Validity of approximative treatments of Δ-isobar degrees of freedom

A perturbative approach to Δ-isobar components of the three-nucleon ground-state wave function is developed. It is based on the transition potential connecting nucleon-nucleon partial waves and partial waves with isobars. The perturbative results with single Δ-isobar excitation are compared to those of an exact coupled-channel calculation and turn out to be in satisfactory agreement for many Δ-effects. Within the perturbative method the role of the ρ-exchange in the transition potential and the importance of double Δ-excitations in the three-nucleon ground state are discussed. Effective nuclear operators such as three-nucleon forces and exchange currents, when simulating Δ-propagation in static approximation, are found to grossly overestimate Δ-effects.