The pressure confined wind of the massive and compact superstar cluster M82-A1

TL;DR

This paper models the wind dynamics of the young superstar cluster M82-A1, showing that radiative cooling and high interstellar pressure confine the wind, affecting the cluster's evolution and surrounding HII region.

Contribution

It introduces a bimodal hydrodynamic model for cluster winds considering radiative cooling and external pressure, providing analytical and semi-analytical solutions.

Findings

Cluster wind is confined by high external pressure.

Radiative cooling reduces energy and mass outflow.

Analytical models match observed HII region parameters.

Abstract

The observed parameters of the young superstar cluster M82-A1 and its associated compact HII region are here shown to indicate a low heating efficiency or immediate loss, through radiative cooling, of a large fraction of the energy inserted by stellar winds and supernovae during the early evolution of the cluster. This implies a bimodal hydrodynamic solution which leads to a reduced mass deposition rate into the ISM, with a much reduced outflow velocity. Furthermore, to match the observed parameters of the HII region associated to M82-A1, the resultant star cluster wind is here shown to ought to be confined by a high pressure interstellar medium. The cluster wind parameters, as well as the location of the reverse shock, its cooling length and the radius of the standing outer HII region are derived analytically. All of these properties are then confirmed with a semi-analytical integration of the flow equations, which provides us also with the run of the hydrodynamic variables as a function of radius. The impact of the results is discussed and extended to other massive and young superstar clusters surrounded by a compact HII region.