A generalized semi-analytic model for the combined outflow from a cluster of stars with strong stellar winds

Abstract

We derive semi-analytic solutions for the thermally driven cumulative outflow from a dense stellar cluster that consists of a large number of stars with strong stellar winds, under the key assumption that their mass and energy deposition can be treated as a continuous source of thermalized wind material. Our approach provides explicit analytic forms for the flow critical radius and sound speed, which then allows for full solution by inward/outward integration from this critical radius. Application to previous models that assume either a power-law or exponential mass deposition allows a direct comparison of their velocity solutions. We then obtain solutions for new models that have mass depositions with a gaussian or the empirically motivated forms derived by Plummer and Elson. Comparisons show that, when cast in terms of the radius scaled by the critical radius, the overall velocity laws are all quite similar, asymptotically approaching the energy-averaged stellar wind speed $V_w$, and anchored to passing through a common critical/sonic speed $v_c = V_w/2$. By deriving associated variations in outflow density and temperature, we obtain scaling forms for the expected X-ray emission, and show that radiative cooling generally represents only a small correction to the assumed cooling from adiabatic expansion. An initial analysis of incomplete wind thermalization suggests this should likewise have modest effect on the overall outflow in dense clusters. The simplified semi-analytic method presented here can be readily applied to alternative mass depositions and so form a basis for interpreting results from detailed numerical simulations of outflows from young stellar clusters.