The Formation of Shells in the Wind from P Cygni

Abstract

A mechanism is proposed for the formation of shells around P Cygni. These shells seem to arise at approximately two monthly intervals and each appears to undergo a steady acceleration of some 0.6 cm s^-2 during the six months that it is present. In the process to be described the formation of the shells takes place in a stellar wind that moves outwards ballistically from the photosphere. Small fluctuations in wind speed are introduced there and eventually lead to the condensation of shells at a radial distance of the order of 10¹⁴ cm. The density in the shells is large enough to account for their acceleration by the Lyman continuum emitted by the star. It is also proposed here that the observed time scale of two months is set by the dynamics of the flow field in interaction with the random fluctuations imposed on it at the photosphere.