Impurity effect of the Λ particle on the structure of 18F and Λ19F

We perform three-body model calculations for a sd-shell hypernucleus Λ19F (Λ17O+p+n) and its core nucleus 18F (16O+p+n), employing a density-dependent contact interaction between the valence proton and neutron. We find that the B(E2) value from the first excited state (with spin and parity of Iπ=3⁺) to the ground state (Iπ=1⁺) is slightly changed by the addition of a Λ particle, which exhibits the so called shrinkage effect of Λ particle. We also show that the excitation energy of the 3⁺ state is reduced in Λ19F compared to 18F, as is observed in a p-shell nucleus 6Li. We discuss the mechanism of this reduction of the excitation energy, pointing out that it is caused by a different mechanism from that in Λ7Li.