6 dent of centrality. A centrality dependence of observables appears because the relative fraction of core and corona contribution varies with centrality. Core particles are in statistical equilibrium. Their interaction with corona particles as well as the interaction among corona particles has to be unimportant otherwise momentum would be transferred between the different species and eq. 2 would not be valid anymore. This suggest also a very limited number of collisional interactions between hadrons from core and corona. Acknowledgements: We would like to thank Drs. R. Bellwied, M. Estienne, U. Heinz, J.Y. Ollitrault, S. Shi, R. Snellings, A. Timmings for valuable discussions. The success of the core - corona model suggests the following reaction scenario: The experimental data are a superposition of two contributions: a corona contribu- tion which is given by pp physics and a core contribution which agrees with the assumption that the core forms an equilibrated source. The data are compatible with the as- sumption that the properties of this source are indepen- The model predicts even the experimentally observed correlations between peripheral heavy ion and pp colli- sions. Such a correlation is alien to any hydrodynamical approach. Also the larger v$_{2}$/ϵ$_{part}$ value for CuCu as compared to AuAu for the same number of N$_{part}$ ques- tions whether the centrality dependence of v$_{2}$/ϵ$_{part}$ can be related to the viscosity of the plasma. [17] S.S. Shi (STAR collaboration), private communication [16] C. Alt et al. [NA49 Collaboration] Phys. Rev. C 68 , 034903 (2003) [arXiv:nucl-ex/0303001] [15] B. I. Abelev et al. [STAR Collaboration] Phys. Rev. C 77 , 054901 (2008) [arXiv:0801.3466 [nucl-ex]] [8] B. I. Abelev et al. [STAR Collaboration], Phys. Rev. C [7] B. I. Abelev et al. [STAR Collaboration] Phys. Rev. C 79 , 034909 (2009) [arXiv:0808.2041 [nucl-ex]] [6] T. Anticic et al. [NA49 Collaboration] arXiv:0906.0469 [hep-ex] [5] H. Song and U. W. Heinz Phys. Rev. C 78 , 024902 (2008) [arXiv:0805.1756 [nucl-th]] [14] B. Alver et al. [PHOBOS Collaboration] Phys. Rev. Lett. 98 , 242302 (2007) [arXiv:nucl-ex/0610037] [13] B. I. Abelev et al. [STAR Collaboration] Phys. Rev. C 75 , 064901 (2007) [arXiv:nucl-ex/0607033] [12] S. S. Adler et al. [PHENIX Collaboration], Phys. Rev. C 69 (2004) 034909 [arXiv:nucl-ex/0307022]. [11] A. Timmins, STAR collaboration, private communica- tion [10] M. Estienne, PhD Thesis, University of Nantes, [9] B. I. Abelev et al. [STAR Collaboration] Phys. Lett. B 673 , 183 (2009) [arXiv:0810.4979 [nucl-ex]] [4] M. Luzum and P. Romatschke Phys. Rev. C 78 , 034915 (2008) [Erratum-ibid. C 79 , 039903 (2009)] [arXiv:0804.4015 [nucl-th]] [3] H. J. Drescher, A. Dumitru, C. Gombeaud and J. Y. Ol- litrault Phys. Rev. C 76 , 024905 (2007) [arXiv:0704.3553 [nucl-th]] [2] J. Aichelin and K. Werner, Phys. Rev. C 79 (2009) 064907 [Erratum-ibid. C 81 (2010) 029902] [arXiv:0810.4465 [nucl-th]]. [1] K. Werner, Phys. Rev. Lett. 98 , 152301 (2007) [arXiv:0704.1270 [nucl-th]]. 79 (2009) 064903 [arXiv:0809.4737 [nucl-ex]].