December IV 2006, pp.799-810
Fecha de publicación:
12
2006
Revista
Número de citas
44
Número de citas referidas
39
Descripción
We have conducted deep photometric searches for substellar members of
the Praesepe (0.5-1 Gyr) and σ 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 arcmin2 and 1122 arcmin2 towards
the central regions of Praesepe and σ Orionis, respectively. The
5-σ 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 (σ
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 σ 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 σ Orionis, which is complete from the
star-brown dwarf borderline down to 7 M_Jup, rises smoothly with a slope
of α = 0.6 +0.5-0.1 (dN/dM ˜
M-α). One of the faintest Praesepe candidates for which
we have obtained follow-up near-infrared JHKs 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 σ Orionis mass functions increases
qualitatively for models older than 800 Myr.