NUCLEATION OF P-BRANES AND FUNDAMENTAL STRINGS

We construct a solution to the low-energy string equations of motion i n five dimensions that describes a circular loop of fundamental string exponentially expanding in a background electric H field. Euclideaniz ing this gives an instanton for the creation of a loop of fundamental string in a background H field, and we calculate the rate of nucleatio n. Solutions describing magnetically charged strings and p-branes, whe re the gauge field comes from Kaluza-Klein reduction on a circle, are also constructed. It is known that a magnetic flux tube in four (reduc ed) spacetime dimensions is unstable to the pair creation of Kaluza-Kl ein monopoles. We show that in (4 + p) dimensions, magnetic (p + 1) '' flux-branes'' are unstable to the nucleation of a magnetically char ed spherical p-brane. In ten dimensions the instanton describes the nucl eation of a Ramond-Ramond magnetically charged six-brane in type IIA s tring theory. We also find static solutions describing spherical charg ed p-branes or fundamental strings held in unstable equilibrium in app ropriate background fields. Instabilities of intersecting magnetic flu x-branes are also discussed.