The Formation of Eccentric Compact Binary Inspirals and the Role of Gravitational Wave Emission in Binary-Single Stellar Encounters (1308.2964v1)

Abstract: The inspiral and merger of eccentric binaries leads to gravitational waveforms distinct from those generated by circularly merging binaries. Dynamical environments can assemble binaries with high eccentricity and peak frequencies within the {\it LIGO} band. In this paper, we study binary-single stellar scatterings occurring in dense stellar systems as a source of eccentrically-inspiraling binaries. Many interactions between compact binaries and single objects are characterized by chaotic resonances in which the binary-single system undergoes many exchanges before reaching a final state. During these chaotic resonances, a pair of objects has a non-negligible probability of experiencing a very close passage. Significant orbital energy and angular momentum are carried away from the system by gravitational wave (GW) radiation in these close passages and in some cases this implies an inspiral time shorter than the orbital period of the bound third body. We derive the cross section for such dynamical inspiral outcomes through analytical arguments and through numerical scattering experiments including GW losses. We show that the cross section for dynamical inspirals grows with increasing target binary semi-major axis, $a$, and that for equal-mass binaries it scales as $a^{2/7}$. Thus, we expect wide target binaries to predominantly contribute to the production of these relativistic outcomes. We estimate that eccentric inspirals account for approximately one percent of dynamically assembled non-eccentric merging binaries. While these events are rare, we show that binary-single scatterings are a more effective formation channel than single-single captures for the production of eccentrically-inspiraling binaries, even given modest binary fractions.

• The paper presents that gravitational wave emission during binary-single encounters in dense clusters significantly influences the formation of eccentric compact binaries.
• It reveals a scaling relation where the inspiral cross section increases with the binary's semi-major axis as a^(2/7) for equal-mass binaries.
• The results imply that eccentric inspirals hold distinct gravitational wave signatures, potentially enhancing detection strategies for observatories like LIGO.

The Formation of Eccentric Compact Binary Inspirals and the Role of Gravitational Wave Emission

This paper presents an investigation into the processes responsible for the formation of eccentric compact binary inspirals, particularly focusing on the role of gravitational wave (GW) emission during binary-single stellar encounters in dense stellar environments such as galactic nuclei and globular clusters. The paper is grounded in the fact that the inspiral and merger of eccentric binaries produce GW signals that are distinct from the circular signals typically modeled by detectors such as LIGO, thereby necessitating further understanding of their formation channels and characteristics.

Dynamical Formation and Gravitational Wave Emission

The authors explore binary-single stellar scatterings—common occurrences in densely populated stellar environments—as pivotal channels leading to the formation of eccentric binaries. During these interactions, chaotic resonances occur, characterized by exchanges and close dynamic interactions between the binary and the single star. Through these interactions, significant orbital energy and angular momentum are lost primarily via GW emission, potentially resulting in short merger times for the formed binaries.

The paper provides a significant result: the inspiral cross section for dynamically formed binaries increases with the binary's semi-major axis (SMA) following a scaling relation of $a^{2/7}$ for equal-mass binaries. This counterintuitive result demonstrates that wider binaries contribute more significantly to the population of these eccentric inspirals.

Distinct Characteristics of Eccentric Inspirals

Eccentric inspirals formed through these dynamic interactions possess unique properties compared to non-eccentric mergers. The paper reveals that while these events are relatively rare, accounting for roughly one percent of all dynamically assembled non-eccentric merging binaries, they are primarily formed through binary-single encounters rather than single-single capture events.

This paper suggests that eccentric inspirals maintain their high eccentricity as they enter the frequency bands detectable by LIGO, offering potential new observational targets that require unique waveform templates distinct from standard circular inspiral models. This insight challenges the detection strategies currently employed, emphasizing the need for future detection frameworks that can accurately identify such unique signals.

Implications and Future Directions

In terms of theoretical and practical implications, the paper provides valuable insights into the population synthesis of compact object mergers in dense star clusters. It calls for further investigation into the rates and observational characteristics of these eccentric inspirals, possibly affecting current models of compact binary evolution and associated event rates derived from observational surveys.

For future work, the findings suggest several avenues: extending the analysis to include systems with varying mass ratios and compositions, such as those including white dwarfs; incorporating the effects of dynamical friction and stellar evolution on long-term cluster dynamics; and improving the modeling of binary-single interactions to include more complex physical effects like tidal forces and neutron star matter equations of state.

The research lays out a framework for better understanding the gravitational wave signals that might arise from dense stellar environments, offering the potential to refine our understanding of such environments and the complex gravitational dynamics they involve. These developments hold promise not only for future detections through gravitational wave astronomy but also for broader astrophysical insights into the end stages of stellar evolution in dense stellar systems.