Cold quark matter with heavy quarks and the stability of charm stars

We study the effects of heavy quarks on the equation of state for cold and dense quark matter obtained from perturbative QCD, yielding observables parametrized only by the renormalization scale. We investigate the thermodynamics of charm quark matter under the constraints of $\beta$ equilibrium and electric charge neutrality in a region of densities where perturbative QCD is, in principle, much more reliable. We also analyze the stability of charm stars, which might be realized as a new branch of ultradense hybrid compact stars, and find that such quark stars are unstable under radial oscillations.