Astron. Astrophys. 363, 1005-1012 (2000)

4. Discussion

4.1. Summary of the properties of rotating compact strange stars

We have calculated numerical models of the uniformly rotating strange stars described by the SS1 and SS2 equations of state using the multi-domain spectral methods, which allows a treatment of the density discontinuity at the surface of self-bound stars with even very high . The model used here describes the quark interactions self-consistently. The maximum mass of strange stars within this model is relatively low, and the stars are very compact. We find that the stars within the Dey model can rotate much faster than typical neutron stars, and also the MIT bag model strange stars. The maximum allowed rotational frequency is kHz for SS1 and kHz for SS2. The maximal mass of a rotating configuration is for SS1 and for SS2. The main physical reason for these high values of the rotational frequency and the low gravitational mass is that the parameter is quite large for this EOS.

Some properties of rotating strange stars described by the Dey EOS are universal and characteristic for all self-bound EOS. We find that: i) there are two cases for the maximal mass of a rotating configuration as the rotation frequency increases; first, for low rotation frequencies the increase in the maximal mass is only a second order effect ( is very close to the line of limiting stability against a quasi-radial perturbation limit); second, for higher frequencies the maximal mass configurations are Keplerian; ii), the maximum mass of strange stars given by the MIT model and by SS1 and SS2 at the point of intersection with the line imposed by the Keplerian limit is approximately greater than the maximum mass of a static configuration; iii), the maximum allowed mass is approximately larger than the static maximum mass. This is much higher than for neutron stars; iv), we show that in contrast to normal neutron stars the maximal rotating frequency for both normal and supramassive stars is never the Keplerian one; v), we find that rotating strange stars have a very high ratio . In the case of the Keplerian limit stars the ratio increases with decreasing mass. Large values of (higher than 0.2) imply that it is quite likely that the maximum rotational frequency can in fact be lower than found here.

4.2. Astrophysical aspects of the compact strange stars

The maximum frequency is very high - 2.6 kHz and 2.8 kHz for SS1 and SS2 models, respectively. It is important to remember that the maximum frequency occurs only for one extreme supramassive model and that this model is both on the mass-shed limit and the stability limit. But even for normal evolutionary sequences we reach very high frequencies - higher than 1.8 kHz and 2 kHz in the case of SS1 and SS2 respectively. The periods for stars rotating with maximal frequency can be shorter than half a millisecond, much shorter than the period of the fastest known millisecond pulsar PSR 1937 + 21.

The maximal masses for the SS1 and SS2 EOS are consistent with the observed masses of compact object. All observed pulsars have masses close to and rotate with frequencies lower than the maximal frequencies for the SS1 and SS2 models. In the case of strange stars described by the SS1 and SS2 equations of state the maximum baryon mass are and (the difference between the baryon and gravitational mass is much greater than in the case of neutron stars). One can speculate that for high central densities in the core of a neutron star a phase transition to strange matter can take place. This can be accompanied by large energy release, and possibly a gamma-ray burst (?). A rotating neutron star may become a strange star, conserving its total baryon mass and angular momentum. If the baryon mass of this star is lower than 1.84 it would become a normal sequence compact strange star. Otherwise, depending on its angular velocity, it can become a stable supramassive strange star and after it slows down finally a black hole. Just before transformation from a supramassive strange star to a black hole the star should accelarate (such phenomena was noticed by Cook et al. (1994) in the case of supramassive neutron stars). In Fig. 3 we show an evolutionary sequence with the baryon mass as an example of a low mass supramassive stars. This sequence begins at high J on the Keplerian limit, then losing angular momentum it reaches the maximum mass limit, and finally it spins up to reach the stability limit and collapse to a black hole. If a neutron star goes through a strange star stage and then ends up as a black hole, this may be an explanation as to why we do not observe pulsars with masses much higher than 1.4 (if pulsars are strange stars described by SS1 and SS2 equation of state.)

The masses of compact objects in LMXBs (inferred from kHz QPOs and assuming that the highest QPO frequency observed is related to a marginally stable orbit) are quite large, and extend even above . Such high mass neutron stars can still undergo a phase transition to form a supramassive compact strange star, which would consequently turn into a black hole. Note that the binary might be disrupted during the transition. To find such stars we would be looking for very fast millisecond pulsars, either single or in binaries.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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