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Astrophysics Sector

SISSA International School for Advanced Studies

Section of Astrophysics, Astronomy & Mechanics

Aristotle University of Thessaloniki

Max Planck Institute for Gravitational Physics – Albert Einstein Institute

Astrophysics Sector

SISSA International School for Advanced Studies

The existence of compact stellar objects partially or totally consisting of quark matter was already predicted long ago. Such stars are thought likely to originate as a result of the conversion of regular purely hadronic matter in the interior of a neutron star into a deconfined quark matter phase when the density exceeds a certain threshold.

The collapse following such a phase transition in the central parts of the neutron star leads to the formation of a denser configuration with a quark core, a so-called hybrid quark star. Such collapse events are potential sources of gravitational radiation, which could be detected by future gravitational wave detectors. A detection of such signals could also constrain the properties of matter at extreme densities, the rate of such events, and the rotation state of collapsed hybrid quark stars.

Although this scenario is quite speculative, and the underlying physics of the phase transition remains unclear in detail, these models are also excellent toy models for computer codes which are designed to simulate the collapse of stars to compact objects, in particular if they employ a general relativistic description of gravity. Collapsing neutron stars also allow to investigate interesting physical phenomena like the excitation of pulsations (in the form of quasi-normal modes) during the collapse phase and the nonlinear interaction between those modes.

Recently, Lin and collaborators have presented the first numerical study of this scenario [Lin, et al., 2006]. However, these simulations were restricted to a Newtonian formulation, which yields large inaccuracies of global quantities like total mass and angular momentum of the neutron star models.

We have therefore performed general relativistic simulations of a very similar model set, additionally improving on the mechanism of the phase transition in order to model the underlying physics better [Abdikamalov, et al., 2009]. Consequently, we find that the strong convective motions seen in the old models is an artifact of the way the pressure is reduced during the phase transition, and does not occur if a more consistent way of initiating the collapse is chosen. We can also exclude that the generation of this strong convective motion is responsible for the damping of the pulsations after the collapse seens in several models, as suggested in the previous work. Instead, this damping is caused by an ejection of mass from the surface of the nascent hybrid quark star in the case of rapid rotation, where centrifugal forces are strong and matter can be easily shed into the surrounding atmosphere.

Nevertheless, for our more realistic model set we obtain gravitational radiation waveform which are qualitatively and quantitatively similar to those found by Lin et al. Still, some models feature a strongly increasing gravitational wave amplitude after the initial collapse, which points at some mechanism that amplifies the pulsations excited during the phase-transition-induced collapse. A detailed analysis of the dominant pulsation modes and their frequencies reveals that this strong emission of gravitational waves is caused by a nonlinear resonance of two modes that have very similar frequencies. We show that a powerful, but weakly emitting quasi-radial mode transfers pulsation energy to an initially weakly excited but strongly emitting quadrupolar mode. This is the first ever discovery of this theoretically long ago predicted mechanism in an actual fully dynamic nonlinear simulation of stellar collapse. If this mechanism is active during a stellar collapse, it can strongly enhance the emission of gravitational waves from such an event.

• Abdikamalov, E.B., Dimmelmeier, H., Rezzolla, L., and Miller, J.C.,
  "Relativistic simulations of the phase-transition-induced collapse of neutron stars",
  Mon. Not. R. Astron. Soc., 392, 52, (2009),
  [Article in astro-ph].


• Lin L.M., Cheng K.S., Chu M.C., and Suen W.-M.,
  "Gravitational waves from phase-transition-induced collapse of neutron stars",
  Astrophys. J., 639, 382-396, (2006),
  [Article in astro-ph].