Do Neutron Stars have an Ocean Surface?

Abstract

THIS article draws attention to one of the more important unsolved problems concerning the qualitative nature of the outer layers of a neutron star. We refer to the likelihood of there being a vapour-liquid type phase transition, involving a major density discontinuity by a factor that could be as high as three or more, between the crust (in which starquakes are thought to take place) and the neutron superfluid below. The idea that such a phase discontinuity might exist is not new¹, but the question does not seem to have been taken seriously in recent years. This is due to the absence of discontinuities not only in the highly simplified but historically important equations of state of Harrison and Wheeler² and of Tsuruta and Cameron³ which played such an important role in discussions of neutron star structure in the period extending up to the mid sixties, but also in the much more sophisticated equations of state which have been published in recent years, such as those of Langer, Rosen, Cohen and Cameron⁴, Bethe, Borner and Sato⁵, and Wang, Rose and Schlenker⁶.