We present the results of an optical multi-band survey for low-mass Pre-Main
Sequence (PMS) stars and young Brown Dwarfs (BDs) in the Chamaeleon II (Cha II)
dark cloud. This survey constitutes the complementary optical data to the c2d
Spitzer Legacy survey in Cha II.
Using the Wide-Field Imager (WFI) at the ESO 2.2m telescope, we surveyed a
sky area of about 1.75 square degrees in Cha II. The region was observed in the
Rc, Ic and z broad-bands, in H-alpha and in two medium-band filters centered at
856 and 914 nm. We select PMS star and young BD candidates using
colour-magnitude diagrams (CMDs) and theoretical isochrones reproduced ad-hoc
for the WFI at the ESO2.2m telescope system. The selection criteria are also
reinforced by using the previously known PMS stars in Cha II to define the PMS
locus on the CMDs and by investigating the infrared (IR) colours of the
candidates. By exploiting the WFI intermediate-band photometry we also estimate
the effective temperature and the level of H-alpha emission of the candidates.
Our survey, which is one of the largest and deepest optical surveys conducted
so far in Cha II, recovered the majority of the PMS stars and 10 member
candidates of the cloud from previous IR surveys. In addition, the survey
revealed 10 new potential members. From our photometric characterisation, we
estimate that some 50% of the 20 candidates will result in true Cha II members.
Based on our temperature estimates, we conclude that several of these objects
are expected to be sub-stellar and give a first estimate of the fraction of
sub-stellar objects.Comment: 26 pages, 18 figure