  We have investigated the halo formation in the neutron rich $\rm{Ca}$
isotopes within the framework of recently proposed relativistic mean-field plus
BCS (RMF+BCS) approach wherein the single particle continuum corresponding to
the RMF is replaced by a set of discrete positive energy states for the
calculation of pairing energy. For the neutron rich $\rm{Ca}$ isotopes in the
vicinity of neutron drip-line, it is found that further addition of neutrons
causes a rapid increase in the neutron rms radius with a very small increase in
the binding energy, indicating thereby the occurrence of halos. This is
essentially caused by the gradual filling in of the loosely bound $3s_{1/2}$
state. Interesting phenomenon of accommodating several additional neutrons with
almost negligible increase in binding energy is shown to be due to the pairing
correlations.
