The progenitors of compact-object binaries: impact of metallicity, common envelope and natal kicks (1806.00001v3)

Abstract: Six gravitational wave events have been reported by the LIGO-Virgo collaboration (LVC), five of them associated with black hole binary (BHB) mergers and one with a double neutron star (DNS) merger, while the coalescence of a black hole-neutron star (BHNS) binary is still missing. We investigate the progenitors of double compact object binaries with our population-synthesis code MOBSE. MOBSE includes advanced prescriptions for mass loss by stellar winds (depending on metallicity and on the Eddington ratio) and a formalism for core-collapse, electron-capture and (pulsational) pair instability supernovae. We investigate the impact of progenitor's metallicity, of the common-envelope parameter $\alpha{}$ and of the natal kicks on the properties of DNSs, BHNSs and BHBs. We find that neutron-star (NS) masses in DNSs span from 1.1 to 2.0 M$\odot$, with a preference for light NSs, while NSs in merging BHNSs have mostly large masses ($1.3-2.0$ M$\odot$). BHs in merging BHNSs are preferentially low mass ($5-15$ M$\odot$). BH masses in merging BHBs strongly depend on the progenitor's metallicity and span from $\sim{}5$ to $\sim{}45$ M$\odot$. The local merger rate density of both BHNSs and BHBs derived from our simulations is consistent with the values reported by the LVC in all our simulations. In contrast, the local merger rate density of DNSs matches the value inferred from the LVC only if low natal kicks are assumed. This result adds another piece to the intricate puzzle of natal kicks and DNS formation.

• The paper finds that low natal kicks are crucial to matching observed DNS merger rates.
• It employs the MOBSE population synthesis code to simulate the impact of metallicity and common envelope efficiency on binary evolution.
• Results show that metallicity strongly influences BH mass distributions, with low-metallicity environments producing heavier black holes.

Overview of the Progenitors of Compact-Object Binaries

The paper by Giacobbo and Mapelli offers an in-depth analysis of the progenitors of double compact object binaries, focusing on the impact of metallicity, common envelope (CE) efficiency, and natal kicks on the evolution of double neutron stars (DNSs), black hole-neutron star binaries (BHNSs), and black hole binaries (BHBs). Utilizing the population synthesis code {\sc MOBSE}, the paper is pivotal in analyzing how these factors influence the formation of compact-object binaries and their subsequent merger rates.

A critical assessment of the mass distributions and merger rates indicates that the masses of neutron stars (NSs) in DNSs typically range from 1.1 to 2.0 M$_\odot$. BHNSs demonstrate significant tendencies, with NS masses concentrated between 1.3 and 2.0 M$_\odot$, and the black holes (BHs) in these systems are generally low mass (5-15 M$_\odot$). Notably, the BHB's BH masses reveal a marked dependence on progenitor metallicity, with masses spanning from approximately 5 to 45 M$_\odot$.

In terms of merger rates, the derived local merger rate densities of BHNSs and BHBs align with LIGO-Virgo reported values across all simulations. However, discrepancies arise for DNSs; their local merger rate density aligns with LIGO-Virgo only under the assumption of low natal kicks.

The paper concludes that natal kicks significantly impact DNS formation, suggesting low natal kicks are necessary to match observed DNS merger rates. This supports previous hypotheses that suggest lower kicks in binaries due to reduced asymmetries during supernovae events. Moreover, the paper confirms that metallicity profoundly impacts the formation of BH systems; low-metallicity environments favor the formation of more massive BHs due to reduced stellar winds.

Practically, this research informs the astrophysical community's understanding of binary star evolution and contributes to ongoing debates regarding natal kicks and CE efficiency. It also emphasizes the critical role of metallicity, which influences mass loss and, consequently, BH mass in systems.

Looking ahead, the implications suggest possible refinements in population synthesis codes, integrating more nuanced models of metallicity evolution in the universe. This would refine the predictions of merger rates and help interpret future gravitational wave observations. The paper indirectly points to the utility of multi-messenger astronomy in resolving the natal kick conundrum and improving the accuracy of compact-object binary models.

In sum, this paper provides significant insights into the progenitors of compact-object binaries, leveraging population synthesis models to elucidate the complex interplay of factors influencing their evolution and merger rates. These findings not only corroborate certain aspects of the current theoretical frameworks but also challenge assumptions that need further investigation through both observational and computational approaches.