Issue |
A&A
Volume 460, Number 3, December IV 2006
|
|
---|---|---|
Page(s) | 799 - 810 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20065909 | |
Published online | 26 September 2006 |
A search for substellar members in the Praesepe and
Orionis clusters
1
Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), PO 50727, 28080 Madrid, Spain e-mail: bmgg@laeff.inta.es;mosorio@iac.es
2
XMM-Newton Instrument Controller at ESAC, INSA, Madrid, Spain
3
Instituto de Astrofísica de Canarias (IAC), vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
4
Consejo Superior de Investigaciones Científicas (CSIC), Spain
Received:
26
June
2006
Accepted:
4
September
2006
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 and
mag,
which according to state-of-the-art evolutionary models
corresponds to masses of 50-55 MJup (Praesepe) and
6 MJup (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 MJup, rises smoothly with a
slope of
(dN/d
). 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 MJup) 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.
Key words: stars: late-type / stars: low-mass, brown dwarfs / stars: luminosity function, mass function / stars: pre-main sequence / open clusters and associations: individual: σ Orionis / open clusters and associations: individual: Praesepe
© ESO, 2006
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