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

1. Introduction

The possibility of the existence of quark matter dates back to the early seventies. Bodmer (1971) remarked that matter consisting of deconfined up, down and strange quarks could be the absolute ground state of matter at zero pressure and temperature. If this is true then objects made of such matter so-called "strange stars" could exist (Witten 1984). Most of the previous calculations of the properties of strange stars (static models - see e.g. Alcock et al.,1986; Haensel et al., 1986, for rotating models, see references in Gourgoulhon et al. 1999) were done for an equation of state based on the MIT bag model, in which quark confinement and asymptotic freedom were postulated from the very beginning, and the deconfinement of quarks at high densities was not obvious.

Several authors have addressed the question of the existence of strange stars in the Galaxy. A binary merger of two strange stars (or of a strange star with a neutron star) might contaminate the entire Galaxy with strange matter seeds, thus precluding the formation of young neutron stars in supernovae (?). However, Alpar (1987) noted that glitching radio pulsars are not strange stars, but young neutron stars. Hence, it would appear, that strange stars are not present in binaries of the Hulse-Taylor type. By extension, it seems unlikely that strange stars are born in ordinary supernovae, as a fraction of supernovae must in fact occur in progenitor systems of Hulse-Taylor type binaries. However, strange stars could exist as millisecond pulsars and be formed from neutron stars in low-mass X-ray binaries through a phase transition (?).

Strange stars described by the simple MIT bag model with massless and non-interacting quarks have orbital frequencies in the marginally stable orbit which are higher than the lowest maximum frequency of kHz quasiperiodic oscillations observed in LMXBs (Bulik et al. 1999a,b) . However the corresponding frequencies for more sophisticated models (MIT bag model with massive strange quarks and lowest order QCD interactions, and/or rotation (Stergioulas et al. 1999; Zdunik et al. 2000a,b) do allow frequencies as low as 1 kHz, in agreement with observations.

Recently Dey et al. (1998) derived an EOS for strange matter which has asymptotic freedom built in and describes deconfined quarks at high density and confinement at zero density. Restoration of chiral quark masses at high density is incorporated in this model and using the model parameter for this restoration one can calculate the density dependence of the strong coupling constant (Ray et al. 2000). This model, with an appropriate choice of the EOS parameters, gives absolutely stable strange quark matter. This equation of state was used to calculate the structure of static strange stars and the mass-radius relations. Later, it was suggested by Li et al. (1999a) that the millisecond X-ray pulsar SAX1808.4-3658 is a strange star. This equation also allowed the explanation of the observed properties of other objects: an analysis of semi-empirical mass-radius relations in 4U 1728-34 (Li et al. 1999b) and 4U 1820-30 (Bombaci 1997), Her X-1 leads to the suggestion that these objects host strange stars (Dey et al. 1998). Two cases of this model have been used in these papers, which will be denoted as SS1 and SS2 equations of state. They both give a rather low value for the maximum gravitational mass for SS1 and for SS2. Very recently, the Dey model was used for calculating frequencies of marginally stable orbits around static strange stars and strange stars rotating with frequency 200 and 580 Hz (Datta et al. 2000). The authors conclude that very high QPO frequencies in the range of 1.9-3.1 kHz would imply existence of a non-magnetized strange star X-ray binary rather than a neutron star X-ray binary.

In this paper, we construct both normal and supramassive constant baryon mass equilibrium sequences in the frame of general relativity. To this end we use the well tested code for rapidly rotating compact objects, for description see Gourgoulhon et al. (1999). Calculations of equilibrium sequences of rapidly rotating compact stars are crucial to understand various astrophysical phenomena and objects, such as LMXBs, QPO and millisecond pulsars. In this paper we calculate the properties of rapidly rotating strange stars described by the models SS1 and SS2. We study which of these are characteristic for stars within the Dey et al. 1998 model and which are common for all models described by self-bound EOS. We compare the properties of compact rotating strange stars with those for neutron stars. We find the upper limits on observable astrophysical quantities, such as masses and frequencies of rotating stars. We locate the rotating maximum mass model and the maximum angular velocity model.

In Sect. 2 we outline briefly the equation of state used throughout this work and compare static model SS1 and SS2 stars with the MIT bag model strange stars. In Sect. 3 we describe the rotating configuration of the compact strange stars, and in Sect. 4 we discuss the results.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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