We study the observational constraints of mass and redshift on the properties of the equation of state(EOS)for quark matter in compact stars based on the quasi-particle description.We discuss two scenarios:strange stars and hybrid stars.We construct the equa- tions of state utilizing an extended MIT bag model taking the medium effect into account for quark matter and the relativistic mean field theory for hadron matter.We show that quark matter may exist in strange stars and in the interior of neutron stars.The bag constant is a key parameter that affects strongly the mass of strange stars.The medium effect can lead to the stiffer hybrid-star EOS approaching the pure hadronic EOS,due to the reduction of quark matter,and hence the existence of heavy hybrid stars.We find that a middle range coupling constant may be the best choice for the hybrid stars being compatible with the observational constraints.
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