Fundamental strings and NS5-branes from unstable D-branes in supergravity

By using the non-supersymmetric p-brane solutions delocalized in arbitrary number of transverse directions in type II supergravities, we show how they can be regarded as interpolating solutions between unstable Dp-branes (a non-BPS D-brane or a pair of coincident D-brane-anti-D-brane) and fundamental strings and also between unstable Dp-branes and NS5-branes. We also show that some of these solutions can be regarded as interpolating solutions between NS5/NS¯5 and Dp-branes (for p⩽5). This gives a closed string description of the tachyon condensation and lends support to the conjecture that the open string theory on unstable D-branes at the tachyonic vacuum has soliton solutions describing not only the lower-dimensional BPS D-branes, but also the fundamental strings as well as the NS5-branes.