Circumbinary disks in post common envelope binary systems with compact objects

TL;DR

This work addresses the formation and dynamical impact of circumbinary disks (CBDs) formed after common envelope evolution (CEE) in massive binaries containing a giant and a compact object (NS/BH). It couples a toy CBD formation model to the COMPAS population synthesis data, using a CBD angular-momentum criterion j_CBD = β J_total,RL / M_env,RL with a formation threshold j_CBD ≥ ζ sqrt{G(M_core+M_CO,f) R_CBD,in}, where R_CBD,in ≈ 2.5 a_f and ζ encodes the disk mass distribution. The study finds CBDs form most readily in BH-BH and NS-NS systems that merge within a Hubble time, but CBD–binary torques generally reduce the number of DCO mergers, potentially producing CEJSN-like transients if contraction drives a core–compact object merger. These results imply CBD formation must be accounted for in population-synthesis predictions to obtain reliable gravitational wave event-rate estimates and to anticipate associated luminous transients; they also highlight sensitivities to metallicity and disk-property parameters that motivate future coupled simulations.

Abstract

We conduct a population synthesis study using the binary population synthesis code compas to explore the formation of circumbinary disks (CBDs) following the common envelope evolution (CEE) phase of a giant star and a neutron star (NS) or black hole (BH). We focus on massive binary systems that evolve into double compact object (DCO) binaries after the exposed core of the giant collapses to form a second NS or BH. A CBD around the binary system of the giant's core and the compact object alters the orbital evolution of the binary. We parameterize the conditions for CBD formation in post-CEE binaries and present characteristics of DCO progenitors that are likely or unlikely to form CBDs. We find that CBD formation is most common in BH-BH binaries and NS-NS binaries that are expected to merge within Hubble time. Furthermore, we find that the interaction of the CBD with the core - NS/BH system at the termination of the CEE reduces the expected rate of DCO mergers, regardless of whether these binaries tighten or expand due to this interaction. If the binary system loses angular momentum to the CBD, it may produce a luminous transient due to a merger between the NS/BH and the core of the giant rather than gravitational wave sources. Thus, accounting for post-CEE CBD formation and its interaction with the binary system in population synthesis studies is significant for obtaining reliable predictions of the gravitational wave event rates expected by current detectors.

Figures (7)

• Figure 1: The percentage of NS-NS (blue dots), BH-BH (green dots), and NS-BH (red dots) systems that can form a CBD, from all systems that go through NS/BH-giant CEE phase and end as NS-NS systems, BH-BH systems, and NS-BH systems, respectively, taking a more restrictive condition for CBD formation ($\zeta = 4$). Assuming the minimum specific angular momentum needed to form a thin ring that can subsequently viscously spread into a CBD (i.e., $\zeta = 1$), the corresponding $\beta$-axis should be rescaled downwards by a factor of four.
• Figure 2: Properties of NS-NS binary systems for $\beta=1.859$ (see equation \ref{['eq:beta_updated']}) with (orange bins) and without (blue bins) post-CEE CBDs. The histograms show the distributions of the orbital semi-major axis $a_{\rm RL}$ (left panels), and the mass ratio $q \equiv M_{\mathrm{CO},RL}/ M_{\rm *,RL}$ (middle panels) at the onset of RLOF, and of the orbital separation after CEE $a_{\rm f}$. The vertical axis represents the number of systems per bin, normalized such that the total sum over all bins is equal to one. The upper row displays the distributions of systems that will merge within Hubble time and the bottom row presents distributions for systems that are not expected to merge. The factor $f_{\rm CCSN}$ is the fraction of the systems relative to all core-collapse supernovae.
• Figure 3: Similar to Figure \ref{['fig:Fig_NSNS']} for BH-BH binary systems with $\beta$=0.656 .
• Figure 4: Similar to Figure \ref{['fig:Fig_NSNS']} for NS-BH binary systems with $\beta$=0.713 .
• Figure 5: Orbital period vs total mass of the binary systems at the onset of CEE for progenitors of NS-NS binaries (blue dots), BH-BH binaries (green dots), and NS-BH binaries (red dots) that form a post-CEE CBD. Above and to the right: histograms showing the distributions of both quantities for the different DCO binaries subclasses. We normalized the histograms such that in each sub-panel the sum over the bins is equal to one.