Thermodynamic Volumes for AdS-Taub-NUT and AdS-Taub-Bolt

In theories of semi-classical quantum gravity where the cosmological constant is considered a thermodynamic variable, the gravitational mass of a black hole has been shown to correspond to the enthalpy of the thermodynamic system, rather than the energy. We propose that this should be extended to all spacetime solutions, and consider the meaning of this extension of gravitational thermodynamics for the Taub-NUT and Taub-Bolt geometries in four dimensional locally anti-de Sitter spacetime. We present formulae for their thermodynamic volumes. Surprisingly, Taub-NUT has negative volume, for which there is a natural dynamical explanation in terms of the process of formation of the spacetime. A special case corresponds to pure AdS_4 with an S^3 slicing. The same dynamical setting can explain the negative entropy known to exist for these solutions for a range of parameters.
Comment: 15 pages. Two embedded figures. One trumpet. (v3: Some sentences and layout of section 2 slightly adjusted for clarity; new section 2.3 added showing a naive integral derivation of the volume formula. Title changed to match journal version.)