[Paper Review] Shedding light on the black hole mass spectrum

Mario Spera, Nicola Giacobbo+1 more|arXiv (Cornell University)|Jun 10, 2016

Pulsars and Gravitational Waves Research6 citations

TL;DR

This paper presents SEVN, a new population synthesis code that integrates up-to-date stellar wind models and supernova explosion mechanisms to predict the stellar black hole (BH) mass spectrum. It shows that at low metallicities (Z ≲ 2×10⁻⁴), BH masses can reach up to ∼100 M⊙ when pair-instability supernovae (PISNe) are included, significantly exceeding predictions from older models.

ABSTRACT

The mass spectrum of stellar black holes (BHs) is highly uncertain. Theoretical models of BH formation strongly depend on the efficiency of stellar winds of the progenitor star and on the supernova (SN) explosion mechanism. We discuss the BH mass spectrum we obtain using SEVN, a new public population-synthesis code that includes up-to-date stellar-wind prescriptions and several SN explosion models. Our models indicate a sub-solar metallicity environment for the progenitors of the gravitational wave source GW150914. We show that our models predict substantially larger BH masses (up to ~100 Msun) than other population synthesis codes, at low metallicity. In this proceeding, we also discuss the impact of pair-instability SNe on our previously published models.

Motivation & Objective

• To resolve uncertainties in the stellar black hole mass spectrum by improving population synthesis models with modern stellar evolution and SN explosion prescriptions.
• To investigate the role of pair-instability supernovae (PISNe) in shaping the upper end of the BH mass function.
• To compare SEVN's predictions with existing codes like SSE and observational constraints from GW150914 and dynamical measurements.
• To assess the impact of metallicity on the formation of massive black holes, particularly in low-Z environments.

Proposed method

• SEVN uses tabulated PARSEC stellar isochrones for on-the-fly interpolation of stellar evolution tracks across a grid of initial masses and metallicities.
• It incorporates five SN explosion models, including delayed, rapid, and compactness-based mechanisms (O’Connor & Ott 2011; Ertl et al. 2016), to predict compact remnant masses.
• The code models PISNe by assuming complete disruption of stars with CO core masses ≥45 M⊙ (helium mass ≥65 M⊙), preventing BH formation in that mass range.
• Stars with helium masses >135 M⊙ are assumed to directly collapse into BHs without explosion.
• SEVN is coupled with N-body codes (e.g., starlab, higpus) for use in dynamical simulations of stellar populations.
• Predictions are validated by comparing SEVN outputs with SSE and observational data, including GW150914 and dynamical BH mass measurements.

Experimental results

Research questions

• RQ1 What is the maximum black hole mass that can form in low-metallicity environments when updated stellar wind and SN explosion models are used?
• RQ2 How do pair-instability supernovae affect the upper end of the black hole mass function in population synthesis models?
• RQ3 How do the predictions of SEVN differ from those of older population synthesis codes like SSE, particularly in the mass gap between neutron stars and black holes?
• RQ4 What metallicity environment is most consistent with the progenitor of GW150914, based on BH mass predictions?
• RQ5 To what extent do different SN explosion models (e.g., rapid vs. delayed) influence the formation of low-mass black holes and the existence of a mass gap?

Key findings

• At Z = 0.02, SEVN predicts a maximum black hole mass of ∼25 M⊙, increasing to ∼60 M⊙ at Z = 0.002 and ∼100 M⊙ at Z = 2×10⁻⁴ when PISNe are included.
• For initial masses up to 350 M⊙, the maximum BH mass reaches ∼275 M⊙ at Z ≲ 2×10⁻⁴, indicating that extremely massive black holes can form in low-metallicity environments.
• The inclusion of PISNe reduces the maximum BH mass in the 150 M⊙ ZAMS limit from ∼130 M⊙ (without PISNe) to ∼100 M⊙ (with PISNe), due to complete disruption of intermediate-mass progenitors.
• The rapid SN model produces a distinct mass gap between ∼2 M⊙ and ∼5 M⊙, consistent with observational hints of a gap between the heaviest neutron star and lightest black hole.
• SEVN predicts substantially higher BH masses than older codes like SSE, especially at low metallicities, due to improved stellar wind and SN explosion prescriptions.
• The progenitor of GW150914 is best explained by a sub-solar metallicity environment (Z ≲ 0.001), consistent with the model's prediction of massive BH formation at low Z.