Charge density of light exotic nuclei and $\rho NN$ tensor coupling

We use a relativistic mean field model to study the charge density distributions of exotic oxygen isotopes. Nonlinear isoscalar-isovector terms which are not constrained by the present data are considered and the $\rho NN$ tensor coupling that does not affect the symmetry energy in the mean field model is included. Strong correlations between the neutron radius of $^{208}$Pb and the charge radius of $^{12,13,23,24}$O are found. The $\rho NN$ tensor coupling explicitly enhances the radius correlations between $^{23,24}$O and $^{208}$Pb. This enhancement is due to the neutron occupation of the orbital $2s_{1/2}$. The charge radius is sensitive to the change of the density dependence of the symmetry energy as the isotope goes towards the proton drip line.