Supersonic Line Broadening within Young and Massive Super Star Clusters

TL;DR

This paper explains the origin of supersonic nebular lines in young massive star clusters as resulting from repressurizing shocks and cluster winds, influenced by thermal instabilities and heating efficiency, supported by simulations and observations.

Contribution

It introduces a model linking supersonic line broadening to repressurizing shocks and cluster wind dynamics, emphasizing the role of thermal instabilities and heating efficiency in young super star clusters.

Findings

Supersonic lines originate from repressurizing shocks and cluster winds.

Thermalization efficiency in clusters above the threshold line is below 20%.

Model predictions match observed line widths and profiles.

Abstract

The origin of supersonic infrared and radio recombination nebular lines often detected in young and massive superstar clusters are discussed. We suggest that these arise from a collection of repressurizing shocks (RSs), acting effectively to re-establish pressure balance within the cluster volume and from the cluster wind which leads to an even broader although much weaker component. The supersonic lines are here shown to occur in clusters that undergo a bimodal hydrodynamic solution (Tenorio-Tagle et al. 2007), that is within clusters that are above the threshold line in the mechanical luminosity or cluster mass vs the size of the cluster (Silich et al. 2004). The plethora of repressurizing shocks is due to frequent and recurrent thermal instabilities that take place within the matter reinserted by stellar winds and supernovae. We show that the maximum speed of the RSs and of the cluster wind, are both functions of the temperature reached at the stagnation radius. This temperature depends only on the cluster heating efficiency ($\eta$). Based on our two dimensional simulations (Wunsch et al. 2008) we calculate the line profiles that result from several models and confirm our analytical predictions. From a comparison between the predicted and observed values of the half-width zero intensity of the two line components we conclude that the thermalization efficiency in SSC's above the threshold line must be lower than 20%.