Is there Quark Matter in (Low-Mass) Pulsars?

The effect of the QCD phase transition is studied for the mass-radius relation of compact stars and for hot and dense matter at a given proton fraction used as input in core-collapse supernova simulations. The phase transitions to the 2SC and CFL color superconducting phases lead to stable hybrid star configurations with a pure quark matter core. In supernova explosions quark matter could be easily produced due to β-equilibrium, small proton fractions and nonvanishing temperatures. A low critical density for the phase transition to quark matter is compatible with present pulsar mass measurements. The nuclear equation of state serves as a crucial input for several astrophysical systems. In supernovae simulations temperatures of T = 20 MeV and higher are reached with a baryon number density slightly above normal nuclear matter density at bounce [1]. In the subsequent evolution of the hot proto-neutron star the central density increases to several times normal nuclear matter density. In neutron star mergers the temperatures and densities reached are also in ranges where one expects the QCD phase transition from hadronic matter to a chirally restored phase. Hunting down strange quark matter in the heavens is coming of age and several signals have been suggested (see [2] for a review) as exotic mass-radius relations of compact stars, rapidly rotating pulsars due to a r-mode stability window, enhanced cooling of neutron stars, gamma-ray bursts by transition to strange quark matter, and gravitational wave signals from phase transitions in binary neutron star collisions, in the collapse of neutron star to a hybrid star or in the r-mode spin-down of hybrid stars. Interestingly, core-collapse supernovae have not been studied in that detail with regard to the QCD phase transition. One recent analysis in connection with quark stars has been done on the x-ray burster EXO 0748676 stating that the mass and the radius should be in the range M ≥ 2.10 ± 0.28M⊙ and R ≥ 13.8 ± 1.8 km [3]. Contrary to the claims made by the author of [3], these constraints do not rule out quark stars or hybrid stars but rather soft equations of state [4]. In addition, a multiwavelength analysis arrives at the conclusion that the data is more consistent with a mass of M = 1.35M⊙ [5]. Also, the mass of the pulsar J0751+1807 has been corrected from (2.1± 0.2)M⊙ down to a value of (1.26± 0.14)M⊙ ‡ talk given at the QM2008 conference, Jaipur, India, February 4–10, 2008 Is there Quark Matter in (Low-Mass) Pulsars? 2 10 12 14 16 18 R@kmD 0.5 1 1.5 2