The mass spectrum of compact remnants from the parsec stellar evolution tracks

Abstract

The mass spectrum of stellar mass black holes (BHs) is highly uncertain. Dynamical mass measurements are available only for few (∼10) BHs in X-ray binaries, while theoretical models strongly depend on the hydrodynamics of supernova (SN) explosions and on the evolution of massive stars. In this paper, we present and discuss the mass spectrum of compact remnants that we obtained with sevn, a new public population-synthesis code, which couples the parsec stellar evolution tracks with up-to-date recipes for SN explosion (depending on the carbon–oxygen mass of the progenitor, on the compactness of the stellar core at pre-SN stage and on a recent two-parameter criterion based on the dimensionless entropy per nucleon at pre-SN stage). sevn can be used both as a stand-alone code and in combination with direct-summation N-body codes (starlab, higpus). The parsec stellar evolution tracks currently implemented in sevn predict significantly larger values of the carbon–oxygen core mass with respect to previous models. For most of the SN recipes we adopt, this implies substantially larger BH masses at low metallicity (≤2 × 10−3), than other population synthesis codes. The maximum BH mass found with sevn is ∼25, 60 and 130 M⊙ at metallicity Z = 2 × 10−2, 2 × 10−3 and 2 × 10−4, respectively. Mass loss by stellar winds plays a major role in determining the mass of BHs for very massive stars (≥90 M⊙), while the remnant mass spectrum depends mostly on the adopted SN recipe for lower progenitor masses. We discuss the implications of our results for the transition between neutron star and BH mass, and for the expected number of massive BHs (with mass >25 M⊙) as a function of metallicity.