New Research Note on the peak GW frequency emitted by an eccentric binary
December 2 2021
This Research Note of the AAS (link) presents a numerical fit to the peak harmonic gravitational wave (GW) frequency emitted by an eccentric binary system in the post-Newtonian approximation. This fit significantly improves upon a previous commonly-used fit, in particular for eccentricities ~< 0.8, with potentially important implications for some previous predictions of eccentric GW sources in both the LIGO and LISA detector bands. The new fit should be useful for future studies of eccentric sources which are motivated by the anticipation of the detection of eccentric sources in future GW observations.
Visitors from NBIA and mini-workshop
November 11 2021
The Multiple-Star Evolution Group at MPA was happy to receive a group of visitors from the Niels Bohr International Academy (NBIA) consisting of Johan Samsing, Dan D'Orazio, Bin Liu, and Alejandro Vigna-Gómez during the week of November 8. On the occasion of their visit and in collaboration with MPA Director Selma de Mink, a successful mini-workshop on Gravitational Wave Astrophysics was held at MPA on November 9. See here for a few photographs.
New paper on first and second generation BH and NS mergers from 2+2 quadruples
July 26 2021
In Hamers et al. (2021; url), we study black hole (BH) and neutron star (NS) mergers from stellar quadruple system in the 2+2 configuration (two binaries orbiting each other's centre of mass). In such systems, secular evolution can accelerate the merger of one of the inner binaries; subsequently, the merger remnant may interact with the companion binary, yielding a second-generation merger. We model the initial stellar and binary evolution of the inner binaries as isolated systems. In the case of successful compact object formation, we subsequently follow the secular dynamical evolution of the quadruple system. When a merger occurs, we take into account merger recoil, and model subsequent evolution using direct N-body integration. With different assumptions on the initial properties, we find that the majority of first-generation mergers are not much affected by secular evolution, with their observational properties mostly consistent with isolated binaries. A small subset shows imprints of secular evolution through residual eccentricity in the LIGO band, and retrograde spin-orbit orientations. Second-generation mergers are ~1e7 times less common than first-generation mergers, and can be strongly affected by scattering (i.e., three-body interactions) induced by the first-generation merger. In particular, scattering can account for mergers within the low-end mass gap, although not the high-end mass gap. Also, in a few cases, scattering could explain highly eccentric LIGO sources and negative effective spin parameters. In this work, gravitational dynamics and stellar/binary evolution were treated as decoupled effects, and this assumption can break down depending on the system parameters. This will be addressed in future work.
New group member
July 1 2021
The group welcomes a new PhD student, Abinaya Swaruba Rajamuthukumar, who previously completed her Masters in physics at Christ University Bangalore, India. She will be working on white dwarf interactions in multiple-star systems.
Multiple Stellar Evolution (MSE): code paper and semi-public release
January 29 2021

The group announces the semi-public release of the Multiple Stellar Evolution (MSE) code. MSE is designed to rapidly model the stellar, binary, and dynamical evolution of multiple-star systems. It includes a number of new features not present in previous population synthesis codes: 1) an arbitrary number of stars, as long as the initial system is hierarchical, 2) dynamic switching between secular and direct N-body integration for efficient computation of the gravitational dynamics, 3) treatment of mass transfer in eccentric orbits, which occurs commonly in multiple-star systems, 4) a simple treatment of tidal, common-envelope, and mass transfer evolution in which the accretor is a binary instead of a single star, 5) taking into account planets within the stellar system, and 6) including gravitational perturbations from passing field stars. The code is written primarily in the C++ language and has few prerequisites; a convenient Python interface is provided. A detailed description of MSE is given in Hamers et al., (2021; url). For access to the GitHub repository (currently private), please contact A. Hamers. The code will be made publicly available in the future. 

New paper on von Zeipel-Lidov-Kozai oscillations in triple systems
November 9 2020

In Hamers (2020; url), we consider von Zeipel-Lidov-Kozai oscillations in triple systems, and focus on the case when the angular momentum in the inner orbit is not negligible compared to that of the outer orbit (as opposed to the "test particle" approximation). This case applies for example to systems with comparable masses such as stellar triples. We characterise the nature of the oscillations as a function of γ, a ratio of inner-to-outer orbital angular momenta variables. In particular, eccentricity oscillations are more effective at retrograde orientations for non-zero γ; assuming zero initial inner eccentricity, the maximum eccentricity peaks at cos(irel,0) = −γ, where irel,0 is the initial relative inclination. We provide a Python script which can be used to quickly compute these properties. 

