2. Strange stars
Quark stars are likely to exist if the ground state of matter at large atomic number is in the form of a quark fluid, which would then necessarily be composed of about equal numbers of up, down, and strange quarks (Bodmer, 1971). Today, such matter is called strange matter. Its thermodynamic properties have been discussed in detail within the bag model by Farhi and Jaffe (1984) in the context of quantum chromodynamics. The first relativistic model of stars composed of quark matter was computed by (Brecher and Caporaso, 1976). The cosmological consequences of the presumed existence of strange matter were first discussed in detail by (Witten, 1984), who also showed that the maximum mass of a static strange star scales as and is for . Detailed models of strange stars have been constructed by Alcock et al. (1986) and Haensel et al. (1986).
Astrophysical implications of these ideas are not yet clear. It has been pointed out that young, glitching radio pulsars are probably neutron stars (Alpar, 1987), and that strange stars are unlikely to be present in Hulse-Taylor type binaries, as their coalescence may lead to dispersal of nuggets of quark matter which would have precluded the formation of young neutron stars in the Galaxy (Madsen 1988, Caldwell and Friedman 1991). However, there seems to be no objection to millisecond pulsars or the compact objects in LMXBs being strange stars (Kluzniak, 1994; Cheng and Dai, 1996), and it has even been suggested (Madsen 1999) that millisecond pulsars can be formed directly in supernovae, if they are strange stars [unlike neutron stars, whose rotation rate would be quickly damped by the r-mode instability: Lindblom et al. (1998), Andersson et al. (1999), Kokkotas and Stergioulas (1999)].
Our work is informed by the question whether the presence of strange stars in LMXBs can be excluded on the basis of the observed timing properties of these sources. 2 Specifically, it has been asked whether the observed frequency of the kHz QPOs in sources such as 4U 1820 -30 (1.07 kHz) is not too low to be compatible with the maximum mass of strange stars (Bulik et al., 1999). For this reason, in constructing our models of strange stars, we have focussed on an equation of state which yields the largest masses of static strange star models:
We have also investigated the more general case, where the factor of 1/3 in Eq. (1) is replaced by a different positive constant, .
© European Southern Observatory (ESO) 1999
Online publication: December 2, 1999