  Proton radii of $^{12-19}$C densities derived from first accurate charge
changing cross section measurements at 900$A$ MeV with a carbon target are
reported. A thick neutron surface evolves from $\sim$ 0.5 fm in $^{15}$C to
$\sim$ 1 fm in $^{19}$C. The halo radius in $^{19}$C is found to be 6.4$\pm$0.7
fm as large as $^{11}$Li. Ab initio calculations based on chiral
nucleon-nucleon and three-nucleon forces reproduce well the radii.

Nuclear Experiment;Nuclear Theory