Exotic Nuclear Rod Formation Induced by Superfluid Vortices in Neutron Star Crusts

Abstract

The inner crust of a neutron star consists of a lattice of nuclei, relativisticdegenerate electrons, and a neutron superfluid. Since the star isrotating, there are a number of quantized vortices in the superfluid.Mochizuki & Izuyama pointed out that such vortex lines may induce nuclearmatter rods along the vortex cores. The exotic nuclear structure ispossible in a very limited region of the inner crust. However, the shortnuclear rod can completely pin the vortex line and can be an origin ofvortex accumulation. The accumulation goes on until the local Magnusforce among the accumulated vortices reaches a critical magnitudefor unpinning of the trapped vortices. The unpinning at this stage,which must be collective, leads to pulsar glitches. In this paper,we first reconsider the energetics of this vortex-induced nuclearmatter rod given by Mochizuki & Izuyama. We present a picture in whichthe nuclear rod is constructed by successive captures of certainneighboring nuclei into the vortex core, and subsequent fusion reactionsof the captured nuclei with those inside the vortex core. We foundthat the marginal region, where the induced nuclear rods are stable,is not trivial. As the next step, the possibility of nuclear rodformation should be warranted by dynamical analysis. The crucial partof the dynamics is the Coulomb potential barrier, which seems atfirst sight to prohibit fusion if one neglects the screening due toelectrons. Then we also report our theoretical estimate of this Coulombbarrier, i.e., the energy increase when a nucleus deviates form itsequilibrium site to the onset position of nuclear fusion. The screening ofnuclear charge by the background electrons is accurately taken intoconsideration. We found that the Coulomb barrier against the rodformation is of the order of 1 MeV throughout the marginal region. Theobtained barrier is only several times as large as the zero-pointenergy of the nuclei-forming crystalline lattice. This result suggeststhat pycnonuclear reactions for the rod formation are feasible.