Nucleation ofp-branes and fundamental strings

We construct a solution to the low-energy string equations of motion in five dimensions that describes a circular loop of fundamental string exponentially expanding in a background electric H field. Euclideanizing this gives an instanton for the creation of a loop of fundamental string in a background H field, and we calculate the rate of nucleation. Solutions describing magnetically charged strings and p-branes, where the gauge field comes from Kaluza-Klein reduction on a circle, are also constructed. It is known that a magnetic flux tube in four (reduced) spacetime dimensions is unstable to the pair creation of Kaluza-Klein monopoles. We show that in (4+p) dimensions, magnetic (p+1) ``flux-branes'' are unstable to the nucleation of a magnetically charged spherical p-brane. In ten dimensions the instanton describes the nucleation of a Ramond-Ramond magnetically charged six-brane in type IIA string theory. We also find static solutions describing spherical charged p-branes or fundamental strings held in unstable equilibrium in appropriate background fields. Instabilities of intersecting magnetic flux-branes are also discussed. {} 1996 The American Physical Society.