Educational book Moving Planets Around released
September 1 2020

The educational book Moving Planets Around by J. Roa, A.S. Hamers, M.X. Cai, and N.W.C. Leigh (link at The MIT Press), offers both an introduction to the laws of celestial mechanics and a step-by-step guide to developing software for direct use in astrophysics research. The text is written engagingly in dialogue form. It not only educates students on the laws of Newtonian gravity, it also provides all that they need to start writing their own software, from scratch, for simulating the dynamical evolution of planets and exoplanets, stars, or other heavenly bodies. The first half of the book develops a fully functional N-body integrator, using state-of-the art integration techniques. The second half of the book focuses on using an advanced integration scheme to conduct real research, leading students in an investigation of the long-term dynamical stability of extrasolar circumbinary planets. At the end of the journey, students will be ready to design, conduct, and publish peer-review quality research. In addition to the physical copy, the book is available online as Open Access (link).  

New group member
September 1 2020

The group welcomes PhD student Pavan Vynatheya, who recently obtained his MSc degree at the Indian Institute of Science Education and Research in Kolkata, India. Pavan will be working on making predictions for gravitational wave sources in multiple-star systems such as triple and quadruple systems, using population synthesis methods. 

New group member
July 8 2020

The group welcomes Holly Preece, who recently obtained her PhD at the University of Cambridge and Armagh Observatory and Planetarium under supervision of Christopher Tout and Simon Jeffery. Holly is an expert on tidal interactions in binary systems.  

New paper on a quadruple origin of GW190412
June 18 2020

In Hamers & Safarzadeh (2020; url), we investigate whether the gravitational wave source GW190412, consisting of two black holes with unusually unequal masses (mass ratio 0.28), could have formed from a "double merger" event in a hierarchical 3+1 quadruple system. In this scenario, the innermost binary merges first due to secular chaotic evolution. In the resulting triple, secular Lidov-Kozai evolution could subsequently lead to a second merger. We show that this scenario produces characteristics that are consistent with GW190412, although its likelihood is highly uncertain due to influences from stellar and binary evolution that were ignored here. 

New paper on MS interactions in systems of the Multiple Star Catalogue
April 22 2020

In Hamers (2020; url), we study the long-term dynamical main-sequence (MS) evolution of hierarchical systems with 3 to 6 stars based on Andrei Tokovinin's Multiple Star Catalogue. We estimate statistical probabilities for strong interactions during the MS such as tidal evolution and mass transfer, and the onset of dynamical instability. Our results indicate that a significant fraction of high-multiplicity systems interact or become dynamically unstable already during the MS, with an increasing importance as the number of stars increases. 

New paper on suborbital effects in multiple systems
April 20 2020

In Hamers (2020; url), we present an update to the SecularMultiple code, which integrates the long-term dynamical evolution of multiple systems with any number of bodies and hierarchical structure, provided that the system is composed of nested binaries. In the update, we incorporate hybrid integration techniques, allowing orbits to be either directly integrated (N-body method), or averaged over (secular method). We also implement orbit-averaging correction terms. The updated code is freely available on GitHub 

New paper on binary-binary scattering
March 9 2020

In Hamers & Samsing (2020; url), we study the gravitational effects of scattering of two binaries in the "weak" limit, i.e., in which the two binaries approach each other at a relatively large distance. We find that, to good approximation, one can use previous analytic results for binary-single scattering in the weak limit, and replace the mass of the single interloping body with the total mass of the interloping binary. In other words, the point mass approximation works well for weak binary-binary scattering. 

New group member
March 1 2020

The group welcomes Patrick Neunteufel, previously a postdoc a the University of Leicester, and who completed his PhD with Norbert Langer at the Argelander Institute in Bonn in 2017. Patrick is an expert on binary star evolution. 

New web site launched
October 23 2019

A new web site has been launched for the group. Please note that it is still under construction.