Lower limits on the maximum orbital frequency around rotating strange stars

Observations of kHz quasi-periodic oscillations (QPOs) in the X-ray fluxes of low-mass X-ray binaries (LMXBs) have been used in attempts to constrain the external metric of the compact members of these binaries, as well as their masses and the equation of state of matter at supranuclear denisties. We compute the maximum orbital frequency of stable circular motion around uniformly rotating strange stars described by the MIT bag model. The calculations are performed for both normal and supramassive constant baryon mass sequences of strange stars rotating at all possible rates. We find the lower limits on the maximum orbital frequency and discuss them for a range of masses and for all rotational frequencies allowed in the model considered. We show that for slowly and moderately rotating strange stars the maximum value of orbital frequency can be a good indicator of the mass of the compact object. However, for rapidly rotating strange stars the same value of orbital frequency in the innermost stable circular orbit is obtained for stars with masses ranging from that of a planetoid to about three solar masses. At sufficiently high rotation rates of the strange star, the rotational period alone constrains the stellar mass to a surprisingly narrow range.