Hot scalar radiation around a cosmic string setting bounds on the   coupling parameter $\xi$
  In this work, by investigating the low temperature behaviour of a scalar field around a cosmic string, and assuming stable thermodynamic equilibrium, it is shown that the coupling parameter of the field with curvature must be restricted to a certain range of values whose bounds are determined by the deficit angle of the associated conical geometry. 
E. S. Moreira Jr
gr-qc astro-ph.CO hep-th